JavaScript has evolved as a programming language giant from the 90's to till date. It is one of the most popular programming languages, which is widely used for both client-side and server-side development. If you're preparing for a JavaScript interview, it’s essential to have a deep understanding of various concepts and practical implementations. Here we have some important JavaScript interview questions with brief explanations that help you to understand the key concepts and also help you to crack an interview. Basic JavaScript Interview Questions 1. What is JavaScript? JavaScript is a high-level, interpreted programming language used primarily to create and control dynamic website content. It allows developers to implement complex features on web pages, such as interactive forms, animations, and real-time updates. 2. How does JavaScript differ from Java? Despite the similar names, JavaScript and Java are distinct languages with different purposes. Java is an object-oriented, compiled language used for building applications, while JavaScript is an interpreted, lightweight scripting language primarily used for enhancing web pages. Java requires a Java Virtual Machine (JVM) to run, whereas JavaScript is executed within the browser. 3. Explain the difference between var, let, and const. var: Function-scoped and can be redeclared. let: Block-scoped and cannot be redeclared within the same block. const: Block-scoped and must be initialized at the time of declaration. The value cannot be reassigned, but the object it points to can be modified. 4. What is negative infinity? Negative Infinity is a number in JavaScript which can be derived by dividing the negative number by zero. 5. Which company developed JavaScript? Netscape is the software company who developed JavaScript. 6. What are Frames in JavaScript? Frames allow you to divide the page into several rectangular areas and to display a separate document in each rectangle. Each of those rectangles is called a "frame". 7. What are undeclared and undefined variables? Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered. And undefined variables are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned. 8. Is there any difference between JavaScript and JScript? No, Netscape provided the JavaScript language. But, Microsoft changed the name and called it JScript to avoid the trademark issue. In other words, you can say JScript is the same as JavaScript, but it is provided by Microsoft. 9. What is BOM? BOM stands for Browser Object Model. It provides interaction with the browser. The default object of the browser is a window. 10. What is DOM? DOM stands for Document Object Model. It is a programming interface for HTML and XML documents. It represents the document structure as a tree of objects, allowing scripts to update the content, structure, and style of a document dynamically. 11. What is the use of a document object? A document object represents the HTML document. It can be used to access and change the content of HTML. 12. How to create objects in JavaScript? There are 3 ways to create an object in JavaScript - By object literal; By creating an instance of Object; By Object Constructor. 13. How to create an array in JavaScript? There are 2 ways to create an array in JavaScript - By array literal - const newArray = []; By using an array constructor - const newArray = new Array(); 14. Is JavaScript a case-sensitive language? Yes! JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters. 15. What are closures in JavaScript? A closure is a function that retains access to its lexical scope even when the function is executed outside that scope. This allows a function to remember the environment in which it was created. Advanced JS Interview Questions 16. What is the purpose of the this keyword in JavaScript? The this keyword refers to the object from which a function was called. Its value can vary depending on how and where it is invoked, such as within a method, a constructor, or globally. 17. What is the difference between == and ===? ==: Abstract equality operator that performs type coercion if the values being compared are of different types. ===: Strict equality operator that checks for both value and type equality, without type coercion. 18. What are JavaScript Promises? Promises are objects representing the eventual completion or failure of an asynchronous operation. They allow developers to write cleaner, more manageable asynchronous code using methods like then(), catch(), and finally(). 19. What is the event loop in JavaScript? The event loop is a mechanism that allows JavaScript to perform non-blocking operations. It continuously checks the message queue and executes tasks (like callback functions) when the call stack is empty, enabling asynchronous behavior. 20. What is the difference between null and undefined? null: An assigned value indicating the intentional absence of any object value. undefined: Indicates that a variable has been declared but not yet assigned a value. 21. Explain what hoisting is in JavaScript. Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their containing scope during the compilation phase. This means you can use variables and functions before they are declared in the code. 22. What are arrow functions, and how do they differ from regular functions? Arrow functions provide a more concise syntax for writing functions. They do not have their own this context and inherit this from the surrounding lexical context. They also lack a prototype property and do not have their own arguments object. 23. What is a JavaScript prototype? A prototype is an object from which other objects inherit properties and methods. In JavaScript, each object has a prototype, and objects can share properties and methods through their prototype chain. 14. Explain what async and await are. async: A keyword used to declare an asynchronous function that returns a Promise. await: Pauses the execution of an async function until the Promise is resolved, making asynchronous code look and behave more like synchronous code. 25. What is JSON, and how do you parse and stringify it in JavaScript? JSON (JavaScript Object Notation) is a lightweight data interchange format. To parse JSON into a JavaScript object, use JSON.parse(). To convert a JavaScript object into a JSON string, use JSON.stringify(). 26. What is event delegation? Event delegation is a technique where a single event listener is added to a parent element to manage events for multiple child elements. This is efficient for dynamically added elements and reduces the number of event listeners. 27. Explain the concept of currying in JavaScript. Currying is a functional programming technique where a function with multiple arguments is transformed into a sequence of functions, each taking a single argument. It allows partial application of functions. 28. What is the spread operator, and how is it used? The spread operator (...) allows an iterable (like an array or object) to be expanded into individual elements. It is used for copying arrays, concatenating arrays, and spreading elements in function arguments. 29. What are modules in JavaScript, and why are they important? Modules are reusable pieces of code that can be imported and exported between different files. They help in organizing code, avoiding global namespace pollution, and making code maintainable. 30. What is the difference between synchronous and asynchronous code? Synchronous code: Executes sequentially, blocking the execution of subsequent code until the current operation completes. Asynchronous code: Allows the execution of other code while waiting for an operation to complete, enabling non-blocking behavior. 31. What is the debounce function, and how does it work? A debounce function limits the rate at which a function can be called. It delays the execution of the function until after a specified wait time has elapsed since the last time the function was invoked. This is useful for optimizing performance during events like window resizing or keypresses. 32. How does this typeof operator work? The typeof is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand. The typeof operator evaluates to "number", "string", or "boolean" if its operand is a number, string, or boolean value and returns true or false based on the evaluation. Conclusion Being well-prepared for a JavaScript interview requires a deep understanding of the language's core concepts and the ability to explain them clearly. Reviewing these common questions and answers will help you build a solid foundation and increase your confidence during the interview process. Remember to stay updated with the latest JavaScript developments and practice coding regularly to refine your skills. Good luck!