In the ever-evolving world of web development, JavaScript stands as one of the most critical programming languages. Whether you are a budding developer or a seasoned professional, mastering JavaScript can open numerous doors in your career. Preparing for a JavaScript interview requires understanding core concepts, staying updated with the latest features, and practicing coding challenges.
Top 15 JavaScript Interview Questions with Answers
1. What is JavaScript, and how is it different from Java?
Answer:
JavaScript is a lightweight, interpreted, or just-in-time compiled programming language with first-class functions. While Java is a statically typed, class-based language typically used for backend development, JavaScript is dynamically typed and primarily used for client-side scripting. JavaScript code can be run in the browser, whereas Java applications need a virtual machine or browser plugin.
2. What are the data types supported by JavaScript?
Answer:
JavaScript supports several data types:
- Primitive types: Number, String, Boolean, Null, Undefined, Symbol, and BigInt.
- Object types: Objects, Arrays, Functions, and more.
3. Explain closures in JavaScript.
Answer:
A closure is a function that retains access to its outer lexical environment, even after the outer function has finished executing. This allows the inner function to access variables and parameters of the outer function.
4. What is the difference between == and === in JavaScript?
Answer:
== is the equality operator that performs type coercion if the variables are of different types before comparing them. === is the strict equality operator that compares both the value and the type, ensuring they are identical without performing type conversion.
5. What is an Immediately Invoked Function Expression (IIFE)?
Answer:
An IIFE is a function that is executed immediately after its creation. The syntax is typically as follows:
(function() {
// code here
})();
6. How does prototypal inheritance work in JavaScript?
Answer:
Prototypal inheritance is a feature in JavaScript where objects can inherit properties and methods from another object, called the prototype. Every JavaScript object has a __proto__ property that points to its prototype object.
7. What are arrow functions, and how do they differ from regular functions?
Answer:
Arrow functions, introduced in ES6, provide a more concise syntax for writing functions. They do not have their own context and cannot be used as constructors.
8. What is the purpose of the let and const keywords?
Answer:
let and const are block-scoped variable declarations introduced in ES6. let allows reassigning the variable value, while const declares a constant that cannot be reassigned after its initial assignment.
9. Explain the event loop in JavaScript.
Answer:
The event loop is a mechanism that allows JavaScript to perform non-blocking operations by offloading tasks to the browser’s APIs and handling asynchronous code. It continuously checks the call stack and the task queue to determine if any function needs to be executed.
10. What are Promises, and how do they work?
Answer:
Promises are objects that represent the eventual completion or failure of an asynchronous operation. They have three states: pending, fulfilled, and rejected. A promise provides then(), catch(), and finally() methods to handle asynchronous operations.
11. What is this keyword in JavaScript?
Answer:
The ‘this’ keyword refers to the context in which a function is called. In the global context, ‘this’ refers to the global object (window in browsers). Inside methods, ‘this’ refers to the object the method belongs to.
12. What is the spread operator, and how is it used?
Answer:
The spread operator (...) allows an iterable such as an array or object to be expanded in places where multiple elements/variables/arguments are expected.
13. How do you handle asynchronous code in JavaScript?
Answer:
Asynchronous code in JavaScript can be handled using callbacks, Promises, and the async/await syntax. Async/await provides a more readable and straightforward way to work with asynchronous code.
14. What are JavaScript modules, and why are they useful?
Answer:
JavaScript modules are reusable pieces of code that can be imported and exported between different files. Modules help in organizing code, maintaining a clean global namespace, and improving code reusability and maintainability.
15. What is the difference between null and undefined?
Answer:
undefined is a type and value assigned to variables that have been declared but not yet assigned a value. null is an assignment value that represents no value or an intentional absence of an object.
Conclusion
Preparing for a JavaScript interview can be daunting, but with a solid understanding of core concepts and practice, you can confidently tackle any challenge. For tech recruiters, finding candidates who can seamlessly answer these questions and demonstrate practical knowledge is crucial.
That's where Tech for Hire services come into play. We specialize in connecting skilled JavaScript developers with companies in need of top-tier talent. Whether you're a company looking to hire or a developer seeking new opportunities, Tech for Hire is here to meet your needs!