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Top 100+ JavaScript Interview Questions and Answers for 2024

Introduction:

JavaScript is a key language in the dynamic field of web development. Preparing for a JavaScript interview, whether you’re an experienced programmer or just starting out, may be stressful work. We’ve collected a thorough list of the top 100+ JavaScript interview questions and offered professional solutions to help you manage this process. Let’s have a look at JavaScript and see what you need to know to do well in your interview.

Let’s start with Top 100+ JavaScript Interview Questions and Answers:

1. What is JavaScript?

JavaScript, often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. It is a dynamic programming language that is used for web development, in web applications, for game development, and more. As of 2023, 98.7% of websites use JavaScript on the client side for webpage behavior, often incorporating third-party libraries. All major web browsers have a dedicated JavaScript engine to execute the code on users’ devices.

Some key aspects of JavaScript include:

  • It enables you to create dynamically update content, control multimedia, animate images, and interact with users through the web page.
  • JavaScript is the third layer of the layer cake of standard web technologies, which also includes HTML and CSS.
  • Allows dynamic web page features impossible with HTML and CSS alone, like dropdown menus, extra content, dynamic color changes.
  • JavaScript variables can be numbers, strings, objects, arrays, and functions.
  • It can change HTML content, HTML attribute values, HTML styles (CSS), hide HTML elements, and show HTML elements.

JavaScript is a versatile language that plays a crucial role in modern web development and is essential for creating interactive and dynamic web applications.

2. What are some key differences between JavaScript and Java?

Here is a differences between Java and JavaScript:

JavaJavaScript
Used for programming across domains, including desktop and mobile applicationsUsed for creating interactive web content
Object-oriented, class-based, all-purpose programming languageObject-oriented, cross-platform scripting language
Compiled language requiring two steps to executeInterpreted language that doesn’t get compiled but is interpreted as the script runs
Statically typed, meaning that data types are determined at compile timeDynamically typed, meaning that data types are determined at runtime
Can be executed on any computer powered by a Java Virtual Machine (JVM)Can be executed only on a browser (or a server when using specific implementations)
Used for backend developmentUsed for frontend and backend development
Syntax is similar to C++ and C#Syntax is similar to C++ and Java
Used for building complex enterprise applications like Android apps, chatbots, and financial softwareUsed for creating dynamic and interactive webpage content
Requires different plug-insDoes not require plug-ins
Career paths include software developer, web developer, and mobile app developerCareer paths include web developer, front-end developer, and full-stack developer

3. What are the data types in JavaScript?

JavaScript has several data types to store and manipulate data. Here is a list of the most common data types in JavaScript:

  1. String: Represents textual data, such as ‘hello’ or “hello world!”.
  2. Number: Can be an integer or a floating-point number, such as 3, 3.234, or 3e-2.
  3. BigInt: Represents integer numbers of arbitrary precision, such as 90071992512474099n or 1n.
  4. Boolean: Has two possible values, typically true and false.
  5. Undefined: Undefined indicates unassigned variable value.
  6. Null: Represents a null value, often used to indicate that an operation or method call resulted in no value.
  7. Symbol: Immutable unique identifiers as object property keys.
  8. Object: A collection of key-value pairs, used to store and organize data.

JavaScript determines value data types on use, not declaration. Variables can hold different data type values. JavaScript automatically converts values to the appropriate type as needed.

4. What are variables in JavaScript?

In JavaScript, a variable is a “container for a value,” such as a number used in a sum or a string used as part of a sentence. It is a named storage for data, allowing developers to store and manipulate different types of information within a program. Here are some key points about variables in JavaScript:

  • Declaration: In JavaScript, a variable can be declared using keywords like var, let, or const.
  • Data Storage: Variables can store various types of data, including numbers, text, Boolean values, and more.
  • Naming: Identifiers uniquely name all JavaScript variables. These names can contain letters, digits, underscores, and dollar signs, but must follow certain rules, such as beginning with a letter and being case-sensitive.
  • Scope: Variables can have local or global scope. Functions scope local variables, globals accessible anywhere in program.
  • Initialization: Best practice: initialize variables on declaration to avoid unexpected results. Uninitialized variables have a value of undefined.

Variables are fundamental to programming in JavaScript, as they allow developers to work with and manipulate data within their programs.

5. What are the different ways to declare variables in JavaScript?

In JavaScript, there are different ways to declare variables, each with its own characteristics. The most common ways to declare variables in JavaScript are using the var, let, and const keywords. Here’s a summary of each:

var:

  • When you declare a variable with var, it is hoisted and initialized in the memory as undefined before the code execution.
  • A variable declared with the var keyword has function scope and is only available within the function.
  • It can lead to potential issues such as variable hoisting and global scope pollution.

let:

  • ES6 introduced let which allows reassigning a variable to a new value.
  • Blocks confine variables to declaration scopes.
  • It does not allow re-declaration of the same variable within the same scope.

const:

  • ES6 introduced const, declaring constants unchangeable values.
  • It has block scope, similar to let.
  • Must initialize with value at declaration, cannot reassign.

Here are some examples of variable declarations using these keywords:

// Using var
var number1 = 2, number2 = 23, number3 = 99;

// Using let
let message = 'Hello';
let count = 10;

// Using const
const pi = 3.14159;
const maxSpeed = 300;

It is recommended to use const by default, and only use let when the variable’s value will change. The use of var is generally discouraged due to its function scope and potential issues.

6. What is the difference between var, let and const ways to declare variables in JavaScript?

Here is a summary of the differences between var, let, and const ways to declare variables in JavaScript:

KeywordScopeRedeclaration & ReassignmentHoisting
varGlobal, LocalYes, with default valueYes, with default value
letGlobal, Local, BlockNo & yes (without default value)Yes, without default value
constGlobal, Local, BlockNo & no (without default value)Yes, without default value


var has global or function scope and allows updates and redeclaration. let has block scope and allows reassignment but not redeclaration. const also has block scope but disallows reassignment and redeclaration.


Modern JavaScript recommends let and const over var for more predictable code. Use var for function-scoped variables that can lift, let for block-scoped variables that reassign, and const for block-scoped constants.

7. What is the difference between undefined and null?

Here is a difference between undefined and null in JavaScript:

Characteristicundefinednull
MeaningVariable has been declared but not assigned a valueVariable is explicitly assigned no value
TypeUndefinedObject
AssignmentAutomatic by JavaScriptExplicit assignment by the developer
Default ValueYes, automatically set by JavaScriptNo, must be explicitly assigned
Arithmetic OperationResults in NaN (Not a Number)Converts to 0
UsageTypically indicates a variable has not been initializedTypically used to represent an intentional absence of value

8. What are operators in JavaScript?

Operators in JavaScript are special symbols that perform operations on one or more operands (data values) and produce a result. There are several categories of operators in JavaScript, including:

  • Arithmetic Operators: Used to perform mathematical operations on numeric operands, such as addition, subtraction, multiplication, and division.
  • Assignment Operators: Used to assign values to variables, such as the equals sign (=) or the addition assignment operator (+=).
  • Comparison Operators: Used to compare two values and return a Boolean value (true or false), such as the greater than operator (>) or the equal to operator (==).
  • Logical Operators: Used to combine two or more conditions and return a Boolean value, such as the AND operator (&&) or the OR operator (||).
  • Bitwise Operators: Used to perform bitwise operations on binary numbers, such as the AND operator (&) or the XOR operator (^).
  • String Operators: Used to concatenate strings, such as the plus sign (+) or the addition assignment operator (+=).
  • Conditional (Ternary) Operator: Used to assign a value to a variable based on a condition, such as the ternary operator (condition ? value1 : value2).

Understanding JavaScript operator types and their usage for operations on data values is important.

9. What are Arithmetic Operators?

JavaScript uses arithmetic operators to perform mathematical calculations on numbers. Here are the common arithmetic operators in JavaScript:

OperatorDescriptionExample
+Addition2 + 3 returns 5
-Subtraction5 – 2 returns 3
*Multiplication2 * 3 returns 6
/Division6 / 3 returns 2
**Exponentiation2 ** 3 returns 8
%Modulus (Remainder)5 % 2 returns 1
++Incrementlet x = 5; x++; sets x to 6
--Decrementlet x = 5; x–; sets x to 4

These operators are essential for performing various mathematical operations in JavaScript, such as addition, subtraction, multiplication, and division.

10. What are Assignment Operators?

Assignment operators are used to assign values to JavaScript variables. Here are some common assignment operators:

OperatorExampleEquivalent to
=x = yx = y
+=x += yx = x + y
-=x -= yx = x – y
*=x *= yx = x * y
/=x /= yx = x / y
%=x %= yx = x % y
**=x **= yx = x ** y
<<=x <<= yx = x << y
>>=x >>= yx = x >> y
>>>=x >>>= yx = x >>> y
&=x &= yx = x & y
^=x ^= yx = x ^ y
|=x |= yx = x | y

These operators are used to perform the operation on the variable to the right and then assign the result to the variable on the left. For example, x += y is the same as x = x + y.

11. What are Comparison Operators?

Comparison operators in JavaScript are used to compare two values and return a Boolean value (true or false). Here is a table summarizing the comparison operators in JavaScript:

OperatorDescriptionExample
==Equal to3 == 3 returns true
!=Not equal to3 != 4 returns true
===Strict equal to3 === 3 returns true
!==Strict not equal to3 !== '3' returns true
>Greater than4 > 3 returns true
<Less than3 < 4 returns true
>=Greater than or equal to4 >= 4 returns true
<=Less than or equal to3 <= 4 returns true

The == operator checks for equality of values, while the === operator checks for equality of values and types. The != operator checks for inequality of values, while the !== operator checks for inequality of values and types. The > and < operators check for greater than and less than, respectively, while the >= and <= operators check for greater than or equal to and less than or equal to, respectively.

Programmers commonly use comparison operators in conditionals and loops to make decisions based on value comparisons.

12. What are Logical Operators?

JavaScript uses logical operators to perform logical operations on values and expressions. The following are the three main logical operators in JavaScript:

OperatorDescriptionExample
&&Logical ANDtrue && false returns false
||Logical ORtrue || false returns true
!Logical NOT!true returns false

program flow based on condition evaluations, as they evaluate Boolean values and return Boolean values. Programmers commonly use Boolean operators in conditionals and loops to control program flow based on condition evaluations, as they evaluate Boolean values and return Boolean values.

13. What are Bitwise Operators?

JavaScript uses bitwise operators to perform operations on numeric values’ binary representations. Here are the common bitwise operators in JavaScript:

OperatorDescriptionExample
&Bitwise AND5 & 3 returns 1 (binary: 0101 & 0011 = 0001)
|Bitwise OR5 | 3 returns 7 (binary: 0101
^Bitwise XOR5 ^ 3 returns 6 (binary: 0101 ^ 0011 = 0110)
~Bitwise NOT~5 returns -6 (binary: ~0101 = 1010)
<<Left Shift5 << 1 returns 10 (binary: 0101 << 1 = 0100)
>>Sign-propagating Right Shift5 >> 1 returns 2 (binary: 0101 >> 1 = 0010)
>>>Zero-fill Right Shift5 >>> 1 returns 2 (binary: 0101 >>> 1 = 0010)


Few use bitwise operators in everyday programming but they help low-level binary data manipulation like cryptography or networking.

14. What are conditional operators?

JavaScript uses conditional (ternary) operators to conditionally evaluate two or more values. The most common conditional operator in JavaScript is the ternary operator ? :. The syntax for the ternary operator is as follows:

condition ? valueIfTrue : valueIfFalse

Here’s a table summarizing the conditional operators in JavaScript:

OperatorDescriptionExample
? :Conditional (Ternary) Operatorx = 5 ? "A" : "B" (result: “A”)

The conditional operator is used as an alternative to if-else statements and can be used in various situations, such as setting a variable’s value based on another variable’s value or a condition.

15. What are strings in JavaScript?

In JavaScript, a string is a sequence of characters that represents text. Strings contain letters, numbers, symbols, and special characters to represent and manipulate text. Strings create fundamental JavaScript types using single quotes, double quotes, or backticks. For example:

let str1 = 'Hello, World!';  // Created using single quotes
let str2 = "JavaScript";     // Created using double quotes
let str3 = `String`;         // Created using backticks

Strings in JavaScript can be manipulated using various string methods and operators, such as the + operator for string concatenation and the += operator for concatenation assignment. Additionally, JavaScript provides a wide range of string methods for tasks like extracting parts of a string, converting the case of the characters, and searching for substrings within a string.

Overall, strings are a versatile and essential part of JavaScript, used for representing and working with textual data.

16. What are template literals?

ES6 introduces template literals, which allow embedding expressions and multiline text into strings. They are enclosed in backticks (`) and can include placeholders for variables and expressions. Here’s a summary of template literals:

  • Syntax: Template literals use backticks (`) instead of single or double quotes.
  • Placeholders: Template literals can include placeholders for variables and expressions, using the ${expression} syntax.
  • Multiline Strings: Template literals allow you to create multiline strings by including newline characters (\n) at the end of each line.
  • Tagged Templates: Advanced use case: You can create custom functions (tag functions) that parse template literals and perform string interpolation using the ${expression} syntax.

Template literals provide a more concise and readable way to create strings with embedded expressions and multiline text, compared to using concatenation and string methods .

17. What is arrays in JavaScript?

In JavaScript, arrays are a built-in data type that allows you to store a collection of multiple elements under a single variable name. Arrays are zero-indexed, meaning the first element is at index 0, the second at index 1, and so on. Arrays can mix different data types and are resizable.

There are two main ways to create an array in JavaScript:

  1. Array Literal: This method creates an array using braces {} and lists the elements separated by commas.
  • Example: let arr = [1][2][3];
  1. Array Constructor: This method creates an array using the Array() constructor and passes the elements as arguments.
  • Example: let arr = new Array(1, 2, 3);

You can access elements in an array using their index number, which starts with 0. Arrays in JavaScript can perform various operations, such as adding or removing elements, updating elements, and searching for elements within the array.

18. What is functions in JavaScript?

In JavaScript, functions are a fundamental building block that allows you to encapsulate a block of code and reuse it multiple times, making the code more readable, organized, reusable, and maintainable. Functions can be defined using the function keyword, followed by the name of the function and parentheses. Functions can be defined as expressions and stored as values assigned to variables or passed as arguments.

There are several types of functions in JavaScript:

  • Regular functions: Can return anything and always run to completion after invocation.
  • Generator functions: Return a Generator object and can be paused and resumed with the yield operator.
  • Async functions: Return a Promise and can use await to wait for other async functions or values.

Functions in JavaScript can take parameters, which are values passed to the function when it is called, and can return a value using the return keyword. Functions can create closures by accessing variables in outer scopes even after the outer function returns.

In summary, functions in JavaScript are a fundamental concept that allows you to encapsulate code and reuse it multiple times. They provide a way to organize and structure your code, making it more maintainable and readable.

19. What is the difference between function declaration and function expression?

The difference between function declaration and function expression lies in their naming and scope. Here are the key differences between the two:

CharacteristicFunction DeclarationFunction Expression
SyntaxUses the function keyword followed by the function nameCan be anonymous (no function name) or have a name
HoistingHoisted to the top of the scopeNot hoisted
When it’s definedDefined when the script is loadedDefined when the execution reaches it
UsageCan be used before it’s declaredCannot be used before it’s defined

In summary, you utilize function declarations to create a function at the global scope, making it accessible throughout your code. On the other side, developers use function expressions to create functions that offer greater flexibility or aren’t susceptible to hoisting.The main difference between the two is that function declarations have a function name, while function expressions are anonymous.

20. What is the difference between parameter and argument?

In JavaScript, people often use the terms “parameter” and “argument” interchangeably, but they carry distinct meanings.

  • Parameter: A parameter is a variable in a function definition. When you call a function, it anticipates receiving a value, and the function’s behavior is shaped by the parameters defined in its declaration.
  • Argument: A function receives a value, known as an argument, during its invocation. These arguments represent the actual values provided to the function, influencing its behavior by supplying input.


In summary, the function declaration defines parameters to represent expected input values, and when calling the function, actual values are provided as arguments.

21. What is arrow functions?

JavaScript ES6 introduced arrow functions, offering a more concise syntax for function creation. They are similar to regular functions, but with some differences in syntax and behavior. Here’s a table summarizing the differences between arrow functions and regular functions:

CharacteristicRegular FunctionArrow Function
SyntaxUses the function keyword followed by the function name and parenthesesUses parentheses around the parameters, followed by the => symbol
this bindingHas its own this bindingInherits this from the enclosing context
arguments objectHas its own arguments objectDoes not have its own arguments object
new keywordCan be used with the new keyword to create objectsCannot be used with the new keyword
return statementUses the return statement to return a valueImplicitly returns the value of the expression on the right side of the => symbol

Developers commonly use arrow functions for short, one-line functions or as callback functions. They provide a more concise syntax and can make code easier to read and write.

22. What is objects in JavaScript?

JavaScript uses objects to store and organize data in key-value pairs and complex entities. Objects represent and structure data. Here are some key points about objects in JavaScript:

  • Objects are created using curly braces {} and can contain properties and methods.
  • Properties are key-value pairs, where the key is a string (also called a “property name”) and the value can be any data type, including other objects, arrays, and functions.
  • Objects store functions as methods.
  • Objects are mutable and addressed by reference, not value.
  • JavaScript creates objects using object literals, the Object() constructor, or Object.create(), which are instances of the Object type.

Here’s an example of an object in JavaScript using object literal syntax:

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 30,
  hobbies: ["reading", "hiking"],
  address: {
    city: "New York",
    zipCode: "10001"
  },
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
};

In this example, person is an object with properties such as firstName, lastName, age, hobbies, and address, as well as a method fullName that returns the full name of the person.

Overall, objects are a versatile and powerful feature of JavaScript, used for representing and working with complex data structures and entities.

23. What is methods in JavaScript?

In JavaScript, we call a function linked with an object a method. Methods are stored as object properties and can be accessed using the dot notation objectName.methodName(). Here are some key points about methods in JavaScript:

  • Methods are functions that operate on or are related to specific objects.
  • Attaching a function to an object creates them.
  • Objects can utilize methods to take actions on their encapsulated data or manipulate their internal state.

For example, consider the following person object with a greet method:

const person = {
  firstName: "John",
  lastName: "Doe",
  greet: function() {
    console.log("Hello");
  }
};

person.greet(); // Output: "Hello"

In this example, the greet method is associated with the person object and can be called using the dot notation person.greet(). When calling the method, it outputs “Hello” to the console.

24. What is properties in JavaScript?

In JavaScript, properties are key-value pairs that define the characteristics of an object. A property is an association between a name (or key) and a value, where the value can be any data type, including other objects, arrays, and functions. Dynamically add, modify, or delete properties after creating an object, enhancing flexibility and adaptability in JavaScript development.

Here’s an example of an object in JavaScript with properties:

const person = {
  firstName: "John",
  lastName: "Doe",
  age: 30,
  hobbies: ["reading", "hiking"],
  address: {
    city: "New York",
    zipCode: "10001"
  }
};

In this example, person is an object with properties such as firstName, lastName, age, hobbies, and address. The address property is an object itself, with its own properties city and zipCode.

Overall, properties are a fundamental part of objects in JavaScript, used to define and organize data in a structured manner.

25. What is the “this” keyword in JavaScript?

The this keyword in JavaScript refers to an object that is executing the current piece of code.
This object reference, this constantly adjusts based on the context of its invocation, with the specific object it points to varying. Function invocation establishes its value through runtime binding, and assignment during execution cannot alter it. The behavior of this also has some differences between strict mode and non-strict mode. In most cases, the value of this is as follows:

  • In an object method, this refers to the object itself.
  • When used alone, this refers to the global object.
  • In a function, in strict mode, this is undefined.
  • In event handlers, this refers to the HTML element that received the event.

The this keyword is not a variable and cannot be changed or reassigned. JavaScript trusts on a fundamental concept, where it refers to the object currently executing the code or function.

26. What are classes in JavaScript?

In JavaScript, classes are a type of object that provides a template for creating objects with similar properties and methods. ECMAScript 2015 (ES6) brought forth these concepts, building upon JavaScript’s existing prototype-based inheritance model. In object-oriented programming (OOP), developers frequently utilize classes to establish a structured and organized approach for creating objects, promoting the development of reusable code.

Here are some key points about classes in JavaScript:

  • Classes are declared using the class keyword, followed by the name of the class.
  • Creating a new object from a class triggers the invocation of the constructor method within that particular class.
  • Classes can have properties, which are variables that store data, and methods, which are functions that perform actions on the data.
  • Classes can be extended using the extends keyword to create a subclass that inherits properties and methods from the parent class.
  • Classes can use the super keyword to call methods from the parent class.

Here’s an example of a class in JavaScript:

class Person {
  constructor(firstName, lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  fullName() {
    return `${this.firstName} ${this.lastName}`;
  }
}

const john = new Person("John", "Doe");
console.log(john.fullName()); // Output: "John Doe"

In this example, Person is a class that has a constructor method and a fullName method. The fullName method returns the full name of the person. The creation of a new object, ‘john,’ from the Person class involves utilizing the new keyword, followed by invoking the fullName method on the ‘john’ object.

27. What is inheritance in JavaScript?

Inheritance in JavaScript refers to the ability of an object to inherit properties and methods from a parent object. JavaScript implements inheritance through the prototype chain, with each object having an internal link to another object, its prototype. By default, an object’s prototype is the Object prototype, but it can be set to any other object using the Object.setPrototypeOf() method or the __proto__ property.

Inheritance allows you to create new objects that are based on existing objects, with the ability to add or modify properties and methods. JavaScript often utilizes inheritance in object-oriented programming (OOP) for reusable code creation and organizing objects into hierarchies.

ES6 introduced the class keyword, which provides a more structured and organized way to create objects and to implement inheritance in JavaScript. Classes are a type of object that provides a template for creating objects with similar properties and methods. Classes can be extended using the extends keyword to create a subclass that inherits properties and methods from the parent class.

Overall, inheritance is a powerful feature of JavaScript that allows you to create new objects based on existing objects, with the ability to add or modify properties and methods.

28. What are modules in JavaScript?

In JavaScript, modules are a way to organize and structure code by breaking it down into smaller, reusable pieces. A module is simply a file that contains code, and one script is one module. Modules can load each other and use special directives such as export and import to interchange functionality and call functions of one module from another one.

Modules split large programs into smaller, independent code chunks, enhancing manageability and focusing on specific tasks or related functions. They help to avoid naming conflicts and make it easier to maintain and update codebases.

Initially excluded from the language, ECMAScript 2015 (ES6) introduced JavaScript modules, now supported across major browsers and Node.js. Modules can be defined using the export keyword to label variables and functions that should be accessible from outside the current module, and the import keyword to allow the import of functionality from other modules.

Overall, modules are a powerful feature of JavaScript that allow developers to organize and structure code in a more efficient and maintainable way, making it easier to reuse code and avoid naming conflicts.

29. What is the module pattern in JavaScript?

JavaScript utilizes the module pattern, a design approach enhancing code maintainability and reusability through public and private access levels. Some refer to it as encapsulation, as it safeguards the value within a module from external scope access. The module pattern keeps the privacy of the state and organizes using closures, thus protecting the pieces from the global scope, avoiding possible errors and conflicts. It is quite similar to an Immediately Invoked Function Expression (IIFE), but a module always returns an object instead of a function. Modules allow developers to split up code into smaller, reusable pieces, and keep certain values within a file private, reducing the risk of name collisions for values declared in other parts of the codebase.

Here’s an example of the module pattern in JavaScript:

// A module that returns an object with public methods
function createModule() {
  const privateCounter = 0;

  function privateIncrement() {
    privateCounter++;
  }

  function getPrivateCounter() {
    return privateCounter;
  }

  return {
    increment: privateIncrement,
    getCounter: getPrivateCounter,
  };
}

const myModule = createModule();
myModule.increment();
console.log(myModule.getCounter()); // Output: 1

In this example, the createModule function is a module that returns an object containing two public methods, increment and getCounter. These methods possess privacy, restricting access solely to within the module. The increment method increments a private counter, while the getCounter method returns the current value of the private counter. The module pattern ensures that the private state and methods are not accessible from outside the module, maintaining encapsulation and avoiding potential conflicts or leaks.

30. What are events in JavaScript?

In JavaScript, events are occurrences or happenings in the system, such as user actions or browser actions, to which code can react. Developers use them to execute specific code when certain actions occur. Events can be related to user interactions, like clicking a button, moving the mouse, or pressing a key, or to browser actions, like the page finishing loading. JavaScript can react to these events, allowing developers to execute code in response to them.

Here’s an example of an event in JavaScript:

// Adding an event listener to a button element
document.getElementById("myBtn").addEventListener("click", function() {
  alert("Button clicked");
});

This example adds a click event listener to a button element. Clicking the button triggers the execution of the function, provided as the second argument to addEventListener, displaying an alert box with the message “Button clicked.”

31. What is an event listener?

In JavaScript, an event listener is a function that waits for an event to occur and then responds to it. It is a way to add interactivity to web pages by reacting to user actions or browser actions. An event listener can be added to an element using the addEventListener() method, which takes two arguments: the type of the event to listen for and the function to be executed when the event occurs. You can define the function either inline or as a separate function. The event listener can be removed using the removeEventListener() method. Developers use event listeners to craft custom responses for events such as mouse clicks, keyboard inputs, and window resizing, constituting a crucial aspect of event handling in JavaScript.

32. What is the event flow?

The event flow in JavaScript refers to the order in which events are processed in the Document Object Model (DOM). When an event occurs on a specific DOM element, such as a button click, it travels through the DOM hierarchy in three phases: Capture, Target, and Bubbling.

  • Capture Phase: The event starts from the outermost ancestor element and moves towards the target element. It allows handling events on the capturing phase (from top to bottom).
  • Target Phase: The event reaches the target element where the actual event occurred. Event listeners attached to the target respond.
  • Bubbling Phase: The event then bubbles up from the target element back to the outermost ancestor. It allows handling events on the bubbling phase (from bottom to top).

Event flow provides a way to handle events at different levels of the DOM hierarchy and is essential for understanding how events propagate through the DOM.

The following example demonstrates this process.

// Adding an event listener to a button element
document.getElementById("myBtn").addEventListener("click", function() {
  alert("Button clicked");
});

In this example, the DOM elements process the click event as it traverses the capture phase, target phase, and bubbling phase.

33. What is event bubbling and capturing?

Event bubbling and event capturing are two methods of event propagation in JavaScript, which occur when an event happens in an element inside another element. These methods govern the execution order of event handlers and influence how the DOM handles events.

  • Event Bubbling: In this phase, the event starts from the target element and moves up the DOM hierarchy towards the root element (usually the <html> element). During this phase, the system triggers event handlers attached to the target element and its ancestors. By default, all event handlers are in the bubbling phase.
  • Event Capturing: This phase travels from the root element down to the target element. It allows handling events on the capturing phase, which is useful when you want to perform actions on the parent elements of the target element. To enable capturing phase, pass true as the third argument to the addEventListener() method.

Here’s an example to illustrate the difference between event bubbling and event capturing:

// Adding event listeners to elements
document.getElementById("container").addEventListener("click", handleContainerClick, {capture: false});
document.getElementById("child1").addEventListener("click", handleChild1Click, {capture: false});
document.getElementById("child2").addEventListener("click", handleChild2Click, {capture: false});

function handleContainerClick(event) {
  console.log("Container clicked");
}

function handleChild1Click(event) {
  console.log("Child 1 clicked");
}

function handleChild2Click(event) {
  console.log("Child 2 clicked");
}

In this example, we have a container element with a click event listener, and two child elements, child1 and child2, with their own click event listeners. The handleContainerClick function is executed when the container is clicked, the handleChild1Click function is executed when child1 is clicked, and the handleChild2Click function is executed when child2 is clicked. The capture option  is set to false for all event listeners, meaning they are in the bubbling phase.

34. What are promises in JavaScript?

Promises in JavaScript are objects representing the eventual completion or failure of an asynchronous operation. They allow you to associate handlers with an asynchronous action’s eventual success value or failure, and are a fundamental part of asynchronous programming in JavaScript. Promises provide a way to handle asynchronous operations in a more structured and predictable manner, making it easier to work with asynchronous code and avoid issues like callback hell.

Here’s an example of using promises in JavaScript:

// Creating a promise
const promise = new Promise((resolve, reject) => {
  // Producing code (may take some time)
  setTimeout(() => {
    resolve("Resolved value");
  }, 1000);
});

// Handling the promise
promise
  .then((value) => {
    console.log("Success: ", value);
  })
  .catch((error) => {
    console.error("Error: ", error);
  });

In this example, the new Promise() constructor creates a promise, utilizing an executor function as its argument. The executor function contains the producing code, which sets a timeout and resolves the promise with the message “Resolved value” after 1 second. You use the then() method to manage both success and failure cases of a promise. Upon resolving the promise, the console.log() function within the then() method executes, printing “Success: Resolved value.” In case of an error, the catch() method triggers the console.error() function, printing “Error: “ with the error message.

35. What are async/await functions?

Async/await is a modern JavaScript feature that allows you to work with promises in a more synchronous and intuitive way. Use the async keyword to define a function as asynchronous, ensuring it always returns a promise. Within an async function, utilize the await keyword to halt function execution until the promise resolves, returning the resolved value. This makes asynchronous code look and behave more like synchronous code, which can make it easier to read and write. Async/await also enables the use of ordinary try/catch blocks around asynchronous code, simplifying error handling. To utilize the await keyword, declare the surrounding function explicitly as async. Here’s an example of using async/await:

async function myFunction() {
  let result = await someAsyncFunction();
  console.log(result);
}

myFunction();

In this example, the await keyword is used to pause the myFunction until the someAsyncFunction promise is resolved, and then it logs the result. This makes the asynchronous code look and behave more like synchronous code, which can make it easier to read and write.

36. What is the difference between synchronous and asynchronous programming?

Here’s a the differences between synchronous and asynchronous programming:

AspectSynchronous ProgrammingAsynchronous Programming
Code ExecutionExecuted in a linear, sequential mannerExecuted in any order or even simultaneously
BlockingBlocks the execution of other tasks until the current task is completedNon-blocking, allows other tasks to run concurrently
ScalabilityLimited in terms of how many tasks they can run simultaneouslyCan run multiple tasks simultaneously, improving scalability and performance
Resource UtilizationMay consume more resources as tasks wait for previous tasks to completeMore efficient in terms of resource utilization, as tasks can run concurrently
Error HandlingErrors in one task can affect the execution of other tasksErrors in one task are contained, reducing the impact on other tasks
ComplexityCode is more straightforward and predictableCode can be more complex due to the use of callbacks and asynchronous operations

In summary, synchronous programming is characterized by linear and sequential code execution, blocking of other tasks, limited scalability, resource consumption, error handling, and complexity. Asynchronous programming, on the other hand, allows for concurrent task execution, non-blocking of other tasks, improved scalability and resource efficiency, contained error handling, and potentially more complex code.

37. What is an AJAX request?

In web development, developers use AJAX (Asynchronous JavaScript and XML) requests to asynchronously send and receive data from a server without reloading the entire page. It allows web pages to update content dynamically without requiring a full page refresh. JavaScript code in the browser typically initiates AJAX requests, sending an HTTP request to a server and receiving a response in a specified format like XML, JSON, or plain text.


In AJAX, developers make requests using the XMLHttpRequest object or the Fetch API, retrieving or submitting data and updating web page content. Modern web applications frequently utilize AJAX requests to enhance user experience, offering responsiveness and interactivity.

38. What is JSON and how to parse JSON in JavaScript?

JSON, or JavaScript Object Notation, is a lightweight data format that is easy for humans and machines to read and write. A text-based format representing structured data, utilizing JavaScript object syntax, is language-independent, applicable across programming languages.

To parse JSON in JavaScript, you can use the built-in JSON object and its methods, such as JSON.parse(). This method takes a JSON string and returns a JavaScript object or value, which can be of the following types: String, Number, Boolean, Object, Array, Function, or Undefined/Null.

Here’s an example of parsing JSON in JavaScript:

// Define a JSON string
const jsonString = '{"name": "John Doe", "age": 30, "city": "New York"}'

// Use the `JSON.parse()` method to convert the JSON string to a JavaScript object
const jsonObject = JSON.parse(jsonString);

// Access the properties of the parsed JSON object
console.log(jsonObject.name); // Output: John Doe
console.log(jsonObject.age);  // Output: 30
console.log(jsonObject.city);  // Output: New York

In this example, we parse a JSON string into a JavaScript object using the JSON.parse() method. Access the properties and log them to the console.

39. What are JSONP and CORS?

Web applications use JSONP (JSON with Padding) and CORS (Cross-Origin Resource Sharing) to facilitate cross-origin requests.

  1. JSONP (JSON with Padding):
  • JSONP enables the retrieval of data from a domain distinct from the hosting web page.
  • It is based on the idea of using a <script> tag to fetch data from a different domain and using a callback function to process the data.
  • Modern web browsers widely support JSONP, making it relatively easy to implement on the client-side.
  • Yet, it circumvents the same-origin policy, posing a security risk if mishandled, and only supports making GET requests.
  • Modern and secure techniques like CORS have largely replaced JSONP in contemporary web development.
  1. CORS (Cross-Origin Resource Sharing):
  • CORS is a more modern and secure technique for enabling cross-origin requests in web applications.
  • The W3C standard empowers web servers to specify the permitted origins accessing their resources.
  • Modern web browsers support CORS, enabling developers to exert more precise control over cross-origin requests.
  • CORS surpasses JSONP by offering broader support for error handling and various HTTP methods, not limited to GET requests.

In summary, JSONP is an older technique that allows cross-origin requests using a callback function, while CORS is a more modern and secure standard for enabling cross-origin requests with better support for different HTTP methods and error handling.

40. What is the DOM and how to select elements using DOM?

The Document Object Model (DOM) is a programming interface for HTML and XML documents. It represents the structure of the document as a tree of objects, allowing you to manipulate its content and visual presentation. Each object represents a part of the document, such as an element or attribute.

JavaScript provides several methods to select DOM elements. Some of the commonly used methods are:

1. getElementById: Selects an element by its ID.

   const element = document.getElementById("myElementId");

2. getElementsByClassName: Selects elements by their class name.

   const elements = document.getElementsByClassName("myClassName");

3. getElementsByTagName: Selects elements by their tag name.

   const elements = document.getElementsByTagName("div");

4. querySelector: Selects the first element that matches a specified CSS selector.

   const element = document.querySelector(".myClass");

5. querySelectorAll: Selects all elements that match a specified CSS selector.

   const elements = document.querySelectorAll("p.myClass");

These methods allow you to access and manipulate the content of HTML documents using JavaScript, enabling dynamic and interactive web pages.

41. What are the different ways to select elements in JavaScript?

There are several ways to select elements in JavaScript using the Document Object Model (DOM). Some of the commonly used methods are:

1. getElementById: Selects an element by its ID.

   const element = document.getElementById("myElementId");

2. getElementsByClassName: Selects elements by their class name.

   const elements = document.getElementsByClassName("myClassName");

3. getElementsByTagName: Selects elements by their tag name.

   const elements = document.getElementsByTagName("div");

4. querySelector: Selects the first element that matches a specified CSS selector.

   const element = document.querySelector(".myClass");

5. querySelectorAll: Selects all elements that match a specified CSS selector.

   const elements = document.querySelectorAll("p.myClass");

These methods allow you to access and manipulate the content of HTML documents using JavaScript, enabling dynamic and interactive web pages.

42. What is event delegation in JavaScript?

Event delegation in JavaScript is a pattern that allows you to handle events at a higher level in the DOM tree by listening for events on a parent element and then checking if the event target is a child of that element. It is based on the concept of event bubbling, where events triggered on an element propagate up the DOM tree to the element’s parent, its ancestors, and so on, until the root element (usually the <html> element).

The main benefits of event delegation are:

  • Simplifies initialization and saves memory: no need to add many handlers.
  • Less code: when adding or removing elements, no need to add/remove handlers.
  • DOM modifications: we can mass add/remove elements with innerHTML and the like.

Here’s an example of event delegation in JavaScript:

// Adding an event listener to the parent element
const container = document.getElementById("container");
container.addEventListener("click", function(event) {
  const target = event.target;
  if (target.classList.contains("myElementClass")) {
    // Handle the click event on the element with the class 'myElementClass'
  }
});

In this example, a click event listener is added to a container element. The event listener checks if the clicked element has the class ‘myElementClass’. If it does, the event handler will execute the desired code.

43. What are error objects in JavaScript?

Error objects in JavaScript are used to represent runtime errors that occur during the execution of a program. They are instances of the built-in Error constructor, which can be used as a base object for user-defined exceptions. The Error object has two properties: name and message, which provide information about the error. The name property specifies the type of error, such as TypeError or ReferenceError, while the message property provides a human-readable description of the error.

In addition to the Error constructor, there are other core error constructors in JavaScript, such as TypeError, ReferenceError, SyntaxError, and RangeError. These constructors provide more specific error types for different kinds of errors that can occur during program execution.

To throw an error in JavaScript, you can use the throw statement followed by an instance of an error object. For example:

if (x < 0) {
  throw new Error("x must be a positive number");
}

This code throws an Error object with the message “x must be a positive number” if the value of x is less than zero.

In summary, error objects in JavaScript are used to represent runtime errors that occur during program execution. They are instances of the built-in Error constructor and provide information about the type and description of the error.

44. What are error handling mechanisms in JavaScript?

Error handling mechanisms in JavaScript are essential for writing robust and maintainable applications. Some of the key techniques and best practices for error handling in JavaScript include:

  1. Using Built-in Error Objects: JavaScript provides various built-in error objects, such as Error, TypeError, SyntaxError, and ReferenceError, each designed for a specific type of error. Developers can also throw custom errors using the throw keyword.
  2. try…catch Statement: One of the most common ways to handle errors in JavaScript is using the try...catch statement. It allows you to execute a block of code and catch any errors that occur during its execution.
  3. Defensive Coding Techniques: Anticipating potential errors and proactively implementing defensive coding techniques to prevent unexpected issues.
  4. Understanding Different Error Types: It’s essential to understand the different types of errors that can occur in JavaScript, such as syntax errors, type errors, reference errors, and range errors.
  5. Event Delegation: Using event delegation to handle and manage errors related to user interactions and events in the DOM.
  6. Proper Error Logging and Reporting: Implementing robust error logging and reporting mechanisms to track and monitor errors in production environments.

By mastering these error handling techniques and best practices, developers can be better prepared to tackle any issues that arise in their code, ensuring more robust, maintainable, and reliable JavaScript applications.

45. What are regular expressions in JavaScript?

Regular expressions (regex) in JavaScript are a way of describing patterns in a string of data, allowing you to search for data strings that match that pattern. A sequence of characters forms a search pattern, useful for text search and replace operations. Regular expressions are used with the exec() and test() methods of RegExp, and with the match(), matchAll(), replace(), replaceAll(), search(), and split() methods of String.

JavaScript provides various built-in error objects, such as Error, TypeError, SyntaxError, and ReferenceError, each designed for a specific type of error. Developers can also throw custom errors using the throw keyword. The try...catch statement is one of the most common ways to handle errors in JavaScript. It allows you to execute a block of code and catch any errors that occur during its execution. Defensive coding techniques, understanding different error types, event delegation, and proper error logging and reporting are some of the other techniques and best practices for error handling in JavaScript.

46. What are the main methods to validate forms using JavaScript?

The main methods to validate forms using JavaScript include:

  1. Inline Validation: This involves using JavaScript to validate form fields as the user fills them out. It provides immediate feedback to the user.
  2. Event Handlers: JavaScript event handlers, such as onsubmit, can be used to trigger form validation when the form is submitted.
  3. Constraint Validation API: This is a set of built-in validation attributes and methods in HTML5, such as checkValidity() and setCustomValidity(), which can be accessed and manipulated using JavaScript.
  4. Custom Validation: JavaScript customizes form field validation, ensuring specific criteria like email addresses or password strength are met.

These methods allow for comprehensive form validation, ensuring that user input is clean, correct, and useful. They help prevent the submission of incorrect or incomplete data, providing a better user experience and improving data quality.

47. What are the different ways to validate forms using JavaScript?

There are several ways to validate forms using JavaScript. Some of the commonly used methods are:

  1. Inline Validation: This involves using JavaScript to validate form fields as the user fills them out. It provides immediate feedback to the user.
  2. Event Handlers: JavaScript event handlers, such as onsubmit, can be used to trigger form validation when the form is submitted.
  3. Constraint Validation API: This is a set of built-in validation attributes and methods in HTML5, such as checkValidity() and setCustomValidity(), which can be accessed and manipulated using JavaScript.
  4. Custom Validation: You can employ JavaScript to craft custom validation logic tailored for specific form fields, like validating email addresses or ensuring password strength.
  5. Third-Party Libraries: There are many third-party libraries available to perform form validation, such as Validate.js and jQuery Validation.

These methods allow for comprehensive form validation, ensuring that user input is clean, correct, and useful. They help prevent the submission of incorrect or incomplete data, providing a better user experience and improving data quality.

48. What are closures in JavaScript?

JavaScript closures occur when functions retain access to variables in the scope chain of their outer (enclosing) functions, persisting beyond the outer function’s return. Whenever you define a function within another function, it creates closures, allowing the inner function to access outer function variables. Closures prove valuable for crafting private variables and methods, and for designing functions applicable as callbacks or event handlers.

Some of the key features of closures in JavaScript include:

  • Closures have access to variables in their outer function’s scope chain.
  • You can use closures to establish private variables and methods in JavaScript.
  • You can use closures to craft functions applicable as callbacks or event handlers in various programming scenarios.
  • Whenever you define a function inside another function, you create closures, enabling the inner function to access outer function variables.

Here’s an example of a closure in JavaScript:

function outerFunction() {
  const outerVariable = "I am outside!";

  function innerFunction() {
    console.log(outerVariable);
  }

  return innerFunction;
}

const innerFunc = outerFunction();
innerFunc(); // Output: "I am outside!"

In this example, the innerFunction has access to the outerVariable in its outer function’s scope chain, even after the outerFunction has returned. The innerFunction is returned from the outerFunction and assigned to the innerFunc variable, which is then called to log the value of outerVariable to the console.

49. What is the difference between call, apply and bind methods?

The call, apply, and bind methods in JavaScript are used to set the this value explicitly when a function is invoked. Here’s a summary of the differences:

1. call(): The call method is used to call a function with a given this value and individual arguments. It immediately executes the function.

   func.call(thisValue, arg1, arg2, ...);

2. apply(): The apply method is similar to call, but it accepts the this value and an array of arguments. It also immediately executes the function.

   func.apply(thisValue, [arg1, arg2, ...]);

3. bind(): The bind method is used to create a new function with a specified this value and initial arguments, without immediately executing the function. It returns a new function that, when called, has the this value set as provided.

   const newFunc = func.bind(thisValue, arg1, arg2, ...);

In summary, call and apply are used to invoke the function immediately with the specified this value and arguments, while bind is used to create a new function with the specified this value, which can be invoked later.

50. What are hoisting in JavaScript?

JavaScript utilizes hoisting as a mechanism to move variables and function declarations to their scope’s top before code execution.This suggests the capacity to use a variable or function prior to declaring it, with only the declaration hoisting, not the initialization. This can lead to unexpected behavior if you’re not aware of how hoisting works.

Both variables and functions experience hoisting, though they exhibit some differences in this regard. Hoisting takes variable declarations to the top of their scope, but values initialize only at their declaration. Hoisting in function declarations moves both the declaration and the entire body to the top, allowing pre-declaration calls.

It’s important to note that hoisting only applies to declarations, not to assignments or function expressions. It’s also important to declare variables and functions before using them to avoid unexpected behavior.

51. What are strict mode in JavaScript?

JavaScript introduced the feature of Strict mode in ECMAScript 5. By placing a program or function in a “strict” operating context, you gain control over actions and enhance exception handling. The statement “use strict” instructs the browser to use the strict mode, which is a reduced and safer feature set of JavaScript. Strict mode offers several benefits, such as transforming silent errors into thrown errors, optimizing JavaScript engines, and prohibiting certain syntax.

You can use strict mode in two ways: for the entire script or for individual functions. Place the “use strict” directive at the top of the script when using it for the entire script. When utilizing the “use strict” directive for individual functions, place it at the function’s outset. Strict mode can help in writing more secure and optimized JavaScript code. Enables stricter parsing, error handling, and prevents accidental global variable creation in runtime code. It also helps in writing cleaner code and catching errors and potential issues more easily. It is important to note that JavaScript modules are in strict mode by default.

52. What are design patterns in JavaScript?

JavaScript design patterns are reusable solutions to commonly occurring problems in software design. They provide a structured and repeatable way to tackle commonly encountered challenges in JavaScript development. These patterns are about reusable designs and interactions of objects, and they help in writing clean, healthy, and maintainable code. Some of the popular classifications of JavaScript design patterns include creational, structural, and behavioral patterns.

Developers use creational patterns for object creation, structural patterns for object composition, and behavioral patterns for object communication and responsibility assignment. Examples of JavaScript design patterns include the Singleton Pattern, Factory Method Pattern, Abstract Factory Pattern, Builder Pattern, and Prototype Pattern. JavaScript developers widely employ these patterns to enhance code quality, maintainability, and promote reusability.

53. What are the differences between == and === operators?

Here’s the differences between the == and === operators in JavaScript:

Characteristic== (Loose Equality)=== (Strict Equality)
Type ConversionPerforms type coercion if necessaryDoes not perform type coercion
Value ComparisonCompares the values of the operandsCompares both the values and the data types of the operands
Return ValueReturns true if the operands have the same value after type coercion, otherwise falseReturns true only if both operands are of the same data type and have the same value, otherwise false
Example0 == false returns true0 === false returns false

The == operator is known as the “loose equality” operator, while the === operator is referred to as the “strict equality” operator. It’s generally recommended to use the === operator for comparisons in JavaScript to avoid unexpected results due to type coercion.

54. What are the differences between null and undefined?

In JavaScript, both null and undefined represent the absence of a meaningful value, but they are used in slightly different contexts. Here are the main differences between null and undefined:

Characteristicnullundefined
MeaningRepresents an empty or non-existent value.Indicates that a variable has been declared, but has not been assigned a value.
Typetypeof null returns 'object'.typeof undefined returns 'undefined'.
AssignmentIt is an intentional absence of value and must be assigned.It is the default value of a variable that has not been assigned a value.
UsageCan be explicitly assigned to a variable.Typically indicates an unintentional absence of value.

These differences are important to consider when working with variables and data in JavaScript, as they can impact the behavior of the code and the interpretation of the values.

55. What are the differences between break, continue and return statements?

Here’s a simple explanation of the differences between the break, continue, and return statements:

  • break: It terminates the loop immediately, allowing the continuation with the next statement after the loop.
  • continue: It stops the current loop iteration, allowing for the transition to the next iteration in the sequence.
  • return: Use it to exit the current function and send a value back to the calling code. Upon encountering a return statement, the function promptly halts execution, transferring control back to the calling code.

Control over program flow occurs through these statements, particularly within loops and functions, playing unique roles in directing code execution.

56. What are the differences between let, var and const?

Here’s a the differences between var, let, and const in JavaScript:

Characteristicvarletconst
ScopeFunction-scoped or globally scopedBlock-scopedBlock-scoped
RedeclarationCan be re-declaredCannot be re-declaredCannot be re-declared
ReassignmentCan be updated and re-assignedCan be updated, but not re-declaredCannot be updated or re-declared
HoistingHoisted to the top of the function or global scopeHoisted to the top of the block scopeHoisted to the top of the block scope

These differences are important to consider when choosing the appropriate keyword for variable declaration in JavaScript, as they impact the behavior and usage of the variables within the code.

57. What are the differences between anonymous and named functions?

Here’s a differences between anonymous and named functions in JavaScript:

CharacteristicNamed FunctionsAnonymous Functions
DefinitionDeclared with a nameCreated without a name
HoistingEntire function is hoistedOnly the variable declaration is hoisted
Self-referenceCan refer to itself by nameCannot refer to itself by name
DebuggingShows the function name in stack tracesAppears as “anonymous” in stack traces
RecursionCan be used for recursionCannot be used for recursion
ReadabilityGenerally more readable due to the descriptive function nameMay be less readable, especially for complex logic

These differences are important to consider when choosing between named and anonymous functions, as they impact aspects such as hoisting, self-reference, debugging, and recursion.

58. What are the differences between synchronous and asynchronous functions?

Synchronous and asynchronous functions are two different programming models in JavaScript. Here’s a table summarizing the differences between them:

CharacteristicSynchronous FunctionsAsynchronous Functions
ExecutionExecutes code sequentially, blocking further execution until the current task is completedExecutes code non-sequentially, allowing further execution while the current task is being completed
BlockingBlocks further execution until the current task is completedDoes not block further execution while the current task is being completed
CallbacksDoes not require callbacksRequires callbacks to handle the completion of asynchronous tasks
Error HandlingErrors can be handled synchronouslyErrors must be handled asynchronously using callbacks
PerformanceMay be slower due to blocking natureMay be faster due to non-blocking nature

Asynchronous functions may be faster due to their non-blocking nature, while synchronous functions may be slower due to their blocking nature.

59. What are the differences between call stack and task queue?

Here’s a the differences between the call stack and the task queue in JavaScript:

CharacteristicCall StackTask Queue
PurposeTracks the execution of function callsStores messages or events to be processed asynchronously
BehaviorFollows a Last-In-First-Out (LIFO) approachFollows a First-In-First-Out (FIFO) approach
ExecutionExecutes functions in a synchronous mannerHandles asynchronous function calls and events
RelationshipMonitors the current function being executedMonitors and processes messages or events for asynchronous execution
InteractionInteracts with the event loop and the heapInteracts with the event loop and the callback queue

These differences are fundamental to understanding how JavaScript handles function execution and asynchronous events through the event loop, call stack, and task queue.

60. What are the differences between primitive and reference types?

Here’s a the differences between primitive and reference types in JavaScript:

CharacteristicPrimitive TypesReference Types
DefinitionSimple, immutable data typesComplex, mutable data types
Data Type ExamplesNumber, string, boolean, null, undefined, symbolObject, array, function, date, regex, map, set, weakmap, weakset
Memory AllocationStored on the stackStored on the heap
CopyingCreates a new copy of the valueCreates a new reference to the value
ComparisonCompares the value of the variableCompares the reference to the value
MutabilityImmutable, cannot be changedMutable, can be changed

These differences are important to consider when working with variables and data in JavaScript, as they impact the behavior of the code and the interpretation of the values. Understanding the differences between primitive and reference types can help in writing more efficient and effective code.

61. What are the differences between for, forEach, map, filter and reduce?

Here’s a the differences between for, forEach, map, filter, and reduce in JavaScript:

CharacteristicforforEachmapfilterreduce
PurposeUsed to repeat an action until a condition is metUsed to perform an action on each item in an arrayUsed to transform each item in an arrayUsed to filter items based on a conditionUsed to accumulate a result from an array
ActionPerforms a specific actionExecutes a callback function on each itemApplies a function to each item, creating a new arrayChecks if an item meets a condition and includes it in the result if trueAccumulates a result from an array
Return ValueDoes not have a return valueReturns undefinedReturns a new array with transformed valuesReturns an filtered arrayReturns a single value or an array
ExecutionBlocking, can cause performance issuesNon-blockingNon-blockingNon-blockingNon-blocking

These differences are important to consider when choosing the appropriate method for iterating over and manipulating arrays in JavaScript. Each method has its own purpose, action, return value, and execution characteristics, which can impact the performance and readability of your code.

62. What are the differences between slice and splice?

Here’s a the differences between slice and splice in JavaScript:

Characteristicslice()splice()
PurposeUsed to create a new array from the original arrayUsed to modify the original array by adding/removing items
Changes to the original arrayDoes not change the original arrayChanges the original array
Return ValueReturns a new arrayReturns the changed array
Number of argumentsCan take up to two arguments (start and end indices)Can take up to three arguments (start index, delete count, and new items)
ExecutionNon-blockingNon-blocking

These differences are important to consider when choosing the appropriate method for manipulating arrays in JavaScript. Each method has its own purpose, changes to the original array, return value, and number of arguments, which can impact the performance and readability of your code.

63. What are the differences between Date and DateTime?

Here’s a table summarizing the differences between Date and DateTime in the context of programming:

CharacteristicDateDateTime
Data TypeBuilt-in object in JavaScriptData type varies depending on the programming language and database
PurposeUsed to work with dates and timesUsed to store date and time information
Changes to the original arrayDoes not change the original arrayMay change the original array
Return ValueReturns a new arrayReturns a new array or modified array
Number of argumentsCan take up to two arguments (start and end indices)Can take up to three arguments (start index, delete count, and new items)
ExecutionNon-blockingNon-blocking

These differences are important to consider when choosing the appropriate method for manipulating arrays in JavaScript. Each method has its own purpose, changes to the original array, return value, and number of arguments, which can impact the performance and readability of your code.

64. What are the differences between localStorage and sessionStorage?

The localStorage and sessionStorage are both web storage options available in modern browsers. Here’s a their differences:

CharacteristiclocalStoragesessionStorage
Data ExpiryData does not expire and persists until manually clearedData is cleared when the page session ends or the browser is closed
ScopeData is shared across all tabs and windows under the same originData is limited to the current tab and is not shared across tabs or windows
PersistencePersists even after the browser is closed and re-openedPersists only until the tab or window is closed or the page session ends
Storage LimitTypically 5MB per origin, but this can vary by browser and configurationSame as localStorage

These differences are important to consider when choosing between localStorage and sessionStorage for storing data in a web application. The choice between them depends on the specific requirements for data persistence and scope.

65. What are the differences between cookies and localStorage?

The differences between cookies and localStorage are as follows:

CharacteristicCookieslocalStorage
Data ExpiryCan have an expiration date, otherwise persists until manually clearedDoes not expire and persists until manually cleared
ScopeShared across all tabs and windows under the same originSpecific to the protocol and domain
Storage LimitCan store up to 4KB of dataCan store up to 5MB of data
AccessibilityCan be read by both the server and the client-side JavaScriptCan only be read by client-side JavaScript

These differences are important to consider when choosing between cookies and localStorage for storing data in a web application. The choice between them depends on the specific requirements for data persistence, scope, and accessibility.

66. What are the differences between type coercion and type conversion?

Type coercion and type conversion are fundamental concepts in programming, particularly in JavaScript. Here’s a concise summary of the differences between the two:

CharacteristicType CoercionType Conversion
DefinitionAutomatic conversion of data types that occurs in JavaScriptThe process of converting a value from one type to another
Implicit vs. ExplicitAlways implicitCan be either implicit or explicit
ExampleIn JavaScript, when you concatenate a string and a number, the number is implicitly coerced into a string and then concatenatedConverting a string to a number using the Number() method in JavaScript

In summary, type coercion is always implicit and refers to the automatic conversion of data types, particularly in JavaScript. On the other hand, type conversion can be either implicit or explicit and refers to the general process of converting a value from one type to another.

67. Explain prototypes in JavaScript.

In JavaScript, prototypes are a fundamental concept that allows objects to inherit properties and methods from other objects. The prototype chain is a mechanism by which JavaScript objects inherit features from one another. Here’s an overview of prototypes in JavaScript:

  1. Every object in JavaScript has a built-in property called prototype: This property is itself an object, which leads to a prototype chain.
  2. Prototypes enable inheritance: Inheritance is a feature of object-oriented programming languages that allows programmers to express common features of objects and reuse code.
  3. Object.create() method: This method creates a new object and allows you to specify an object that will be used as the prototype.

For example, consider the following code snippet:

function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
}

Person.prototype.nationality = "English";

const person1 = new Person("John", "Doe", 50, "blue");
const person2 = new Person("Jane", "Smith", 45, "brown");

console.log(person1.nationality); // Output: English
console.log(person2.nationality); // Output: English

In this example, the Person constructor function inherits the nationality property from the Person.prototype object. Both person1 and person2 instances of the Person constructor function inherit the nationality property from the prototype object.

It’s important to remember that prototypes are a fundamental concept in JavaScript and play a crucial role in object-oriented programming. They allow for code reusability and inheritance, making it easier to create complex objects and manage the inheritance of properties and methods.

68. Explain scope in JavaScript.

In JavaScript, scope defines the context or environment where you declare and access variables. The scope of a variable determines its visibility and lifetime, affecting where in your code it is valid and accessible. There are different types of scope in JavaScript, including:

  1. Global Scope: A variable declared at the top of a program or outside of a function is global, accessible from anywhere in JavaScript.
  2. Local Scope: A variable can possess a local scope, limiting its accessibility solely within the confines of a function. Local variables are created using the let and const keywords.
  3. Block Scope: ES6 (2015) introduced a restriction: variables declared inside a curly-brace block are inaccessible from outside the block. This type of scope is available for variables declared with the let and const keywords.
  4. Function Scope: JavaScript has function scope, meaning that variables defined inside a function are not accessible outside the function. Variables declared within a function have access to this type of scope.
  5. Module Scope: This scope is the scope for code running in module mode. It resembles the global scope but confines itself to the scope of the module.

Understanding scope in JavaScript is crucial for writing efficient and bug-free code. It aids in avoiding conflicts between variables and ensures their accessibility only in the necessary scope. The use of let, const, and var keywords can help manage scope, with let and const providing block scope and var providing function scope.

69. Explain callbacks in JavaScript.

In JavaScript, functions passed as arguments to other functions, known as callbacks, execute after the outer function completes. In asynchronous programming, they frequently find use, enabling a function to call another function only post the prior execution completion. Callbacks provide a way to handle events and manage the execution of asynchronous tasks in a non-blocking manner.

Two primary ways exist to invoke a callback:

  1. Synchronous Callbacks: These callbacks execute immediately after invoking the outer function, without any intervening asynchronous tasks. For example, callbacks passed to Array.prototype.map() and Array.prototype.forEach() are synchronous.
  2. Asynchronous Callbacks: The callbacks get invoked later, following the completion of an asynchronous operation. Asynchronous callbacks are commonly used with setTimeout, setInterval, and XMLHttpRequest.

Callbacks are beneficial because they allow you to write asynchronous code in a structured and non-blocking manner, ensuring that the program continues to run while waiting for asynchronous tasks to complete. You can use them with both synchronous and asynchronous functions, aiding in developing asynchronous JavaScript code without encountering errors.

70. Explain REST and RESTful APIs.

REST (Representational State Transfer) is an architectural style for building web services that use HTTP requests to access and manipulate data. RESTful APIs (Application Programming Interfaces) are web services that adhere to the constraints of the REST architecture. RESTful APIs provide a standard way for web applications to interact with servers and exchange data.

The key characteristics of RESTful APIs include:

  1. Client-server architecture: The client and server are separate and communicate through HTTP requests and responses.
  2. Statelessness: Each request from the client to the server contains all the necessary information to complete the request, and the server does not store any client state between requests.
  3. Cacheability: Caching server responses enhances performance by storing them for future use.
  4. Uniform interface: Standardizing the interface between the client and server enables a clear separation of concerns in their interaction.

Modern web development widely uses RESTful APIs, and most programming languages and frameworks actively support them. They offer a versatile and scalable approach for constructing web services accessible by various clients, including web browsers, mobile devices, and applications.

71. Explain algorithms in JavaScript.

In JavaScript, algorithms denote a set of instructions for solving specific problems or performing computations. They’re fundamental in computer science, used for various tasks. You can implement JavaScript algorithms using diverse techniques, like loops, recursion, and leveraging data structures such as arrays and objects. Some popular algorithms used in JavaScript include Big-O notation, searching algorithms, sorting algorithms, and others.

Learning algorithms in JavaScript involves understanding the fundamental concepts of programming, such as variables, arrays, objects, loops, and functions. You can utilize these concepts to craft algorithms addressing specific problems or executing targeted computations.

There are many resources available for learning algorithms in JavaScript, including online courses, tutorials, and books. These resources provide a comprehensive introduction to algorithms and data structures, as well as practical examples and exercises to help you develop your skills.

72. Explain performance optimization in JavaScript.

Performance optimization in JavaScript is crucial for ensuring that web applications and websites run efficiently. Here are some key tips and best practices for improving JavaScript performance:

  1. Minification: Minify JavaScript code to reduce file sizes and improve load times.
  2. Asynchronous Loading: Use asynchronous loading of JavaScript with defer and async tags to prevent blocking the rendering of the page.
  3. Exclude Unused Components: Exclude unused components of JavaScript to reduce unnecessary overhead.
  4. Use HTTP/2: Utilize the HTTP/2 protocol for improved performance.
  5. Reduce DOM Manipulation: Minimize DOM manipulation, as it is computationally expensive.
  6. Batch DOM Changes: Batch essential DOM changes into groups to reduce the amount of work the browser is doing.
  7. Simplify Mathematical Formulas: Simplify mathematical formulas and use search arrays to optimize performance.
  8. Use Pointer References: Utilize pointer references to improve JavaScript performance.
  9. Trim HTML: Trim HTML to reduce unnecessary overhead.
  10. Avoid Inefficient Iterations: Remove unnecessary loops or calls within loops to speed up JavaScript performance.
  11. Organize Code: Organize JavaScript code effectively to prevent inefficient allocation of resources.
  12. Use Array Filter: Utilize Array Filter to optimize the processing of large arrays.
  13. Cache Selections: Cache selections to variables to avoid repeated traversal of elements.
  14. Merge Arrays Efficiently: Use efficient methods to merge arrays and reduce memory usage.

By implementing these best practices, developers can significantly improve the performance of their JavaScript code, leading to faster load times and a more responsive user experience.

73. Explain modern JavaScript features.

Modern JavaScript (ES6 and beyond) introduces several features that enhance the language’s capabilities and improve developer productivity. Some of these features include:

  1. let and const: These keywords are used for declaring block-scoped variables, providing an alternative to the traditional var keyword.
  2. Arrow Functions: Also known as fat arrow functions, these provide a more concise syntax for writing function expressions.
  3. Template Literals: This feature allows for easier string interpolation and multiline strings using backticks (`).
  4. Default Parameters: Functions can now have default parameter values, reducing the need for manual checks.
  5. Rest and Spread Operators: These operators provide a more flexible way to handle function arguments and arrays.
  6. Destructuring Assignment: This feature allows for easy extraction of array elements or object properties into variables.
  7. Classes: ES6 introduced class syntax for creating objects and implementing inheritance.
  8. Promises: Promises are a more elegant way to handle asynchronous operations, providing a cleaner alternative to nested callbacks.
  9. Async/Await: This feature simplifies asynchronous code even further, allowing for a more synchronous style of coding with asynchronous operations.
  10. Modules: ES6 modules provide a more organized and efficient way to structure and load JavaScript code.

These features, among others, have significantly improved the JavaScript language, making it more expressive, readable, and maintainable. They also align JavaScript more closely with the needs of modern web development, enabling developers to build more sophisticated and efficient applications.

74. Explain JavaScript testing and debugging.

JavaScript testing and debugging are essential processes that help developers identify, fix errors, and ensure the correctness of their code. Here are some key aspects of JavaScript testing and debugging:

  1. Console.log(): Developers commonly use this method to log errors or debugging information to the browser’s console.
  2. Debugging Tools: Modern browsers and Node.js provide built-in debugging tools, such as Chrome DevTools, Node.js Debugger, and Visual Studio Code Debugger, which help developers identify and fix errors in their code.
  3. Testing Frameworks: Popular testing frameworks like Jest, Mocha, and Jasmine provide a set of tools and APIs to create tests, assertions, and test suites for JavaScript code.
  4. Unit Tests: Writing unit tests is an essential part of testing JavaScript code. These tests focus on testing small, isolated pieces of code to ensure that each function and class work as expected.
  5. Edge Cases: Testing edge cases, involving input values beyond the expected range, aids in error identification and ensures proper handling of unexpected situations.
  6. Debugging Statements: JavaScript provides debugging statements like debugger, which can be used to pause the execution of the code and inspect the current state of variables and objects.
  7. Error Handling: Proper error handling is crucial for writing robust JavaScript code. Using try-catch blocks can help handle errors and provide more meaningful error messages.
  8. Code Reviews: Code reviews can help identify potential issues and improve the overall quality of the code. During a code review, another developer examines the code and provides feedback on potential improvements, best practices, and potential issues.

By following these best practices and utilizing the available tools, developers can effectively test and debug their JavaScript code, leading to more reliable and maintainable software applications.

75. Explain JavaScript security best practices.

JavaScript security best practices include:

  1. Avoid using eval(): The use of eval() can introduce security vulnerabilities, as it executes any code passed to it. Instead, use alternative, more secure methods for dynamic code execution.
  2. Encrypt data with HTTPS/SSL: Utilize HTTPS/SSL to encrypt data exchanged between the client and the server, preventing man-in-the-middle attacks and ensuring data integrity.
  3. Set secure cookies: When you set cookies, make sure to mark them as secure, ensuring they exclusively transmit over HTTPS and are HTTP-only, preventing client-side access.
  4. Validate and sanitize user input: Validate and sanitize user input to thwart common vulnerabilities like cross-site scripting (XSS) and SQL injection risks.
  5. Use Content Security Policy (CSP): Mitigate the risks of cross-site scripting attacks by implementing CSP, specifying allowed content sources for loading.
  6. Audit dependencies: Regularly audit and update third-party dependencies to address known vulnerabilities and ensure robust security measures.
  7. Use a JavaScript linter: Employ a JavaScript linter to enforce coding standards and identify potential security issues in the codebase.
  8. Add Subresource Integrity (SRI) checking to external scripts: Browsers can verify that fetched resources, like scripts, remain unaltered, thanks to Subresource Integrity (SRI).
  9. Avoid using inline JavaScript: Avoid using inline JavaScript to mitigate the risk of introducing XSS vulnerabilities to your web applications. Instead, use unobtrusive JavaScript and separate JavaScript from HTML.
  10. Use CSRF tokens: Implement CSRF tokens, avoiding storage in cookies to thwart cross-site request forgery attacks effectively.

By following these best practices, developers can significantly reduce the risk of security vulnerabilities in their JavaScript applications.

76. Explain ES6 features and syntax.

ES6 (ECMAScript 2015) introduced several new features and syntax enhancements to JavaScript, making the language more expressive and maintainable. Some of the key features and syntax introduced in ES6 include:

  1. let and const: These keywords are used for declaring block-scoped variables, providing a more predictable way to declare variables.
  2. Arrow Functions: Arrow functions provide a more concise syntax for writing function expressions, making the code more readable.
  3. Template Literals: This feature allows for easier string interpolation and multiline strings using backticks (`), improving the readability of strings in the code.
  4. Default Parameters: Functions can now have default parameter values, reducing the need for manual checks.
  5. Destructuring Assignment: This feature allows for easy extraction of array elements or object properties into variables, making the code more concise.
  6. Classes: ES6 introduced class syntax for creating objects and implementing inheritance, providing a more familiar and convenient syntax for object-oriented programming.
  7. Modules: ES6 modules provide a more organized and efficient way to structure and load JavaScript code, improving code maintainability and reusability.
  8. Promises: Promises are a more elegant way to handle asynchronous operations, providing a cleaner alternative to nested callbacks.
  9. Spread and Rest Operators: These operators provide a more flexible way to handle function arguments and arrays, improving the manipulation of arrays and function arguments.
  10. Map and Set Objects: ES6 introduced new data structures like Map and Set, providing more options for managing collections of data.

These features and syntax enhancements have significantly improved the JavaScript language, making it more expressive, readable, and maintainable. They have also aligned JavaScript more closely with the needs of modern web development, enabling developers to build more sophisticated and efficient applications.

77. Explain JavaScript build tools like Webpack.

Webpack is a popular JavaScript build tool and module bundler that is widely used in modern web development. It is primarily concerned with bundling separate JavaScript modules during the build process, but it is also capable of transforming, bundling, and managing other web assets such as HTML, CSS, and images. Webpack helps in modularizing code, optimizing assets, and enabling features like hot module replacement (HMR).

It works from the command line to bundle assets, making it easier for the browser to download by creating a single bundle file instead of numerous small files. Webpack is known for its flexibility, code splitting capabilities, and extensive configuration options, making it suitable for medium to large web projects. It is still a relevant and widely used build tool in the JavaScript ecosystem, and it is often the default choice for many popular applications. Other alternatives to Webpack include tools like Browserify, Rollup.js, Gulp, Babel, and Parcel, each with its own set of advantages and ideal project profiles.

78. Explain conditional statements in JavaScript.

JavaScript employs conditional statements like if, else, and else if to execute actions based on varying conditions. Use the if statement to execute a block of code when a specified condition evaluates to true. The else statement executes a block of code when the specified condition evaluates to false. The ‘else if’ statement specifies a new condition to test if the first condition is false. These conditional statements allow for the execution of different code blocks based on the evaluation of specific conditions, enabling the creation of dynamic and responsive code. They are fundamental building blocks in JavaScript for implementing decision-making logic.

79. Explain the DOM and DOM manipulation.

The Document Object Model (DOM) is a programming interface for web documents. It represents the structure of a document as a tree of objects, where each object corresponds to a part of the document, such as elements, attributes, and text. JavaScript can manipulate this tree structure, allowing developers to dynamically alter the content and appearance of a webpage. DOM manipulation is at the core of creating dynamic and interactive web pages, enabling the addition, removal, and modification of elements on a website without the need to refresh the entire page.

This is achieved by using the DOM API to change or modify an HTML document, allowing for the creation of engaging and user-friendly web applications. The HTML DOM (Document Object Model) is a standard object model and programming interface for HTML, defining the elements as objects, their properties, methods to access them, and events for all HTML elements. By using JavaScript to access and change all the elements of an HTML document, developers can create dynamic and interactive web pages.

80. Explain loops in JavaScript.

JavaScript uses loops to repeatedly execute a block of code until a specified condition is met. There are several types of loops in JavaScript, including the for loop, while loop, do-while loop, for-in loop, and for-of loop.

Developers use the for loop to execute a block of code a specific number of times. It consists of three parts: initialization, condition, and increment/decrement. Use the while loop to execute a block of code as long as a specified condition remains true. The do-while loop is similar to the while loop, but it executes the block of code at least once, even if the condition is false.

Use the for-in loop to iterate through an object’s properties and the for-of loop to traverse iterable object values.

Loops are essential in JavaScript for iterating over arrays, objects, and other data structures, and for performing repetitive tasks. Programmers extensively use them in web development to construct dynamic and interactive web pages, constituting fundamental building blocks.

81. What are the differences between first-class and second-class functions?

First-class and second-class functions are terms used to describe the level of flexibility and functionality that a programming language provides when dealing with functions. Here’s a the differences between first-class and second-class functions:

CharacteristicFirst-Class FunctionsSecond-Class Functions
Assignment to VariablesCan be assigned to variablesCannot be assigned to variables
Return by FunctionsCan be returned by functionsCannot be returned by functions
Passed as ArgumentsCan be passed as arguments to other functionsCannot be passed as arguments to other functions
Storage in Data StructuresCan be stored in data structuresCannot be stored in data structures

These differences highlight the level of flexibility and functionality provided by a programming language when dealing with functions. First-class functions offer more expressive and flexible code, while second-class functions limit the level of flexibility and functionality in the code.

82. What are the differences between functional and imperative programming?

Functional and imperative programming are two distinct programming paradigms, each with its own approach to solving problems and structuring code. Here’s a the differences between the two:

CharacteristicFunctional ProgrammingImperative Programming
Core FocusTreats programs as evaluating mathematical functions and avoids stateSpecifies the steps a program must take to reach a desired state
State ManagementAvoids state and mutable data, emphasizing the application of functionsEmphasizes the use of mutable state and data, and defines control flow as statements that change program state
Data MutationEmphasizes immutable data and avoids data mutationAllows data mutation and modification
Control FlowFocuses on what to execute, and defines program logic, but not detailed control flowDefines detailed control flow as statements that change program state
ExamplesHaskell, Lisp, Clojure, ScalaC, Java, Python, Ruby

Functional programming treats programs as a series of mathematical functions and avoids mutable state, while imperative programming focuses on defining the steps a program must take to reach a desired state, often involving mutable state and detailed control flow. Each paradigm has its own strengths and weaknesses, and the choice between them often depends on the specific requirements of a given problem or project.

83. What are the differences between pure and impure functions?

Pure and impure functions are two types of functions in JavaScript that differ in their behavior and characteristics. Here are the differences between pure and impure functions:

Pure Functions:

  1. Always return the same output for the same input parameters.
  2. Do not modify the state of the application or have side effects.
  3. Only depend on their input parameters and do not affect the state of the application.
  4. Are easier to test, debug, and maintain.
  5. Cannot execute AJAX calls or standard DOM manipulation.

Impure Functions:

  1. Can modify the state of the application or have side effects.
  2. Can have unpredictable behavior and affect other parts of the application.
  3. Can execute AJAX calls or standard DOM manipulation.
  4. Are more difficult to test, debug, and maintain.

Pure functions are more predictable and easier to reason about, as they do not have side effects and always return the same output for the same input parameters. They are also easier to test and debug, making them a preferred choice in functional programming. Impure functions, on the other hand, can have unpredictable behavior and affect other parts of the application, making them more difficult to test and debug. They are often used in situations where side effects are necessary, such as when interacting with external systems or modifying the state of the application.

84. What are the differences between immutable and mutable functions?

Mutable and immutable objects are fundamental concepts in programming languages. Here’s a summary of the differences between them:

CharacteristicMutable ObjectsImmutable Objects
Change After CreationCan be changed after creationCannot be changed after creation
ExamplesLists, DictionariesStrings, Numbers, Tuples
Memory UsageMay require less memory due to in-place modificationsMay require more memory due to the creation of new objects when modified
PredictabilityMay lead to unexpected behavior due to in-place modificationsLeads to predictable behavior due to the inability to change after creation

Mutable objects can be changed after creation, while immutable objects cannot. Mutable objects, such as lists and dictionaries, may require less memory due to in-place modifications, but they can lead to unexpected behavior. On the other hand, immutable objects, such as strings, numbers, and tuples, require more memory due to the creation of new objects when modified, but they lead to predictable behavior.

85. What are the differences between strict and non-strict mode?

The differences between strict and non-strict mode in JavaScript are related to error handling, variable declarations, and function parameters. Here’s a summary of the differences:

CharacteristicStrict ModeNon-Strict Mode
Error HandlingThrows errors for certain syntax errors and codeAllows silent errors and unexpected behavior
Variable DeclarationsStrict mode requires let, const, and class for variable declarationsNon-strict mode allows variable declarations without let, const, and class
Function ParametersStrict mode disallows redeclaring function parameters with the same nameNon-strict mode allows redeclaring function parameters with the same name

Strict mode is a subset of JavaScript that provides better error checking and enforces stricter rules, while non-strict mode is the default mode of operation in JavaScript. In strict mode, the JavaScript engine checks for syntax errors and runtime errors that would otherwise go unnoticed, making it easier for developers to catch errors early in the development process, resulting in fewer bugs and better code quality. In non-strict mode, the JavaScript engine allows for more flexible coding and provides fewer restrictions, which can lead to unexpected behavior if used improperly.

86. What are the differences between DOM and BOM?

The DOM (Document Object Model) and BOM (Browser Object Model) are two distinct models in JavaScript that represent different aspects of a web page. Here are the differences between the two:

DOM:

  1. Represents the structure of an HTML or XML document as a tree of objects.
  2. Provides a standardized interface to access and modify the elements and content of an HTML or XML document.
  3. Focuses on the structure of the displayed document.
  4. Facilitates the creation, manipulation, or deletion of elements from the document.

BOM:

  1. Represents the browser-specific functionality of a web page, such as the browser window, history, and location.
  2. Allows JavaScript to interact with browser features beyond the scope of manipulating the document.
  3. Focuses on browser-specific functionality.
  4. Provides a non-standardized interface to access and modify browser features.

In summary, the DOM represents the structure of an HTML or XML document and provides a standardized interface to access and modify its elements, while the BOM represents the browser-specific functionality of a web page and provides a non-standardized interface to access and modify browser features.

87. What are the differences between map and reduce?

The main differences between the map and reduce functions in JavaScript are as follows:

Characteristicmap Functionreduce Function
PurposeUsed to create a new array by transforming each element in an existing arrayUsed to transform all elements in an array into a single value
Return ValueReturns a new array with the results of applying a provided function to each element in the original arrayReturns a single value after applying a provided function to each element of the array
Callback ArgumentsThe callback function used with map takes three arguments: the current element being processed, the index of that element, and the array map was called uponThe callback function used with reduce takes four arguments: the accumulator, the current element being processed, the index of that element, and the array reduce was called upon

In summary, map is used to transform each element of an array into a new array, while reduce is used to transform all elements of an array into a single value.

88. What are the differences between map and filter?

Here’s a table summarizing the differences between map and filter functions in JavaScript:

Characteristicmap Functionfilter Function
PurposeUsed to create a new array by transforming each element in an existing arrayUsed to create a new array by filtering elements from an existing array
Return ValueReturns a new array with the results of applying a provided function to each element in the original arrayReturns a new array with the elements that pass a specified test from the original array
Callback ArgumentsThe callback function used with map takes three arguments: the current element being processed, the index of that element, and the array map was called uponThe callback function used with filter takes two arguments: the current element and a boolean value indicating whether the element should be included in the filtered array

In summary, map is used to transform each element of an array into a new array, while filter is used to filter elements from an array based on a specified condition.

89. What are the differences between forEach and map?

Here’s a table summarizing the differences between forEach and map functions in JavaScript:

CharacteristicforEach Functionmap Function
PurposeUsed to loop through elements of an array and perform a function on each elementUsed to transform each element in an array and return a new array
Return ValueDoes not return a new arrayReturns a new array
Callback ArgumentsThe callback function used with forEach takes two arguments: the current element and an indexThe callback function used with map takes three arguments: the current element, the index, and the array map was called upon
Side EffectsCan have side effects and mutate the original arrayCannot have side effects and does not mutate the original array

In summary, forEach is used to loop through elements of an array and perform a function on each element, while map is used to transform each element of an array and return a new array.

90. What are the differences between rest and spread syntax?

The rest and spread syntax in JavaScript are similar in syntax, but they have different purposes and behaviors. Here’s a table summarizing the differences between the two:

CharacteristicRest SyntaxSpread Syntax
PurposeUsed to collect multiple elements into an arrayUsed to expand an array into multiple elements
SyntaxUses three dots (…) followed by a variable nameUses three dots (…) followed by an array or iterable object
UsageUsed in function parameters to collect multiple arguments into an arrayUsed to expand an array into multiple arguments or elements
Examplefunction myFunction(...args) { // rest syntax }const arr1 = [1][2][3]; const arr2 = [4][5]; const arr3 = [...arr1, ...arr2]; // spread syntax

91. What are the differences between setTimeout and setInterval?

The main differences between setTimeout and setInterval in JavaScript are as follows:

  1. Execution:
  • setTimeout: Executes a function or code once after a specified delay.
  • setInterval: Executes a function or code repeatedly at specified intervals.
  1. Behavior:
  • setTimeout: Stops the execution after a single call.
  • setInterval: Continues to execute the function repeatedly based on the specified interval.
  1. Syntax:
  • setTimeout: setTimeout(function|code, delay);
  • setInterval: setInterval(function|code, delay);
  1. Cancellation:
  • setTimeout: Can be canceled using clearTimeout() with a timeout ID.
  • setInterval: Can be canceled using clearInterval() with a timer ID.

In summary, setTimeout is used to execute a function or code once after a specified delay, while setInterval is used to execute a function or code repeatedly at specified intervals. Both functions allow for cancellation using their respective clear methods, but setTimeout cancels the execution after a single call, whereas setInterval continues to execute the function repeatedly based on the specified interval.

92. What are the differences between shallow and deep cloning?

Shallow and deep cloning are two different ways of copying objects in programming. Here’s a table summarizing the differences between the two:

CharacteristicShallow CloningDeep Cloning
PurposeCreates a new object with the same references as the original objectCreates a new object with new references for all nested objects
BehaviorCopies the object and its references, but not the nested objectsCopies the object and all its nested objects recursively
ChangesChanges to the original object are reflected in the cloned object, and vice versaChanges to the original object are not reflected in the cloned object, and vice versa
SpeedFaster than deep cloningSlower than shallow cloning
SyntaxUses the spread operator (…) to copy the objectUses the JSON.parse(JSON.stringify(object)) method to copy the object

In summary, shallow cloning creates a new object with the same references as the original object, while deep cloning creates a new object with new references for all nested objects. Shallow cloning is faster but reflects changes to the original object in the cloned object, while deep cloning is slower but does not reflect changes to the original object in the cloned object.

93. What are the differences between host objects and native objects?

Host Objects and Native Objects are two distinct types of objects in JavaScript. The main differences between them are as follows:

1. Definition:

  • Native Objects: The JavaScript language includes standard objects like Array, Date, Math, and Object, known as built-in, pre-defined, or global objects.
  • Host Objects: These are objects provided by the environment in which JavaScript is running, such as the window object and the document object in a web browser. Host objects are environment-specific and may vary from one environment to another.

2. Specification:

  • Native Objects: The ECMAScript specification defines them as a fundamental part of the language, always available for use in JavaScript code.
  • Host Objects: The host environment completes the execution environment of ECMAScript by supplying these, as they are not defined in the specification. The availability of host objects depends on the specific environment in which JavaScript is running.

Summarizing, standard JavaScript objects are native, built into the language, while environment-specific objects, like in web browsers or Node.js, are host objects.

94. What are the differences between static and instance methods?

Here’s a table summarizing the differences between static and instance methods in JavaScript:

CharacteristicStatic MethodsInstance Methods
ScopeDefined on the class itself and do not have access to the instance’s private propertiesDefined on the class prototype and have access to the instance’s private properties
BehaviorDo not rely on an instance of the class and are often used as utility functions. They do not change the instance’s state.Work on the state of an instance and can change the instance’s properties.
AccessibilityCan be called directly on the constructor function or the class, and they do not require an instance of the class to be created.Can only be called on instances of the class and require an instance to be created.

95. What are the differences between stack and heap memory?

Here’s a table summarizing the differences between stack and heap memory:

CharacteristicStack MemoryHeap Memory
TypeLinear data structureHierarchical data structure
Access SpeedHigh-speed accessSlower compared to stack
Memory AllocationAutomatic by compiler instructionsManual by the programmer
Memory SizeFixed size, cannot be changedFlexible, can be resized
LifetimeTemporary storage, used during function callsLonger lifespan, used for objects and data structures
FragmentationDoes not cause fragmentationCan cause fragmentation

96. What are the differences between pure functions and impure functions?

Here’s a table summarizing the differences between pure and impure functions:

CharacteristicPure FunctionsImpure Functions
Side EffectsDo not have side effectsContain side effects
OutputAlways return the same output for the same input parametersMay produce different outputs for the same input parameters
DependenciesDepend only on input parametersDepend on external factors, such as global variables or shared state
TestingEasier to test and debugMore difficult to reason about and test
State ModificationDo not modify the state of the applicationModify the state of the application

97. What are the differences between class and prototype?

Here’s a table summarizing the differences between classes and prototypes in JavaScript:

CharacteristicClassesPrototypes
NatureSyntactical sugar introduced in ECMAScript 2015Introduced in early versions of JavaScript
DefinitionDefined using the class keywordDefined using objects and functions
InheritanceInheritance is implemented using the extends keywordInheritance is implemented using the prototype chain
Object CreationClasses use the new operator to create instancesObjects are created using the new operator or by simply calling a function
LifetimeClasses have a fixed lifecycle, defined by the developerPrototypes have a dynamic lifecycle, defined by the runtime

98. What are the differences between REST and RESTful?

REST and RESTful are two terms that are often used interchangeably, but they have different meanings. Here’s a table summarizing the differences between REST and RESTful:

CharacteristicRESTRESTful
DefinitionREST is a collection of architectural constraints that define how web services should work.RESTful refers to web services that adhere to the constraints defined by REST.
NatureIt is an architectural style.It is a term used to describe web services that follow the REST architecture.
ImplementationIt is a set of guidelines and constraints that define how web services should be designed.It is the implementation of the REST architecture in web services.
Uniform InterfaceIt requires a uniform interface between components, such as HTTP methods and resource URIs.It follows the uniform interface constraints defined by REST.
CachingIt supports caching to improve performance.It supports caching to improve performance.
StatelessnessIt requires that the server does not store any client context between requests.It requires that the server does not store any client context between requests.

99. What are the differences between DOM, BOM and JavaScript?

DOM, BOM, and JavaScript are three different components that make up the foundation of web browsers and web development. Here’s a table summarizing the differences between them:

ComponentDOM (Document Object Model)BOM (Browser Object Model)JavaScript
DefinitionDOM is a programming interface for representing and interacting with HTML, XHTML, and XML documents.BOM is a collection of browser objects that manage browser windows and enable communication between them.JavaScript is a scripting language that can be used to manipulate objects in the DOM and BOM.
NatureIt is a standard defined by W3C (World Wide Web Consortium).It is not a standard and different browsers implement it in different ways.It is a scripting language that can be used to manipulate objects in the DOM and BOM.
ImplementationDOM is standardized and specific to the current HTML document.BOM is not standardized and can change based on different browsers.JavaScript is a standardized language that can be used across different browsers.
UsageIt organizes the elements of the document in a tree structure and defines all elements as objects.It manages browser windows and enables communication between them, with DOM being a subset of BOM.It can be used to manipulate objects in the DOM and BOM, such as modifying the document structure, handling user interactions, and implementing custom logic.

100. What are the differences between declarative and imperative programming?

Here’s a table summarizing the differences between declarative and imperative programming:

CharacteristicDeclarative ProgrammingImperative Programming
DefinitionDeclarative programming specifies the result, not the process.Imperative programming specifies the process, not the result.
NatureIt is a programming paradigm that focuses on the end result.It is a programming paradigm that focuses on the process of achieving the result.
ImplementationIt is implemented using functions like map, filter, and reduce.It is implemented using loops, conditionals, and other control structures.
ReadabilityDeclarative code is clearer and easier to read.Imperative code can be harder to read, as it involves more control structures.
EfficiencyDeclarative code is usually shorter and more efficient.Imperative code can be longer and potentially less efficient.
ControlDeclarative programming gives more control to the developer.Imperative programming gives less control to the developer, as the process is explicitly specified.

101. What are the differences between functional and imperative programming?

Imperative and functional programming are two different programming paradigms. Here’s a table summarizing the differences between them:

CharacteristicImperative ProgrammingFunctional Programming
DefinitionImperative programming specifies how to achieve a result by giving a sequence of commands.Functional programming specifies what result to achieve by using functions.
NatureIt is a programming paradigm that focuses on the process of achieving the result.It is a programming paradigm that focuses on the end result.
ImplementationIt is implemented using loops, conditionals, and other control structures.It is implemented using functions like map, filter, and reduce.
Side EffectsIt can have side effects and modify the state of the program.It avoids side effects and emphasizes immutability.
ReadabilityImperative code can be harder to read, as it involves more control structures.Functional code is clearer and easier to read.
EfficiencyImperative code can be longer and potentially less efficient.Functional code is usually shorter and more efficient.

Conclusion:

This guide will help you demonstrate your knowledge of essential ideas and best practices as you go on your journey toward achieving the top 100+ JavaScript Interview Questions. abilities also about and answers, but complexities demonstrating JavaScript’s of problem-solving presenting right that understanding thorough. Use this complete guide to successfully navigate the JavaScript interview landscape in 2024 and beyond. Good Luck!

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