JavaScript | Interview Questions (2023)

JavaScript interview questions and answers for provides a list of top interview questions. The frequently asked JavaScript interview questions with answers for beginners and professionals are given below.

Javascript Concepts :

Javascript Questions:

Basic
What are JavaScript functions and how do you declare them?
What are JavaScript objects and how do you create them?
What are JavaScript arrays and how do you create them?
What are JavaScript loops and how do you use them?
What is JavaScript scope and how does it work?
What is the difference between let and var in JavaScript?
What are JavaScript callbacks and how do you use them?
What are JavaScript closures and how do they work?
What is the difference between null and undefined in JavaScript
What is the difference between synchronous and asynchronous code in JavaScript?
What are the advantages of using JavaScript frameworks and libraries?
What is JSON and how do you use it in JavaScript?
What is AJAX in JavaScript and how do you use it?
What is the difference between client-side JavaScript and server-side JavaScript?

Variables
What is the difference between var, let, and const in JavaScript?
How to declare a global variable in JavaScript?
How to declare a constant variable in JavaScript?
What is hoisting in JavaScript variables?
How to check if a variable is undefined in JavaScript?
How to check the data type of a variable in JavaScript?
How to convert a string to a number in JavaScript?
How to convert a number to a string in JavaScript?
How to concatenate variables in JavaScript?
What is variable scope in JavaScript?
What is the difference between local and global variables in JavaScript?
How to create a variable using a string value in JavaScript?
What is the difference between null and undefined in JavaScript variables?
How to declare a variable in JavaScript using ES6 syntax?
How to destructure an object and assign its values to variables in JavaScript?
How to destructure an array and assign its values to variables in JavaScript?

Data Types
What is the difference between null and undefined in JavaScript?
What is NaN in JavaScript?
What is the typeof operator in JavaScript?
What is the difference between == and === in JavaScript?
What is the difference between null and undefined in JavaScript?
What is the difference between primitive and non-primitive data types in JavaScript?
What is the difference between let, var, and const in JavaScript?
What are the different types of operators in JavaScript?
How do you check if a variable is an array in JavaScript?
What is the difference between a string and a number data type in JavaScript?
What is the difference between a null and an empty string in JavaScript?
What is the difference between a boolean and a number data type in JavaScript?
How do you convert a string to a number in JavaScript?
What is a NaN value in JavaScript, and how do you test for it?
What are truthy and falsy values in JavaScript?
What is the difference between null, undefined, and NaN in JavaScript?
What is the difference between a symbol and a string in JavaScript?
What is the difference between a boolean and a string in JavaScript?
What is the difference between a number and a string in JavaScript?
How do you check if a variable is a string in JavaScript?
How do you check if a variable is a number in JavaScript?
What is the difference between a string and a boolean data type in JavaScript?
What is the difference between a number and a boolean data type in JavaScript?
How do you check if a variable is null or undefined in JavaScript?

Operators

What is the use of the ‘===’ operator in JavaScript?
What is the difference between ‘==’ and ‘===’ operators in JavaScript?
What is the use of the ‘!=’ operator in JavaScript?
What is the use of the ‘&&’ operator in JavaScript?
What is the use of the ‘||’ operator in JavaScript?
What is the use of the ‘?’ operator in JavaScript?
What is the use of the ‘typeof’ operator in JavaScript?
What is the use of the ‘instanceof’ operator in JavaScript?
What is the use of the ‘delete’ operator in JavaScript?
What is the use of the ‘new’ operator in JavaScript?
What is the use of the ‘typeof’ operator in JavaScript?
What is the use of the ‘++’ operator in JavaScript?
What is the use of the ‘–‘ operator in JavaScript?
What is the use of the ‘!’ operator in JavaScript?
What is the use of the ‘in’ operator in JavaScript?
What is the use of the ‘typeof’ operator in JavaScript?
What is the use of the ‘void’ operator in JavaScript?
What is the use of the ‘yield’ operator in JavaScript?
What is the use of the ‘spread’ operator in JavaScript?
What is the use of the ‘rest’ operator in JavaScript?

Control Structures

What is a conditional statement in JavaScript?
What is the difference between the “==” operator and the “===” operator in JavaScript?
What is the purpose of a loop in JavaScript?
How does the “for” loop work in JavaScript?
How does the “while” loop work in JavaScript?
What is a “do-while” loop in JavaScript?
What is the “switch” statement in JavaScript?
How does the “break” statement work in JavaScript?
How does the “continue” statement work in JavaScript?
What is the difference between the “for” and “forEach” loops in JavaScript?
What is a ternary operator in JavaScript?
How can you loop through an object in JavaScript?
What is a labeled statement in JavaScript?
What is a block statement in JavaScript?
How does the “if…else” statement work in JavaScript?
How does the “try…catch” statement work in JavaScript?
How does the “throw” statement work in JavaScript?
How does the “finally” statement work in JavaScript?
What is the purpose of the “return” statement in JavaScript?
How do you use the “debugger” statement in JavaScript?

Functions

What is a function in JavaScript?
How to define a function in JavaScript?
What is a parameter in a function in JavaScript?
What is an argument in a function in JavaScript?
How to call a function in JavaScript?
What is the return statement in a function in JavaScript?
What is a callback function in JavaScript?
What is a higher-order function in JavaScript?
How to pass a function as a parameter in JavaScript?
How to return a function from another function in JavaScript?
What is an anonymous function in JavaScript?
How to declare and use arrow functions in JavaScript?
What is the “this” keyword in JavaScript functions?
What is the difference between call and apply in JavaScript?
What is the bind method in JavaScript functions?
How to use the bind method to set the value of “this” in a function?
What is a closure in JavaScript?
How to create a closure in JavaScript?
What is the difference between a closure and a regular function in JavaScript?
How to use a closure to create a private variable in JavaScript?
What is a generator function in JavaScript?
How to declare and use a generator function in JavaScript?
What is a Promise in JavaScript?
How to create and use Promises in JavaScript?
What are async/await functions in JavaScript?
How to declare and use async/await functions in JavaScript?

Arrays

How to declare an array in JavaScript?
How to access an element of an array in JavaScript?
How to add an element to the end of an array in JavaScript?
How to add an element to the beginning of an array in JavaScript?
How to remove the last element from an array in JavaScript?
How to remove the first element from an array in JavaScript?
How to get the length of an array in JavaScript?
How to loop through an array in JavaScript?
How to sort an array in JavaScript?
How to check if an array contains a specific value in JavaScript?
How to concatenate two arrays in JavaScript?
How to create an empty array in JavaScript?
How to copy an array in JavaScript?
How to convert a string to an array in JavaScript?
How to convert an array to a string in JavaScript?
How to find the index of an element in an array in JavaScript?
How to remove an element from an array in JavaScript?
How to insert an element into an array at a specific position in JavaScript?
How to check if an object is an array in JavaScript?
How to filter an array in JavaScript?

Objects

What are objects in JavaScript?
How to create an object in JavaScript?
What is the difference between an object and a class in JavaScript?
What is the difference between null and undefined in JavaScript?
How to check if an object is empty in JavaScript?
How to check if a value is an object in JavaScript?
How to clone an object in JavaScript?
How to merge two objects in JavaScript?
How to delete a property from an object in JavaScript?
What is the difference between a for-in loop and a for-of loop in JavaScript?
What is object destructuring in JavaScript?
How to use the spread operator with objects in JavaScript?
What is the difference between Object.assign and the spread operator in JavaScript?
How to get the keys of an object in JavaScript?
How to get the values of an object in JavaScript?
How to convert an object to an array in JavaScript?
What is the prototype in JavaScript?
What is prototypal inheritance in JavaScript?
How to create a constructor function in JavaScript?
What is the difference between a constructor function and a class in JavaScript?

Classes

Inheritance

Regular Expressions

What is a Regular Expression in JavaScript?
How to create a regular expression in JavaScript?
What is a flag in a regular expression in JavaScript?
What are the different flags used in regular expressions in JavaScript?
What is the difference between greedy and lazy quantifiers in regular expressions in JavaScript?
What is a character set in regular expressions in JavaScript?
How to use character sets in regular expressions in JavaScript?
What is a negated character set in regular expressions in JavaScript?
How to use negated character sets in regular expressions in JavaScript?
What is a range in regular expressions in JavaScript?
How to use ranges in regular expressions in JavaScript?
What is a quantifier in regular expressions in JavaScript?
How to use quantifiers in regular expressions in JavaScript?
What is a capturing group in regular expressions in JavaScript?
How to use capturing groups in regular expressions in JavaScript?
What is a non-capturing group in regular expressions in JavaScript?
How to use non-capturing groups in regular expressions in JavaScript?
What is a lookahead assertion in regular expressions in JavaScript?
How to use lookahead assertions in regular expressions in JavaScript?
What is a lookbehind assertion in regular expressions in JavaScript?
How to use lookbehind assertions in regular expressions in JavaScript?
What is a backreference in regular expressions in JavaScript?
How to use backreferences in regular expressions in JavaScript?
What is a named capturing group in regular expressions in JavaScript?
How to use named capturing groups in regular expressions in JavaScript?
What is a boundary in regular expressions in JavaScript?
How to use boundaries in regular expressions in JavaScript?
What is a Unicode property escape in regular expressions in JavaScript?
How to use Unicode property escapes in regular expressions in JavaScript?
What is a flagless regular expression in JavaScript?

Error Handling

What is an error-first callback?
What is the purpose of try-catch blocks?
How can you catch all possible exceptions in a single catch block?
What is the difference between throw and throw new Error()?
How do you handle errors in asynchronous code in Node.js?
How do you create custom error messages in JavaScript?
What is the difference between errors and exceptions in JavaScript?
What is the difference between synchronous and asynchronous error handling in JavaScript?
How can you handle errors in a Promise chain?
How can you handle unhandled rejections in Node.js?
How can you debug JavaScript code that is throwing an error?
How can you handle errors in React applications?
What is the purpose of the finally block in a try-catch-finally statement?
How can you create an error object in JavaScript?
How can you re-throw an exception in JavaScript?
How can you check for errors in JavaScript code without using try-catch blocks?
How can you handle errors in Redux applications?
How can you use the debugger statement to debug JavaScript code?
What is the purpose of the window.onerror function in JavaScript?
How can you handle errors in Angular applications?

DOM Manipulation

What is the difference between the window object and the document object in JavaScript?
How to get the value of an input field using JavaScript?
How to change the text of an element using JavaScript?
How to change the HTML content of a div using JavaScript?
How to change the background color of an element using JavaScript?
How to create an element dynamically using JavaScript?
How to add a class to an element using JavaScript?
How to remove a class from an element using JavaScript?
How to hide an element using JavaScript?
How to show an element using JavaScript?
How to get the value of a selected option in a dropdown list using JavaScript?
How to add an event listener to an element using JavaScript?
How to remove an event listener from an element using JavaScript?
How to prevent default behavior of an element using JavaScript?
How to stop propagation of an event in JavaScript?
How to get the parent element of an element using JavaScript?
How to get the child elements of an element using JavaScript?
How to get the next sibling element of an element using JavaScript?
How to get the previous sibling element of an element using JavaScript?
How to create a new text node using JavaScript?
How to create a new element with text using JavaScript?

Event Handling

What is event bubbling and capturing in JavaScript?
How to add an event listener to multiple elements in JavaScript?
How to remove event listener in JavaScript?
How to prevent default behavior of anchor tag using JavaScript?
How to pass parameters to event handlers in JavaScript?
How to stop event propagation in JavaScript?
How to disable a button using JavaScript?
How to trigger a click event programmatically in JavaScript?
How to detect when the user has stopped typing in a text input using JavaScript?
How to trigger an event on checkbox selection in JavaScript?
How to get the target element of a click event in JavaScript?
How to trigger an event when the user scrolls to a specific element in JavaScript?
How to check if a key is pressed in JavaScript?
How to detect if the user has pressed the enter key in JavaScript?
How to handle keyboard events in JavaScript?
How to handle mouse events in JavaScript?
How to handle touch events in JavaScript?
How to handle drag and drop events in JavaScript?
How to handle form submission using JavaScript?
How to handle resize events in JavaScript?

Asynchronous Programming

What is asynchronous programming in JavaScript?
How does asynchronous programming work in JavaScript?
What is the difference between synchronous and asynchronous programming in JavaScript?
What is a callback function in JavaScript?
How do you create a callback function in JavaScript?
How do Promises work in JavaScript?
What is the difference between Promises and callbacks in JavaScript?
How do you handle errors with Promises in JavaScript?
What is async/await in JavaScript?
How does async/await work in JavaScript?
What is the difference between Promises and async/await in JavaScript?
How do you handle errors with async/await in JavaScript?
How do you use Promise.all in JavaScript?
What is the difference between Promise.all and Promise.race in JavaScript?
How do you use Promise.race in JavaScript?
What is the event loop in JavaScript?
How does the event loop work in JavaScript?
What is the difference between the call stack and the event loop in JavaScript?
What is setTimeout in JavaScript?
How does setTimeout work in JavaScript?
What is setInterval in JavaScript?
How does setInterval work in JavaScript?
What is the difference between setTimeout and setInterval in JavaScript?
How do you cancel a setTimeout in JavaScript?
How do you cancel a setInterval in JavaScript?
What is the purpose of requestAnimationFrame in JavaScript?
How does requestAnimationFrame work in JavaScript?
What is the difference between setTimeout and requestAnimationFrame in JavaScript?
What are Web Workers in JavaScript?
How do Web Workers work in JavaScript?
What is the difference between Web Workers and the main thread in JavaScript?

Promises

What are promises in JavaScript?
What are the states of a promise in JavaScript?
How do you create a promise in JavaScript?
What is the difference between callbacks and promises in JavaScript?
What is promise chaining in JavaScript?
What is the purpose of the then() method in JavaScript promises?
What is the purpose of the catch() method in JavaScript promises?
What is the purpose of the finally() method in JavaScript promises?
How do you handle errors in JavaScript promises?
What is the purpose of Promise.all() method in JavaScript?
What is the purpose of Promise.race() method in JavaScript?
How do you use async/await with JavaScript promises?
What is the difference between a resolved promise and a rejected promise?
What is the purpose of Promise.resolve() method in JavaScript?
What is the purpose of Promise.reject() method in JavaScript?
What is the difference between a promise and an observable in JavaScript?
What is the purpose of the async keyword in JavaScript?
What is the purpose of the await keyword in JavaScript?
How do you handle multiple promises in JavaScript?
How do you cancel a promise in JavaScript?
What is a Promise in JavaScript?
What is a Promise.all() method in JavaScript?
What is Promise.race() method in JavaScript?
How to handle errors in Promise in JavaScript?
What is Promise Chaining in JavaScript?
How to cancel a Promise in JavaScript?
What is the difference between Promise and Callback in JavaScript?
What is Promise.prototype.catch() method in JavaScript?
How to create a new Promise in JavaScript?
What is Promise.resolve() method in JavaScript?
What is Promise.reject() method in JavaScript?
How to use async/await with Promises in JavaScript?
What is Promise.prototype.then() method in JavaScript?
What is Promise.prototype.finally() method in JavaScript?
How to use Promise.allSettled() method in JavaScript?
What is Promise.prototype.catch() method in JavaScript?
How to use Promise.any() method in JavaScript?
What is Promise.prototype.finally() method in JavaScript?
How to use Promise.raceSettled() method in JavaScript?
How to use Promise.try() method in JavaScript?
What is the difference between Promise.all() and Promise.race() in JavaScript?

Async/Await

What is Async/Await in JavaScript and how does it work?
How do you handle errors in Async/Await?
What is the difference between Promises and Async/Await in JavaScript?
How do you use Async/Await with Fetch?
What are the benefits of using Async/Await over Promises in JavaScript?
How do you handle multiple promises in Async/Await?
What is the purpose of the ‘await’ keyword in Async/Await?
How do you convert a Promise to Async/Await in JavaScript?
How do you use Async/Await with Axios?
What are some common pitfalls of using Async/Await in JavaScript?

Callbacks

What is a callback function in JavaScript?
How to pass a callback function as a parameter in JavaScript?
How do you handle errors with callbacks in JavaScript?
What is the difference between synchronous and asynchronous callbacks in JavaScript?
How to use callbacks to handle asynchronous code in JavaScript?
What is a higher-order function in JavaScript and how does it relate to callbacks?
How do you return a value from a callback function in JavaScript?
How to chain multiple callbacks in JavaScript?
What are the benefits of using callbacks in JavaScript?
How to create a custom callback function in JavaScript?
How to handle callback hell in JavaScript?
What is the difference between a Promise and a callback in JavaScript?
How to use callbacks with event listeners in JavaScript?
What is a callback queue in JavaScript and how does it work?
How to pass arguments to a callback function in JavaScript?
How to implement a callback function in a class in JavaScript?
How to handle multiple callbacks in JavaScript?
What is the difference between a synchronous and an asynchronous function in JavaScript, and how do callbacks relate to these concepts?
How to create a callback function that takes multiple arguments in JavaScript?
How to handle errors with multiple callbacks in JavaScript?
Web Storage

Cookies
Localisation
Security
Debugging
Performance Optimization
Best Practices

What are some best practices for writing clean and maintainable JavaScript code?
How can you ensure that your JavaScript code follows best practices for performance?
What are some best practices for working with asynchronous JavaScript?
What are some best practices for organizing large-scale JavaScript projects?
How can you ensure that your JavaScript code is secure and follows best practices for security?
What are some best practices for testing JavaScript code?
What are some best practices for debugging JavaScript code?
What are some best practices for commenting and documenting your JavaScript code?
How can you ensure that your JavaScript code is compatible with different browsers and follows best practices for cross-browser compatibility?
What are some best practices for using third-party libraries and frameworks in your JavaScript code?
How can you ensure that your JavaScript code is maintainable and follows best practices for maintainability?
What are some best practices for version control and collaboration in JavaScript projects?
What are some best practices for optimizing the performance of your JavaScript code?
What are some best practices for error handling in JavaScript code?
What are some best practices for working with JavaScript in a team environment?
What are some best practices for working with JavaScript in a serverless environment?

ECMAScript 6/2015 (ES6)

What is ECMAScript 6?
What are some new features in ECMAScript 6?
What is let and const in ECMAScript 6?
How is block-level scoping implemented in ECMAScript 6?
What are template literals in ECMAScript 6?
How does destructuring work in ECMAScript 6?
What is the spread operator in ECMAScript 6?
How does arrow function syntax differ from regular function syntax in ECMAScript 6?
What are the advantages of using arrow functions in ECMAScript 6?
What is the purpose of default function parameters in ECMAScript 6?
How does the for…of loop work in ECMAScript 6?
What is a generator function in ECMAScript 6?
How is a generator function different from a regular function in ECMAScript 6?
What is a Promise in ECMAScript 6?
How do Promises work in ECMAScript 6?
What is async/await in ECMAScript 6?
How does async/await work in ECMAScript 6?
What are modules in ECMAScript 6?
How do modules work in ECMAScript 6?
What is the purpose of the import statement in ECMAScript 6?
What is the purpose of the export statement in ECMAScript 6?
How does the class syntax work in ECMAScript 6?
What is the purpose of the constructor method in ECMAScript 6 classes?
How do you extend a class in ECMAScript 6?
What is the super keyword in ECMAScript 6 classes?
How do you create static methods in ECMAScript 6 classes?
What are Symbols in ECMAScript 6?
How do Symbols work in ECMAScript 6?
What is the purpose of the new Set() constructor in ECMAScript 6?
What is the purpose of the new Map() constructor in ECMAScript 6?
Testing and Debugging Tools

In conclusion, JavaScript remains an essential skill for any software developer, and these interview questions will help you prepare for your next job interview. It is important to note that these are just a few examples of the types of questions you may encounter, and it is always a good idea to continue learning and practicing your JavaScript skills. Good luck!

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