Top 75 JavaScript Interview Questions and Answers

JavaScript Interview Questions: Programming is one of the booming industries of technology for the year 2024. Among many programming languages, JavaScript is the most widely used in the web development sector. Brendan Eich, in 1995, created a programming language primarily for creating interactive and dynamic functionality on websites. It included features like form validation, animations, and user interface enhancements. 

According to a survey conducted by Stack Overflow in 2022, JavaScript remains the most popular programming language among programmers. If you are interested in beginning your career in web development or looking to switch to a new career job, here are the Top 75 JavaScript Interview Questions and Answers that are important for you. 

25 Java Script Interview Questions For Freshers

Find the answers for JavaScript Questions and Answers for freshers from below and land on the job of your choice:

Question 1: What is JavaScript? Answer: JavaScript is a client-side scripting language used to add interactivity and dynamic behaviour to web pages.

Question 2: Differentiate between JavaScript and Java. Answer: JavaScript and Java are two different programming languages. Java is an object-oriented programming language used for building applications, while JavaScript is a scripting language primarily used for web development.

Question 3: What is the use of the ‘let’ keyword in JavaScript? Answer: The ‘let’ keyword declares a variable with block-level scope, meaning the variable is only accessible within the block in which it is defined.

Question 4: What is the difference between ‘==’ and ‘===’ operators in JavaScript? Answer: The ‘==’ operator checks for value equality, while ‘===’ checks for both value and type equality.

Question 5: How do you declare a function in JavaScript? Answer: You can declare a function using the ‘function’ keyword, followed by the function name, parameters (if any), and the function body.

Question 5: What is an arrow function in JavaScript? Answer: An arrow function is a shorter syntax for writing function expressions, introduced in ES6. It uses the ‘=>’ arrow syntax.

Question 6: What is the purpose of the ‘this’ keyword in JavaScript? Answer: The ‘this’ keyword refers to the object that the current function is a property of. Its value depends on how the function is called.

Question 7: What is the difference between ‘null’ and ‘undefined’ in JavaScript? Answer. ‘null’ is an assigned value that means ‘no value’, while ‘undefined’ means a variable has been declared but not assigned a value.

Question 8: What is the use of the ‘map’ method in JavaScript? Answer: The ‘map’ method creates a new array by applying a provided function to every element of the original array.

Question 9: How do you add an event listener to an element in JavaScript? Answer: You can add an event listener using the ‘addEventListener’ method on the element, specifying the event and a callback function.

Question 10: What is the purpose of the ‘spread’ operator in JavaScript? Answer: The spread operator (…) allows you to spread the elements of an iterable (like an array or string) into individual elements.

Question 11: What is the purpose of the ‘default’ keyword in a switch statement? Answer: The ‘default’ keyword in a switch statement specifies a code block to be executed if none of the cases matches.

Question 12: What is the difference between ‘let’ and ‘const’ in JavaScript? Answer: ‘let’ declares a variable with block-level scope, while ‘const’ declares a constant variable that cannot be reassigned.

Question 13: How do you create an object in JavaScript? Answer: You can create an object using object literal notation ({}) or by using the ‘new Object()’ constructor.

Question 14: What is the purpose of the ‘apply’ method in JavaScript? Answer: The ‘apply’ method calls a function with a given ‘this’ value and an array-like object as arguments.

Question 15: What is the purpose of the ‘filter’ method in JavaScript? Answer: The ‘filter’ method creates a new array containing elements that pass a specified test condition provided by a callback function.

Question 16: What is the purpose of the ‘reduce’ method in JavaScript? Answer: The ‘reduce’ method applies a function against an accumulator and each element of the array to reduce it to a single value.

Question 17: What is the purpose of the ‘try…catch’ statement in JavaScript? Answer: The ‘try…catch’ statement is used for error handling in JavaScript. It allows you to catch and handle exceptions thrown in the ‘try’ block.

Question 18: What is the difference between ‘forEach’ and ‘map’ methods in JavaScript?Answer: The ‘forEach’ method executes a provided function once for each array element, while ‘map’ creates a new array with the results of calling a provided function on every element.

Question 19: What is the purpose of the ‘slice’ method in JavaScript? Answer: The ‘slice’ method returns a shallow copy of a portion of an array into a new array object.

Question 20: How do you check if a variable is an array in JavaScript? Answer: You can use the ‘Array.isArray()’ method or the ‘instanceof’ operator to check if a variable is an array.

Question 21: What is the purpose of the ‘Promise’ object in JavaScript? Answer: A ‘Promise’ is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value.

Question 22. What is the difference between ‘var’ and ‘let’ in JavaScript? Answer. ‘var’ declares a variable with function or global scope, while ‘let’ declares a variable with block-level scope.

Question 23. How do you create an array in JavaScript? Answer. You can create an array using an array literal notation ([]) or by using the ‘new Array()’ constructor.

Question 24. What is the purpose of the ‘includes’ method in JavaScript? Answer. The ´includes´ method in JavaScript is used to check if an array or a string contains a specified value. It returns ´true if the value is found, and false otherwise. This method is case-sensitive when used with strings. Here is an example with an array: const fruits = [‘apple’, ‘banana’, ‘mango’]; console.log(fruits.includes(‘banana’)); // trueconsole.log(fruits.includes(‘grape’));  // false An example with a string: const sentence = ‘The quick brown fox jumps over the lazy dog’; console.log(sentence.includes(‘fox’)); // trueconsole.log(sentence.includes(‘cat’)); // false

Question 25: How do you use ´forEach´ to loop through an array in JavaScript? Answer: You can use ´forEach´ to loop through an array by calling it on the array and passing a callback function that gets executed for each element. The callback function can take three arguments: the current element, the index of the current element, and the array itself.  For Example: const numbers = [1, 2, 3, 4, 5]; numbers.forEach(function(element, index, array) {  console. log(‘Element:’, element, ‘Index:’, index);}); This will log each element and its index to the console: Element: 1 Index: 0Element: 2 Index: 1Element: 3 Index: 2Element: 4 Index: 3Element: 5 Index: 4 You can also use an arrow function for the callback: numbers.forEach((element, index) => {  console.log(`Element: ${element}, Index: ${index}`);});

Also Read: Self-Introduction for Teacher Interview

25 JavaScript Interview Questions For Experienced Professionals

Find the answers for JavaScript Questions and Answers for Experienced Professionals from below and land on the job of your choice:

Question 1. What is closure in JavaScript? Answer. Closure is a feature in JavaScript where a function has access to its outer function’s variables, even after the outer function has finished executing. It allows functions to maintain references to variables in their parent scopes.

Question 2. Explain the concept of prototypal inheritance in JavaScript. Answer. Prototypal inheritance in JavaScript means that objects can inherit properties and methods from other objects. Each object has a prototype object, and if a property or method is not found on the object itself, JavaScript looks for it in the prototype chain.

Question 3. How does asynchronous programming work in JavaScript? Answer. Asynchronous programming in JavaScript allows tasks to be executed without blocking the main thread. It uses callbacks, promises, or async/await to handle asynchronous operations, such as fetching data from a server or waiting for user input.

Question 4. What are the differences between let, var, and const in JavaScript? Answer. `let` and `const` are block-scoped, meaning they are limited to the block (enclosed within curly braces) in which they are defined. `var` is function-scoped, meaning it is limited to the function in which it is defined. Additionally, `const` cannot be reassigned once declared, whereas `let` and `var` can.

Question 5. How do you handle errors in JavaScript? Answer. Errors in JavaScript can be handled using catch blocks, where code that may throw an error is enclosed in a try block, and any errors are caught and handled in the catch block. Additionally, errors can be logged or displayed to the user for debugging purposes.

Question 6. What is event delegation in JavaScript? Answer: Event delegation is a technique in JavaScript where a single event listener is attached to a parent element, instead of attaching multiple event listeners to individual child elements. This allows dynamically added elements to also trigger the same event handler.

Question 7: Explain the difference between == and === operators in JavaScript. Answer: The `==` operator checks for equality after performing type coercion, meaning it converts the operands to the same type before comparison. The `===` operator checks for strict equality without type coercion, meaning both the value and the type of the operands must match for it to return true.

Question 8: What are the different ways to define functions in JavaScript? Answer: Functions in JavaScript can be defined using function declarations, function expressions, arrow functions, and methods within objects.

Question 9: How do you work with promises in JavaScript? Answer: Promises in JavaScript represent asynchronous operations and allow chaining multiple asynchronous operations together. They can be created using the `Promise` constructor and then handled using the `.then()` and `.catch()` methods to handle success and failure respectively.

Question 10: What is the purpose of the ‘use strict’ directive in JavaScript? Answer: The ‘use strict’ directive enables strict mode in JavaScript, which enforces stricter rules and helps catch common coding errors. It prevents the use of undeclared variables, disables certain potentially problematic features, and generally promotes safer and more efficient code.

Question 11: Explain the concept of hoisting in JavaScript. Answer: Hoisting in JavaScript means that variable declarations and function declarations are moved to the top of their containing scope during the compilation phase. This allows variables and functions to be used before they are declared, although it’s recommended to declare them at the top of the scope for clarity.

Question 12: What are arrow functions and when would you use them? Answer: Arrow functions are a concise way to write function expressions in JavaScript. They have a shorter syntax compared to traditional function expressions and automatically bind `this` to the surrounding context. They are commonly used for callbacks, event handlers, and in situations where a concise syntax is preferred.

Question 13: How do you handle CORS (Cross-Origin Resource Sharing) in JavaScript?Answer. CORS in JavaScript can be handled by configuring the server to include appropriate CORS headers, such as `Access-Control-Allow-Origin`, to allow cross-origin requests from specific domains. Alternatively, JSONP (JSON with Padding) or proxy servers can be used to bypass CORS restrictions.

Question 14. What is the role of the ‘this’ keyword in JavaScript? Answer. The ‘this’ keyword in JavaScript refers to the context in which a function is executed. Its value is determined by how a function is called, and it can refer to different objects depending on the invocation context, such as the object owning the method or the global object.

Question 15: How do you convert a string to a number in JavaScript? Answer: Strings can be converted to numbers in JavaScript using the `parseInt()` or `parseFloat()` functions for converting to integers or floating-point numbers respectively. Additionally, the unary plus operator (`+`) can be used to convert a string to a number.

Question 16. Explain the concept of event bubbling and event capturing in JavaScript. Answer. Event bubbling and event capturing are two phases of event propagation in JavaScript. During event bubbling, events are first captured at the target element and then propagate up the DOM tree to the root element. During event capturing, events propagate from the root element down to the target element.

Question 17: What are some common methods for iterating over arrays in JavaScript?Answer. Some common methods for iterating over arrays in JavaScript include `forEach()`, `map()`, `filter()`, `reduce()`, and `for…of` loops. Each method provides a different way to iterate over the elements of an array and perform operations on them.

Question 18. Explain the difference between synchronous and asynchronous code execution in JavaScript. Answer. Synchronous code execution in JavaScript means that statements are successively executed one after the other, blocking the main thread until each statement completes. Asynchronous code execution, on the other hand, allows statements to be executed concurrently without blocking the main thread, typically using callbacks, promises, or async/await.

Question 19. What are higher-order functions in JavaScript? Answer. Higher-order functions in JavaScript are functions that operate on other functions by taking them as arguments or returning them as results. They enable functional programming paradigms such as function composition, currying, and partial application.

Question 20: How do you clone an object in JavaScript? Answer. Objects can be cloned in JavaScript using various methods, such as the `Object.assign()` method, the spread operator (`…`), or by using libraries like lodash. These methods create a shallow copy of the object, meaning nested objects are not cloned recursively.

Question 21: What is the Event Loop in JavaScript? Answer. The Event Loop is a mechanism in JavaScript that allows asynchronous code to be executed in a non-blocking manner. It continuously checks the call stack for functions to execute, and when the stack is empty, it processes pending asynchronous tasks from the task queue, ensuring that the application remains responsive.

Question 22: Explain the difference between null and undefined in JavaScript. Answer. In JavaScript, `null` represents the intentional absence of any value and is typically assigned to variables as a placeholder. `undefined`, on the other hand, indicates that a variable has been declared but has not been assigned a value. It is the default value for uninitialized variables.

Question 23. What is the difference between mutable and immutable data types in JavaScript? Answer. Mutable data types in JavaScript are those that can be modified after creation, such as arrays and objects.  Changes made to mutable data types affect the original data. Immutable data types, on the other hand, cannot be changed after creation. Examples of immutable data types in JavaScript include strings and numbers. Once created, their values cannot be altered.  This property ensures data integrity and simplifies concurrency control in multi-threaded environments.

Question 24. How do you handle memory leaks in JavaScript? Answer. Memory leaks in JavaScript can be handled by identifying and fixing the root cause of the leak. Common causes include circular references, event listeners that are not properly removed, and retaining unnecessary references to objects. Using browser developer tools, such as Chrome DevTools, to analyze memory usage and profile memory can help identify memory leaks.

Question 25: Explain the concept of event-driven programming in JavaScript. Answer: Event-driven programming in JavaScript involves designing applications that respond to events triggered by user interactions or system events. Instead of executing code sequentially, event-driven programs rely on event listeners and callbacks to handle asynchronous events.

Also Read: Introduce Yourself in English for University Interview

25 Advanced Java Script Coding Interview Questions

Find the answers to Advanced Java Script Coding Interview Questions below and land on the job of your choice:

Question 1: Explain var, let, and const in JavaScript. How do their scopes differ? Answer. ´var´ is function-scoped and can be redeclared and updated. ´let´  and ´const´  are block-scoped. ´let´ can be updated but not redeclared. ´const´ cannot be updated or redeclared.

Question 2: Describe prototypal inheritance in JavaScript. Answer: Prototypal inheritance allows objects to inherit properties and methods from another object. This is done using prototypes. Every object has a prototype, which is also an object.

Question 3: What is the difference between ´==´ and ´===´? Answer: ´==´ compares values with type coercion, meaning different types can be considered equal if they represent the same value. ´===´ compares both value and type, without coercion.

Question 4: How does the event loop work in JavaScript? Answer: The event loop is responsible for executing code, collecting and processing events, and executing queued sub-tasks. It allows JavaScript to perform non-blocking operations by offloading operations to the system kernel whenever possible.

Question 5: What are arrow functions? How do they differ from regular functions? Answer: Arrow functions are a concise way to write functions. They do not have their own ´this´, ´arguments´, ´super´, or ´new.target´.

Question 6: What is this keyword? How is it determined? Answer: ´this´ refers to the object it belongs to. Its value is determined by how a function is called in a method, it refers to the owner object; alone, it refers to the global object (in non-strict mode) or undefined (in strict mode).

Question 7: What are the differences between null and undefined? Answer: ´null´ is an assigned value that represents no value. ´undefined´ means a variable has been declared but has not yet been assigned a value.

Question 8: What is the difference between for…in and for…of loops in JavaScript? Answer: ´for…in´ iterates over enumerable property names of an object. ´for…of´ iterates over iterable objects like arrays, strings, etc.

Question 9: What is the difference between ´localStorage´ and ´sessionStorage´? Answer: Both ´localStorage´ and ´sessionStorage´ are used for storing data in the browser. ´localStorage´ has no expiration time, while ´sessionStorage´ is cleared when the page session ends.

Question 10. How do you explain the difference between ´var´, ´let´, and ´const´ in JavaScript? Answer: In JavaScript, ´var´ is function-scoped and can be redeclared and updated, while ´let´ and ´const´ are block-scoped. ´let´ can be updated but not redeclared, while const cannot be updated or redeclared.

Question 11: What are closures in JavaScript? Provide an example. Answer: A closure is a function that retains access to its lexical scope even when the function is executed outside that scope.

Question 12. What is a Promise? How do you use it? Answer: A Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

Question 13. Explain ´async´ and ´await´ in JavaScript. Provide an example. Answer: ´async´ functions enable the use of ´await´, which pauses the execution of the function until the promise resolves.

Question 14: Explain the concept of hoisting in JavaScript. Answer: Hosting is JavaScript is the default behaviour of moving declarations to the top of current scope. Only the declarations are hoisted, not the initializations.

Question 15. What is the purpose of the ‘Array.prototype.map´ method? Answer: ´map´ creates a new array populated with the results of calling a provided function on every element in the calling array.

Question 16. Explain the concept of ´event delegation´ in JavaScript. Answer: Event delegation involves leveraging event bubbling to handle events at a higher level in the DOM rather than adding event listeners to individual elements.

Question 17. What are template literals in JavaScript? Answer: Template literals allow embedded expressions and multi-line strings using backticks (“).

Question 18. Explain call, apply, and bind methods in JavaScript. Answer: ´call´ and ´apply´ methods invoke a function with a specified ´this´ context. ´call´ takes arguments separately while ´apply´ takes arguments as an array. ´bind´ returns a new function, permanently bound to the provided ´this´ context.

Question 19. What is the use of ´fetch´ API in JavaScript? Answer: The ´fetch´ API provides a modern, promise-based way to make asynchronous HTTP requests.

Question 20. Explain the concept of ´debouncing´ in JavaScript. Answer: Debouncing ensures a function is only called once after a certain period, no matter how many times an event is triggered.

Question 21. How can you create a deep copy of an object in JavaScript? Answer: A deep copy can be created using ´JSON.stringify´ and JSON.parse for simple objects or using libraries like Lodash for more complex structures.

Question 22. What are JavaScript modules? How do you export and import them? Answer: JavaScript modules allow code to be split into reusable pieces. They use ´export´ to make parts of the code available to other modules and ´ímports´ to bring those parts in.

Question 23: What is the purpose of ´Symbol´ in JavaScript? Answer: ´Symbol´ is a primitive data type used to create unique identifiers. It helps prevent naming conflicts in objects.

Question 24: What are generators in JavaScript? Answer: Generators are functions that can be paused and resumed, allowing them to produce a sequence of values.

Question 25: Can you describe how closures work in JavaScript with a simple example? Answer: Closures in JavaScript allow inner functions to access variables from their outer scope even after the outer function has finished executing. For example:

Conclusion

In conclusion, landing a job as a web developer through JavaScript interview questions and answers is an exciting opportunity for young professionals. Demonstrating your programming skills can lead to a good salary and a bright future. 

Our questions and answers for freshers, professionals, and advanced candidates will help you showcase your abilities and make a strong impression on the interview panel. 

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