Top 15 best JavaScript interview questions

by | Nov 8, 2023 | Computer Science, Web Development

As a JavaScript developer, you’re likely to encounter a wide array of javascript interview questions during interviews that span the spectrum of complexity. Whether you’re just starting your journey as a JavaScript developer or you’re a seasoned pro, it’s essential to prepare for these interviews by understanding and practicing the core concepts. In this comprehensive guide, we’ll cover JavaScript interview questions categorized by difficulty levels, from beginner to advanced, and provide you with insights into how to approach them effectively.

Beginner Level Questions:

  1. What is JavaScript, and what is it used for?

JavaScript is a versatile, high-level, interpreted scripting language used for making web pages interactive. It allows you to implement dynamic content, handle user input, and modify the document structure and style.

Useful Insights: Start by explaining that JavaScript was initially designed to work in browsers, but now it’s also used on servers (Node.js) and in various other contexts, such as game development or desktop applications.

  1. What are variables, and how do you declare them in JavaScript?

In JavaScript, variables are used to store and manage data. They act as symbolic names for values. You can declare variables using the var, let, or const keywords.

Useful Insights: Highlight the differences between var, let, and const. var has function-level scope, while let and const have block-level scope. Also, mention that const variables cannot be reassigned after declaration.

  1. What are data types in JavaScript?

JavaScript has several data types, including strings, numbers, booleans, null, undefined, objects, and more. These data types are used to store different kinds of information in your code.

Useful Insights: Emphasize the distinction between primitive data types (like strings and numbers) and reference data types (like objects and arrays). Discuss the use of the typeof operator to determine a variable’s data type.

  1. What is the difference between null and undefined?

Undefined is a data type in JavaScript that represents the absence of a value. It’s typically the value of a variable that has been declared but not initialized. Null, on the other hand, is an assignment value representing no value or no object. It’s explicitly assigned by the programmer.

Useful Insights: Explain that it’s common for variables to have an initial value of undefined and that developers can explicitly set a variable to null when needed.

  1. How do you write a basic for loop in JavaScript?

A for loop is a control structure used to repeat a block of code multiple times. It consists of three parts: the initialization, the condition, and the iteration statement. The loop continues until the condition is false.

Useful Insights: Provide an example of a simple for loop that counts from 1 to 10. Explain how to use variables to control the loop and how to iterate over arrays using for loops.

Intermediate Level Questions:

  1. What is the DOM (Document Object Model), and how do you manipulate it with JavaScript?

The Document Object Model (DOM) is a programming interface that represents and interacts with HTML and XML documents. You can use JavaScript to access, manipulate, and modify elements and content within a web page.

Useful Insights: Discuss the hierarchical structure of the DOM, with the document as the entry point. Explain how to select elements by ID, class, or tag name and how to manipulate attributes and content.

  1. Explain the concept of closures in JavaScript.

A closure is a function that has access to variables from its outer (enclosing) function, even after that outer function has finished executing. Closures are commonly used for data encapsulation and creating private variables.

Useful Insights: Provide an example of a closure, like a function that returns another function. Explain how closures capture their lexical scope.

  1. What is the difference between == and === in JavaScript?

In JavaScript, == and === are used for equality comparisons. == performs type coercion, meaning it converts operands to the same type before making the comparison. === performs strict (type and value) comparison.

Useful Insights: Discuss how type coercion works with == and how === is often preferred to avoid unexpected results.

  1. What is the event loop in JavaScript, and how does it work?

The event loop is a central component of JavaScript’s concurrency model. It manages asynchronous operations and callbacks. The event loop continuously checks the message queue for pending events and processes them.

Useful Insights: Explain the call stack, callback queue, and message queue components of the event loop. Discuss the concept of non-blocking code and how promises and async/await fit into the event loop.

  1. What are callbacks, promises, and async/await?

Callbacks are functions passed as arguments to other functions, often used in asynchronous operations. Promises are objects representing the eventual completion (or failure) of an asynchronous operation. Async/await is a modern syntax for handling promises, making asynchronous code more readable and maintainable.

Useful Insights: Provide examples of callback functions, promise chains, and async/await usage. Explain error handling with promises.

Advanced Level Questions:

  1. Explain hoisting in JavaScript.

Hoisting is a mechanism in JavaScript where variable and function declarations are moved to the top of their containing scope during compilation. It’s important to understand how hoisting affects code execution.

Useful Insights: Discuss the differences between variable declarations (var) and variable initializations. Explain that function declarations are fully hoisted, while function expressions are not.

  1. What is the prototype chain in JavaScript, and how does it relate to inheritance?

The prototype chain is a fundamental concept in JavaScript’s object-oriented programming. It defines the way objects inherit properties and methods from their prototype (parent) objects.

Useful Insights: Explain how objects delegate property and method lookups to their prototypes. Provide examples of creating and extending prototypes using constructor functions and classes.

  1. What are the differences between function declarations and function expressions?

Function declarations define named functions that are hoisted to the top of their containing scope, allowing them to be called before their actual code appears. Function expressions define functions as values assigned to variables or properties.

Useful Insights: Show examples of both function declarations and function expressions. Discuss their different use cases and how they are hoisted differently.

  1. What is the purpose of the “this” keyword in JavaScript?

The this keyword refers to the context within which a function is executed. It can have different values based on how a function is called, such as in a method, a constructor, or a simple function invocation.

Useful Insights: Explain the four common rules for determining the value of this in a function. Clarify how arrow functions handle this differently.

  1. What is CORS (Cross-Origin Resource Sharing), and how can you enable it on a server?

CORS is a security feature implemented by web browsers to prevent unauthorized web pages from making requests to a different domain. To enable CORS on a server, you need to include specific HTTP headers that allow cross-origin requests.

Useful Insights: Discuss the same-origin policy, the need for CORS, and the HTTP headers used to control it, such as Access-Control-Allow-Origin and Access-Control-Allow-Methods.

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JavaScript interviews can be challenging, but with the right preparation, you can confidently tackle questions ranging from fundamental language concepts to advanced topics like closures and prototypes. By mastering these concepts and practicing your coding skills, you’ll be well-prepared to showcase your JavaScript expertise to potential employers. Keep in mind that in addition to providing correct answers, demonstrating a deep understanding of JavaScript’s inner workings and practical applications can set you apart in the competitive world of web development. Prepare thoroughly, build projects, and keep learning to stay at the forefront of JavaScript development. Good luck in your interviews!

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