Javascript Interview Questions and Answers

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JavaScript is essential for web development, powering interactive user interfaces. With more than 11,45,000 global job opportunities, JavaScript developers are highly sought after, with salaries between ₹8 to ₹18 lakhs annually. Below are important JavaScript interview questions designed to equip you with the essential knowledge and problem-solving techniques to confidently tackle any JavaScript-related question that comes your way during an interview.

Table of content

• Basic JavaScript Interview Questions for Freshers
• Intermediate JavaScript Interview Questions and Answers
• Advanced JavaScript Interview Questions for Experienced

Most Frequently Asked Javascript Interview Questions

Q1. What is JavaScript, and what are its key features?
Q2. Explain the difference between let, var, and const for declaring variables in JavaScript.
Q3. How do you create and use functions in JavaScript?
Q4. What is the purpose of NaN in JavaScript, and how do you check for it?
Q5. Describe the difference between == and === in JavaScript.
Q6. How do you handle asynchronous operations in JavaScript? Provide an example.
Q7. What are arrays in JavaScript, and how do you access their elements?
Q8. Explain the role of ‘this’ keyword in JavaScript and how it behaves in different contexts.
Q9. How do you loop through an object’s properties in JavaScript?
Q10. What is the use of the type of operator in JavaScript? Give an example.

You can revise all the concepts in this set of top JavaScript interview questions and answers, from basic to advanced questions. Dive into the arena of learning with us through these JavaScript questions. Throughout this blog, we embark on a quest to discover the most common interview inquiries, covering straightforward concepts to intricate scenarios. Explore the fundamental concepts and problem-solving approaches that will equip you to successfully handle any JavaScript-related issue.

Basic JavaScript Interview Questions

1. What is JavaScript, and what are its key features?

JavaScript is a very powerful high-level programming language commonly used to make web pages more interactive and dynamic. It allows developers to create responsive and engaging websites by adding features like animations, form validations, and interactive elements. JavaScript is lightweight and easy to use, making it a popular choice for web development. It supports different programming styles, including object-oriented and functional approaches, giving developers the flexibility to write code in a way that suits their needs best. With JavaScript, developers can breathe life into static web pages, making the user experience more enjoyable and interactive.

2. Explain the difference between let, var, and const for declaring variables in JavaScript.

‘let’ and ‘const’ are block-scoped variables introduced in ES6 (short for ECMAScript 6), while ‘var’ is function-scoped. ‘let’ allows reassigning values, ‘const’ is for constants that cannot be reassigned and ‘var’ can be reassigned but has a broader scope than let.

3. How do you create and use functions in JavaScript?

Functions in JavaScript are like building blocks that allow developers to create reusable blocks of code. 

There are two ways to create functions: 

1. Using the function keyword or 
2. Using the arrow functions (() => {}). 

Functions can take input values, which we call parameters, and perform specific actions or calculations based on these inputs. After performing the desired actions, functions can also return a value using the return keyword. This returned value can then be used or stored for further use in the program. With functions, developers can organize their code into logical modules, making it easier to understand and maintain. They are a fundamental concept in JavaScript programming and play a crucial role in creating dynamic and interactive web applications

4. What is the purpose of NaN in JavaScript, and how do you check for it?

‘NaN’, short for ‘Not a Number’, is a unique value in JavaScript that indicates an invalid result from a mathematical operation. For example, if you try to perform a mathematical calculation that doesn’t yield a valid number, JavaScript will return ‘NaN’. 

To check if a value is ‘NaN’, you can use the ‘isNaN()’, which will return true if the value is indeed ‘NaN’ and false if it is a valid number. This helps developers handle scenarios where the result of a calculation might not be a number, ensuring that the code behaves as expected and avoids unexpected errors. Understanding ‘NaN’ and how to check for it is an essential aspect of working with numerical data in JavaScript and contributes to creating reliable and accurate code.

5. Describe the difference between '==' and '===' in JavaScript.

The ‘==’ is an equality operator that checks for equality after converting the operands to the same type if needed. On the other hand, the ‘===’ is a strict equality operator that checks for equality without any type of conversion.

In simple terms, ‘==’ allows for loose comparison, while ‘===’ enforces strict comparison, ensuring that both the value and the data type must match for it to return true.

6. How do you handle asynchronous operations in JavaScript? Provide an example.

Asynchronous operations are tasks in JavaScript that may take some time to complete, such as fetching data from a server or reading files. Unlike synchronous operations that block the execution of the code until the task is finished, asynchronous operations allow the program to continue executing other tasks while waiting for the asynchronous task to complete.

There are three main ways to handle asynchronous operations in JavaScript:

1. Callbacks: Callbacks are functions passed as arguments to other functions, which will be executed once the asynchronous task is completed. The callback function allows the program to respond to the result of the asynchronous task or handle any errors that might occur. 
2. Promises: Promises are a more structured way to handle asynchronous operations and provide a better alternative to callbacks. A Promise represents a value that may not be available yet but will be resolved in the future.
   It has then() and catch() methods to handle successful and error outcomes, respectively. Promises allow the chaining of asynchronous tasks in a more readable and sequential manner, making the code more organized and easier to understand.
3. Async/Await: Async/Await is a modern and elegant syntax built on top of Promises, introduced in ES2017 (ES8). It allows developers to write asynchronous code that looks and behaves more like synchronous code. By using the async keyword before a function declaration, the function automatically returns a Promise. The await keyword is then used inside the async function to pause execution until the Promise is resolved, making it appear as if the asynchronous operation is synchronous.

7. What are Arrays in JavaScript, and how do you access their elements?

Arrays in JavaScript are a fundamental data structure used to store and organize collections of data. They are ordered, meaning the elements are stored in a specific sequence. Each element in an array is identified by its index, which starts at 0 for the first element, 1 for the second element, and so on.

To access the elements of an array, you can use square brackets ([]) along with the index of the element you want to retrieve.

For example, if you have an array named myArray, you can access its first element using myArray[0], the second element using myArray[1], and so on.

//Creating an array of numbers
const numbers = [10, 20, 30, 40, 50];

//Accessing elements of the array
console.log(numbers[0]);

console.log(numbers[2]);

console.log(numbers[4]);

<strong>Output:</strong>
10
30
50

8. Explain the role of ‘this’ keyword in JavaScript and how it behaves in different contexts.

The ‘this’ keyword in JavaScript plays a pivotal role in referencing the current execution context. Its behavior varies based on the context in which it is used, such as within a function, method, or event handler. ‘this’ typically refers to the object that is called the function/method, but in certain situations, it may point to the global object or remain undefined. Understanding ‘this keyword in JS’ is crucial for precise and effective programming in JavaScript.

9. How do you loop through an object's properties in JavaScript?

When you need to iterate through an object’s properties in JavaScript, you have several options available. One way is to use a ‘for’ in loop, which allows you to loop through each property of the object one by one. Another approach is to use the methods ‘Object.keys()’, ‘Object.values()’, or, ‘Object.entries()’ introduced in ES6. These methods provide arrays of the object’s keys, values, or key-value pairs, respectively, which you can then iterate through using traditional array looping techniques like for or forEach. These techniques offer different ways to access and work with an object’s properties, providing flexibility and convenience when handling complex data structures in JavaScript.

10. What is the purpose of the 'typeof' operator in JavaScript? Give an example.

Developers can determine the data type of a variable or expression with the aid of the ‘typeof’ operator in JavaScript. You can use it to figure out whether a value is a number, text, boolean, object, function, undefined, or any other data type that the language supports. Making judgments in your code and managing data appropriately based on its kind require the use of this knowledge.

let name = 'John';
let age = 30; 
let isMarried = false; 
//Accessing elements of the array
console.log(typeof name);
console.log(typeof age); 
console.log(typeof isMarried); 

Output:

string
number
boolean

Intermediate JavaScript (JS) Interview Questions and Answers

11. Explain hoisting in JavaScript.

Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their scope before code execution. This means functions can be called before they are declared and variables can be accessed before they are declared without throwing errors.

12. What is the 'use strict' mode in JavaScript?

The “use strict” mode is a way to opt into a restricted variant of JavaScript. It catches common coding mistakes and “unsafe” actions. When used, it disables actions like using undeclared variables, deleting variables, and others to prevent bugs. It makes JavaScript code more secure and helps developers write cleaner code.

13. What is the difference between map and foreach in JavaScript?

The map method returns a new array with the results of calling a function on every element in an array, while each simply executes the function on each element without returning a value. Map allows you to transform an array, while each is used only to iterate over elements without modifying the original array.

14. What are ES6 modules in JavaScript?

ES6 modules allow JavaScript code and functions to be split across multiple separate files called modules. Each module handles a specific task and can export functions or objects for other modules to use. This avoids polluting the global namespace and makes code more maintainable by separating concerns into individual modules.

15. Explain the concept of object-oriented programming in JavaScript.

Object-oriented programming (OOP) in JavaScript uses objects and classes to structure code. Objects contain data (properties) and related behaviors (methods). Classes act as templates to create objects with common properties and methods. OOP principles like inheritance, encapsulation, and polymorphism help organize code and make it reusable.

16. What are the different types of errors in JavaScript?

The main types of errors in JavaScript are syntax errors, runtime errors, and logical errors. Syntax errors occur due to incorrect syntax like missing brackets. Runtime errors happen during execution like reference errors. Logical errors produce unintended results even if the code runs without crashing, due to flawed thinking in programming logic.

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

In synchronous programming, tasks are executed sequentially where each subsequent task waits for the previous task to complete before starting. Asynchronous programming doesn’t wait and allows multiple tasks to run concurrently without blocking. JavaScript uses asynchronous programming to make UI responsive and handle long tasks without freezing the browser.

18. What are Closures in JavaScript, and how are they useful? Provide an example.

Closures in JavaScript are functions that have access to variables from their containing (enclosing) function, even after the enclosing function has finished executing. This occurs because the inner function maintains a reference to the environment in which it was created, allowing it to ‘“close over’” and retain access to its outer function’s variables. Closures are powerful and useful for creating private variables, implementing data encapsulation, and achieving functional programming paradigms in JavaScript.

Code:

function outerFunction() {
  let outerVariable = ‘I am from the outerFunction’;
  
  function innerFunction() {
    console.log(outerVariable);
  }
  return innerFunction;
}
const closureFunction = outerFunction();
closureFunction();

Output:

I am from the outer function

19. Explain the concept of Prototypal inheritance in JavaScript.

Prototypal inheritance is the mechanism by which objects in JavaScript inherit properties and methods from other objects, forming a prototype chain. Each object has an internal link to its prototype (also known as __proto__). When a property or method is accessed on an object, JavaScript searches for the property/method in the object itself and then in its prototype chain until it finds the property or reaches the end of the chain (i.e., the root Object.prototype). This concept allows objects to inherit and share functionality, leading to code reusability and a more memory-efficient way of handling similar objects.

20. How do you handle errors and exceptions in JavaScript using try...catch blocks?

The try…catch block is used in JavaScript to handle errors and exceptions gracefully. When code that may cause an error is placed within the try block, if an error occurs,  control is immediately transferred to the corresponding catch block. This prevents the error from causing the entire program to crash. The catch block receives an error object that contains information about the error, allowing developers to log or handle the error appropriately.

21. Describe the concept of Event delegation in JavaScript and its benefits.

Event delegation is a technique in JavaScript where a single event handler is attached to a common ancestor element instead of attaching individual event handlers to multiple child elements. This way, events can be efficiently managed and handled for dynamically added elements or elements that are not yet present in the DOM. Event delegation reduces memory consumption and improves performance by leveraging event propagation (bubbling or capturing) to handle events at a higher level in the DOM tree.

22. What are higher-order functions in JavaScript? Provide a practical example.

In JavaScript, higher-order functions are functions that can take one or more functions as arguments and/or return a function as their result. This concept is derived from functional programming, which treats functions as first-class citizens, meaning they can be assigned to variables, passed as arguments, and returned as values.

Higher-order functions enable developers to write more flexible and reusable code by abstracting common patterns and behaviors into functions that can be manipulated and combined with other functions. They are an essential feature of functional programming and are widely used in modern JavaScript development.

Example of higher-order functions in JavaScript:

Code:

// Original array of numbers
const numbers = [1, 2, 3, 4, 5];

// Higher-order function using map()
const doubledNumbers = numbers.map(function (number) {
  return number * 2;
});
console.log(doubledNumbers);

Output:

[2, 4, 6, 8, 10]

23. How does the async/await syntax simplify asynchronous programming in JavaScript?

Simplifying Asynchronous Programming with async/await:

The async/await syntax is a feature introduced in ES2017 (ES8) that simplifies working with asynchronous code in JavaScript. By marking a function with the async keyword, it automatically returns a Promise. Within an async function, the await keyword can be used to pause execution until a Promise is resolved or rejected. This makes asynchronous code look more like synchronous code, improving code readability and maintainability.

24. Discuss the concept of Callback functions and their role in Asynchronous operations.

Callback functions are functions passed as arguments to other functions and are executed once the asynchronous operation is completed. They are commonly used in older asynchronous patterns (such as callbacks in setTimeout() or XMLHttpRequest) but are gradually being replaced by promises and async/await due to the callback hell problem, where multiple nested callbacks lead to difficult-to-read code.

25. What are Immediately-Invoked Function Expressions (IIFE) in JavaScript, and why would you use them?

Immediately-Invoked Function Expressions (IIFE) is a design pattern in JavaScript where a function is defined and immediately invoked. This pattern is often used to create a private scope, encapsulating variables and preventing them from polluting the global namespace. IIFE is commonly used in older JavaScript code to avoid variable collisions and maintain cleaner code.

26. How do you use Modules in modern JavaScript development? Give an example of importing and exporting modules.

In modern JavaScript development, modules are used to organize code into separate files, each containing a specific piece of functionality. The ES6 module system allows developers to import and export functionality between modules, promoting code reusability and maintainability. Modules help manage dependencies, avoid global scope pollution, and make code easier to manage in large-scale applications.

27. Explain the concept of hoisting in JavaScript and its implications.

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase. This allows variables and functions to be accessed before formally declaring them in the code. However, only the declarations are hoisted, not the initializations. Understanding hoisting is crucial to avoiding unexpected behavior and writing more predictable code.

Advanced JavaScript Interview Questions for Experienced

28. What are the advantages of using JavaScript frameworks/libraries like React or Angular?

Frameworks provide reusable code and components, save development time, and make it easier to build large, complex apps. They handle tasks like DOM manipulation which is difficult in plain JS. Frameworks enforce style guides and best practices. Apps built with frameworks are more organized, scalable, and maintainable compared to vanilla JS code. Frameworks also provide built-in functionality like routing and state management.

29. What is the difference between local storage and session storage in JavaScript?

Local storage stores data with no expiration date, and remains even after browser tab closes. Session storage stores data for one session only, data is lost when tab closes. Local storage does not have a size limit but session storage is limited to 5MB. Local storage is better for permanent storage and session storage for temporary data that needs to be deleted.

30. Explain the concept of the DOM (Document Object Model) in JavaScript.

The DOM represents an HTML document as nodes and objects. It provides programming interfaces to dynamically access and update the content, structure, and style of a document. With DOM, JavaScript can connect to HTML and manipulate how it displays. This allows dynamic changes to webpages without reloading. The DOM tree has the HTML document as the root and all other elements, texts, and images as nodes

31. What is the purpose of the 'bind' method in JavaScript?

The ‘bind’ method creates a new function that, when called, has its ‘this’ keyword set to a specific value, enabling explicit control over the context in which a function is executed.

32. What is the difference between 'null' and 'undefined' in JavaScript?

Null is an assignment value that means empty or no value assigned. Undefined means a variable has been declared but no value assigned to it yet. While null is an object, undefined is a primitive value. A variable with a null value is by design, undefined means a missing reference usually due to programming errors or lack of initialization.

33. What are the differences between arrow functions and regular functions in JavaScript?

Arrow functions in JavaScript are like quick ways to write functions with =>, and are good for short tasks, while regular functions use functions and suit bigger jobs.

For example:

Arrow function:

const arrowFunction = (a, b) = > a + b;

Regular function:

function regularFunction(a, b) {
return a + b;
}

34. Explain the concept of the spread operator in JavaScript.

The spread operator (…) allows iterables like arrays and objects to be expanded in places where zero or more arguments are expected. It copies the elements of an array or properties of an object and places them in a new array/object. This is useful for concatenating arrays, merging objects, function calls and more

35. Describe the concept of the event loop in JavaScript and how it enables asynchronous behavior.

The event loop is a crucial mechanism in JavaScript that enables asynchronous behavior, ensuring that programs can handle time-consuming operations efficiently without becoming unresponsive. JavaScript is single-threaded, which means it executes one task at a time. Asynchronous tasks, such as fetching data from servers or reading files, may take some time to complete, potentially blocking the program’s execution if it were to wait for them. However, the event loop solves this problem by constantly checking a message queue for pending tasks or events. When the call stack is empty, the event loop picks the next task from the queue and executes it, allowing the program to continue processing other tasks while awaiting the completion of asynchronous operations. This way, the application remains responsive and avoids being blocked, providing a smooth user experience.

36. What are Promises in JavaScript, and how do they handle asynchronous operations?

Promises in JavaScript are a feature introduced in ES6 to handle asynchronous operations in a more structured and elegant way than using traditional callbacks. A Promise represents a future value that may or may not be available at a particular moment. It can be in one of three states: pending (initial state), fulfilled (resolved with a value), or rejected (failed with an error). Promises provide two main methods: then() to handle successful outcomes and catch() to handle errors. When an asynchronous operation is complete, the Promise resolves or rejects with the corresponding result or error, respectively. Promises help avoid callback hell and provide a more readable and maintainable way to deal with asynchronous flows.

37. Explain the role of bind, call, and apply methods in managing the context of this.

In JavaScript, we have three methods – bind(), call(), and apply() – that allow us to manage the ‘this’ context when calling a function. These methods are helpful when we want to ensure that a function is executed with a specific object as its ‘this’ value.

1. bind() Method: The bind() method creates a new function that is a copy of the original function, but with a fixed value. This means that whenever the new function is called, the ‘this’ keyword inside it will always refer to the object passed as an argument to bind(). The original function is not affected by bind(); instead, it remains unchanged.
2. call() Method: The call() method allows us to call a function immediately, specifying the object that should be used as the ‘this’ value inside the function. It also allows us to pass individual arguments to the function, which will be received as separate parameters inside the function.
3. apply() Method: Similar to call(), the apply() method also calls a function immediately and sets the ‘this’ value to the specified object. However, it accepts an array of arguments as the second argument, making it useful when the number of arguments is not known in advance.

These methods come in handy when we want to control the context in which a function is executed. For example, when using object methods as event handlers, we can use bind() to ensure that this inside the method refers to the object itself, regardless of how the method is called. Similarly, call() and apply() are useful when we want to borrow methods from one object to use with another object and set the correct ‘this’ context for the borrowed method.

Overall, these methods provide flexibility and control over the ‘this’ context in JavaScript functions, allowing developers to write more organized and maintainable code when working with objects and methods.

38. Discuss the differences between ES5 and ES6 (ECMAScript 2015) in JavaScript.

ES5 and ES6 (also known as ECMAScript 2015) are two different versions of the ECMAScript standard, which is the specification that defines the JavaScript language. ES6 introduced several new features and improvements over ES5, aiming to make JavaScript more powerful, expressive, and developer-friendly. Here are the key differences between ES5 and ES6:

1. Let and const Declarations:
   • ES6 introduced the let and const keywords for declaring variables with block scope, whereas in ES5, variables are declared using var, which has function scope.
   • let allows reassignment of values, similar to var, but it restricts the scope to the block where it’s defined.
   • const is used for defining constants that cannot be reassigned once they are initialized.
2. Arrow Functions:
   • ES6 introduced arrow functions, which provide a concise syntax for writing anonymous functions.
   • Arrow functions have a shorter syntax and automatically capture the surrounding values, making it easier to handle context-related issues.
3. Template Literals:
   • ES6 introduced template literals using backticks ( ), allowing multi-line strings and easy string interpolation.
   • Template literals provide a more readable and expressive way to create strings with variables embedded directly inside them.
4. Class Syntax:
   • ES6 introduced a class syntax for defining classes and constructor functions, making it easier to implement object-oriented programming in JavaScript.
   • Prior to ES6, class-like structures were implemented using constructor functions and prototypes.
5. Destructuring Assignment:
   • ES6 added destructuring assignment, enabling you to extract values from arrays and objects and assign them to variables in a concise and readable way.
6. Default Parameters:
   • ES6 allows you to define default parameter values in function declarations, making it simpler to handle optional arguments in function calls.
7. Spread and Rest Operators
   • ES6 introduced the spread (…) and rest (…) operators for working with arrays and function arguments.
   • The spread operator can expand an array or object into individual elements, while the rest operator collects multiple function arguments into a single array.
8. Modules:
   • ES6 introduced native support for modules, allowing you to export and import functionality between different JavaScript files.
   • Modules provide a cleaner and more organized way to structure and manage code in large applications.
9. Promises:
   • Although promises were initially introduced as a separate library, ES6 standardized them, providing native support for handling asynchronous operations and avoiding callback hell.
10. Other Enhancements:
    • ES6 introduced many other smaller enhancements, such as for…of loops for iterating over iterable objects, Symbols for creating unique identifiers, and Map and Set data structures for managing collections of data.

Overall, ES6 brought significant improvements to JavaScript, making it a more modern and expressive language, and it has become the foundation for subsequent ECMAScript versions. However, it’s essential to be mindful of browser support when using ES6 features, as older browsers may not fully support all ES6 features without transpilation or polyfills.

39. What are Generators in JavaScript, and how can they be useful in certain scenarios?

Generators in JavaScript are a powerful feature  introduced in ES6. They are functions that can be paused and resumed during their execution, allowing for more complex control flows and lazy evaluation of data. Generators are denoted by an asterisk (*) after the function keyword. When a generator function is called, it returns an iterator object that can be used to control the function’s execution. Generators are useful for scenarios where you need to handle sequences of data or control flow in a more sophisticated manner.

40. Describe the concept of Memoization and its application to optimizing function performance.

Memoization in JavaScript  is a technique used to optimize function performance by caching the results of expensive function calls and returning the cached result when the same inputs occur again. It helps reduce redundant computations and improve the efficiency of functions that are frequently called with the same arguments. Memoization is particularly useful for recursive or computationally intensive functions, as it avoids repetitive calculations and significantly speeds up the overall execution time.

41. How do you implement a debounce function in JavaScript, and when is it beneficial?

To create a debounce function in JavaScript, you can design a higher-order function that receives another function as input and produces a new function as output. This new function is responsible for postponing the execution of the original function until a specific time interval has elapsed since its last invocation. If the function is called again during this waiting period, the timer restarts, and the waiting time is reset accordingly. This debounce technique effectively limits the frequency of function calls and helps optimize performance in scenarios where frequent and rapid invocations are expected, such as handling user input events. By using debounce, you can enhance the efficiency of your JavaScript code and prevent unnecessary and excessive function calls, leading to a more responsive and smoother user experience.

A simple debounce function can be created like this:

function debounce(func, delay) {
  let timer;
  return function (…args) {
    clearTimeout(timer);
    timer = setTimeout(() => func.apply(this, args), delay);
  };
}

The debounce function is beneficial in scenarios where you want to optimize performance by reducing the number of function invocations. It is commonly used in scenarios like auto-complete search boxes, resizing events, or handling user input where you want to delay the execution of the function until the user has stopped typing or interacting with the UI. By using debounce, you can prevent excessive and unnecessary function calls, leading to a more responsive and efficient user experience.

42. Explain the concept of Web Workers in JavaScript and how they improve multi-threading in the browser.

Web Workers in JavaScript are a feature that enables multi-threading in the browser. They allow developers to run scripts in the background without affecting the main UI thread. By doing so, Web Workers improve the responsiveness of web applications, as time-consuming tasks can be offloaded to separate threads, preventing the main thread from being blocked. This separation of tasks into different threads enhances the overall performance and user experience, making web applications more efficient and capable of handling computationally intensive operations without causing freezing or delays in the user interface.

43. Discuss the advantages and disadvantages of using arrow functions in JavaScript.

Advantages:

1. Shorter Syntax: Arrow functions provide a concise syntax, making code more readable and reducing boilerplate code, especially for simple one-liner functions.
2. No this Binding: Arrow functions inherit the ‘this’ value from the surrounding lexical context, eliminating the need to use bind() or store the reference to this in a separate variable.
3. Implicit Return: Arrow functions with a single expression automatically return the result of that expression, saving developers from writing explicit return statements.
4. No Binding of arguments: Arrow functions do not bind their own arguments object, which can be beneficial as it avoids potential confusion with other variables named arguments.

Disadvantages:

1. No arguments Object: As mentioned earlier, arrow functions do not have their own arguments object, which means you cannot access the function arguments using arguments within an arrow function.
2. No ‘this’ Manipulation: The automatic lexical ‘this’ binding in arrow functions can be disadvantageous in some cases where dynamic ‘this’ manipulation is required.
3. Not Suitable for Object Methods: Arrow functions are not ideal for defining object methods because they lack their own ‘this’ binding, and using this in such cases would refer to the surrounding context, not the object itself.
4. Limited Use in Constructors: Arrow functions cannot be used as constructors, which means they cannot be instantiated using the new keyword. This can be a limitation in certain scenarios that require constructor functions.

44. What is the role of the Map and Set data structures in modern JavaScript, and how are they different from regular objects and arrays?

In modern JavaScript, the Map and Set data structures serve specific purposes and offer advantages over regular objects and arrays in certain scenarios:

Map: 

1. Key-Value Pairs: Maps are primarily used to store key-value pairs, where both keys and values can be of any data type. This makes them more versatile compared to regular objects, which only allow strings or symbols as keys.
2. Order Preservation: Unlike regular objects, which do not guarantee the order of properties, Map maintains the insertion order of elements, making it suitable for use cases where the order of insertion matters.
3. Key Existence Check: Maps offer a convenient way to check if a specific key exists using the has method, without the need for additional checks or iterating over the object.
4. Iterating and Size Retrieval: Maps have built-in methods like forEach for easy iteration over the key-value pairs, and the size property to quickly retrieve the number of entries.

Set:

1. Unique Values: Sets are designed to store unique values of any data type. If you add a value that already exists, it won’t be duplicated in the Set.
2. Membership Check: Sets provide an efficient way to check whether a particular value is present in the Set using the has method, without the need for custom checks or iterations.
3. Iterating and Size Retrieval: Sets have built-in methods like forEach for easy iteration over the unique values, and the size property to quickly retrieve the number of elements.

Differences from Regular Objects and Arrays:

1. Keys and Indexing:
   • Regular objects use strings or symbols as keys, and values can be accessed using dot notation or bracket notation (e.g., obj.key or obj[‘key’]).
   • Maps allow any value as a key, and values can be accessed using the get method (e.g., map.get(key)).
   • Arrays use numeric indices and allow elements to be accessed using index notation (e.g., arr[0]).
2. Order Preservation:
   • Regular objects do not guarantee the order of properties, and the order of insertion is not preserved.
   • Maps maintain the order of elements based on their insertion order.
   • Arrays preserve the order of elements based on their index.
3. Key Existence Check:
   • For regular objects, you need to manually check for their existence using methods like hasOwnProperty or in.
   • Maps provide a built-in method to check for key existence.
   • Arrays do not have built-in methods for key existence checks, as they are primarily indexed by numeric values.
4. Values and Duplicates:
   • Regular objects and Maps can store any data type as values.
   • Sets only store unique values; duplicates are automatically removed.

In summary, Maps and Sets in modern JavaScript provide specialized data structures for specific use cases, offering advantages like key-value pair storage, order preservation, efficient membership checks, and handling unique values. Regular objects and arrays are more general-purpose data structures and do not offer the same specialized features as Maps and Sets. Choosing the right data structure for computer programming depends on the specific requirements of your problem statement or application.

These top Javascript interview questions and answers cover a wide range of topics and levels varied in difficulty to assess a fresher’s understanding of JavaScript. Preparing for these questions will provide freshers with a solid foundation for JavaScript interviews questions and boost their confidence in handling various JavaScript-related concepts and challenges.

45. Explain WeakMap in javascript.

WeakMap is a collection of key-value pairs where the keys are weakly referenced. This allows key objects to be garbage collected if there are no other references to them. Since keys are weakly held, WeakMaps don’t interfere with the garbage collection of objects. This makes WeakMaps useful for caching and temporary data that don’t need to be strongly referenced.

46. What is a Temporal Dead Zone?

The Temporal Dead Zone refers to the period between entering a block and declaring a variable with let or const where a ReferenceError will be thrown if trying to access the variable. This is because let and const variables are not hoisted like var, and exist only after their declaration in block scope, not for the whole block.

47. Explain the difference between 'map()' and 'filter()' methods in JavaScript.

Map() returns a new array by performing a function on each array element. Filter() returns a new array by keeping elements that pass a test. Map() always returns an array of the same length, while filter() returns an array with a length potentially less than or equal to the original. Map() transforms elements, filter() extracts matching elements based on condition.

48. What is the difference between 'slice()' and 'splice()' methods in JavaScript?

• slice() extracts a portion of an array and returns a new array with the extracted elements without modifying the original array.
• splice() removes elements from an original array and can insert new elements in their place, modifying the original array.

49. Explain the concept of debouncing and throttling in JavaScript.

• Debouncing in JavaScript: Delays the execution of a function until a certain amount of time has elapsed since the last call. Useful for performance to avoid unnecessary executions.
• Throttling in JavaScript: Limits the rate at which a function can be called over time. Executes at most once per period. Useful for limiting frequent calls to improve performance.

50. Explain the difference between 'async' and 'defer' attributes in script tags.

• async:- The async attribute downloads the external script file asynchronously in the background without blocking page rendering. As soon as the script is downloaded, it is executed immediately, even if the HTML document is still parsing. Execution order is not guaranteed with async scripts.
• defer:- The defer attribute also downloads the external script file asynchronously in the background like async. However, it delays the execution of the script until the HTML document has finished parsing. This ensures proper execution order and allows multiple deferred scripts to execute in the order they appear. defer does not block page rendering either.

51. How would you dynamically create a dropdown/select menu in HTML using JavaScript?

function createDropdown() {

var select = document.createElement(‘select’);

var options = [‘Option 1’, ‘Option 2’, ‘Option 3’];

options.forEach(optionText => {

var option = document.createElement(‘option’);

option.value = optionText.toLowerCase();

option.textContent = optionText;

select.appendChild(option);

});

document.body.appendChild(select);
}