JavaScript Interview Quizzes

193 top JavaScript interview questions sourced from greatfrontend/top-javascript-interview-questions.

• Can you offer a use case for the new arrow => function syntax?
• Describe event bubbling in JavaScript and browsers
• Describe event capturing in JavaScript and browsers
• Describe the difference between <script>, <script async> and <script defer>
• Describe the difference between a cookie, sessionStorage and localStorage in browsers
• Difference between document load event and document DOMContentLoaded event?
• Difference between: function Person(){}, const person = Person(), and const person = new Person() in JavaScript?
• Explain Function.prototype.bind in JavaScript
• Explain AJAX in as much detail as possible
• Explain event delegation in JavaScript
• Explain how this works in JavaScript
• Explain how JSONP works (and how it's not really Ajax)
• Explain how prototypal inheritance works in JavaScript
• Explain the concept of "hoisting" in JavaScript
• Explain the concept of this binding in event handlers
• Explain the concept of a callback function in asynchronous operations
• Explain the concept of a microtask queue
• Explain the concept of caching and how it can be used to improve performance
• Explain the concept of code coverage and how it can be used to assess test quality
• Explain the concept of Content Security Policy (CSP) and how it enhances security
• Explain the concept of Cross-Site Request Forgery (CSRF) and its mitigation techniques
• Explain the concept of debouncing and throttling
• Explain the concept of destructuring assignment for objects and arrays
• Explain the concept of error propagation in JavaScript
• Explain the concept of hoisting with regards to functions
• Explain the concept of inheritance in ES2015 classes
• Explain the concept of input validation and its importance in security
• Explain the concept of lazy loading and how it can improve performance
• Explain the concept of lexical scoping
• Explain the concept of partial application
• Explain the concept of scope in JavaScript
• Explain the concept of tagged templates
• Explain the concept of test-driven development (TDD)
• Explain the concept of the Prototype pattern
• Explain the concept of the Singleton pattern
• Explain the concept of the spread operator and its uses
• Explain the concept of the Strategy pattern
• Explain the concept of the Web Socket API
• Explain the concept of tree shaking in module bundling
• Explain the difference between document.querySelector() and document.getElementById()
• Explain the difference between classical inheritance and prototypal inheritance
• Explain the difference between dot notation and bracket notation for accessing object properties
• Explain the difference between global scope, function scope, and block scope
• Explain the difference between mutable and immutable objects in JavaScript
• Explain the difference between shallow copy and deep copy
• Explain the difference between synchronous and asynchronous functions in JavaScript
• Explain the difference between unit testing, integration testing, and end-to-end testing
• Explain the difference in hoisting between var, let, and const
• Explain the differences between CommonJS modules and ES modules in JavaScript
• Explain the differences on the usage of foo between function foo() {} and var foo = function() {} in JavaScript
• Explain the different states of a Promise
• Explain the different ways the this keyword can be bound
• Explain the event phases in a browser
• Explain the Observer pattern and its use cases
• Explain the same-origin policy with regards to JavaScript
• Explain what a single page app is and how to make one SEO-friendly
• Explain why the following doesn't work as an IIFE: function foo(){}();. What needs to be changed to properly make it an IIFE?
• How can closures be used to create private variables?
• How can you avoid problems related to hoisting?
• How can you create custom error objects?
• How can you implement secure authentication and authorization in JavaScript applications?
• How can you optimize DOM manipulation for better performance?
• How can you optimize network requests for better performance?
• How can you prevent clickjacking attacks?
• How can you prevent SQL injection vulnerabilities in JavaScript applications?
• How can you share code between JavaScript files?
• How can you test asynchronous code in JavaScript?
• How do <iframe> on a page communicate?
• How do Sets and Maps handle equality checks for objects?
• How do currying and partial application differ from each other?
• How do you abort a web request using AbortController in JavaScript?
• How do you access the index of an element in an array during iteration?
• How do you add, remove, and modify HTML elements using JavaScript?
• How do you add, remove, and update elements in an array?
• How do you check if an object has a specific property?
• How do you check the data type of a variable?
• How do you convert a Set to an array in JavaScript?
• How do you convert a string to a number in JavaScript?
• How do you create a constructor function?
• How do you detect if JavaScript is disabled on a page?
• How do you get the query string values of the current page in JavaScript?
• How do you handle errors in asynchronous operations?
• How do you handle errors using try...catch blocks?
• How do you import and export modules in JavaScript?
• How do you make an HTTP request using the Fetch API?
• How do you manipulate CSS styles using JavaScript?
• How do you organize your code?
• How do you prevent the default behavior of an event?
• How do you redirect to a new page in JavaScript?
• How do you reliably determine whether an object is empty?
• How do you use window.history API?
• How do you validate form elements using the Constraint Validation API?
• How do you write unit tests for JavaScript code?
• How does hoisting affect function declarations and expressions?
• How does JavaScript garbage collection work?
• How is Promise.all() different from Promise.allSettled()?
• Provide some examples of how currying and partial application can be used
• What advantage is there for using the JavaScript arrow syntax for a method in a constructor?
• What are Sets and Maps and how are they used?
• What are Symbols used for in JavaScript?
• What are callback functions and how are they used?
• What are default parameters and how are they used?
• What are design patterns and why are they useful?
• What are event listeners and how are they used?
• What are iterators and generators in JavaScript and what are they used for?
• What are JavaScript object getters and setters for?
• What are JavaScript object property flags and descriptors?
• What are JavaScript polyfills for?
• What are mocks and stubs and how are they used in testing?
• What are modules and why are they useful?
• What are Progressive Web Applications (PWAs)?
• What are Promises and how do they work?
• What are proxies in JavaScript used for?
• What are rest parameters and how are they used?
• What are server-sent events?
• What are some best practices for handling sensitive data in JavaScript?
• What are some best practices for writing maintainable and effective tests in JavaScript?
• What are some common performance bottlenecks in JavaScript applications?
• What are some common security headers and their purpose?
• What are some of the advantages and disadvantages of using TypeScript and compile-to-JavaScript languages
• What are some popular JavaScript testing frameworks?
• What are some techniques for reducing reflows and repaints?
• What are some tools and techniques for identifying security vulnerabilities in JavaScript code?
• What are some tools that can be used for JavaScript testing?
• What are some tools that can be used to measure and analyze JavaScript performance?
• What are template literals and how are they used?
• What are the advantages and disadvantages of using AJAX?
• What are the advantages of using the spread operator with arrays and objects?
• What are the benefits of using a module bundler?
• What are the benefits of using currying and partial application?
• What are the benefits of using spread syntax in JavaScript and how is it different from rest syntax?
• What are the common pitfalls of using the this keyword?
• What are the differences between Map/Set and WeakMap/WeakSet in JavaScript?
• What are the differences between XMLHttpRequest and fetch() in JavaScript and browsers?
• What are the differences between JavaScript ES2015 classes and ES5 function constructors?
• What are the differences between JavaScript variables created using let, var or const?
• What are the different methods for iterating over an array?
• What are the different types of errors in JavaScript?
• What are the different types of testing in software development?
• What are the different ways to copy an object or an array?
• What are the different ways to make an API call in JavaScript?
• What are the metadata fields of a module?
• What are the potential issues caused by hoisting?
• What are the potential pitfalls of using closures?
• What are the pros and cons of using Promises instead of callbacks in JavaScript?
• What are the various data types in JavaScript?
• What are the various ways to create objects in JavaScript?
• What are Web Workers and how can they be used to improve performance?
• What are workers in JavaScript used for?
• What do you think of CommonJS vs ESM?
• What is 'use strict'; (strict mode) in JavaScript for?
• What is Object.freeze() for?
• What is Object.preventExtensions() for?
• What is Object.seal() for?
• What is a closure in JavaScript, and how/why would you use one?
• What is async/await and how does it simplify asynchronous code?
• What is Cross-Site Scripting (XSS) and how can you prevent it?
• What is currying and how does it work?
• What is recursion and how is it used in JavaScript?
• What is the Intl namespace object for?
• What is the Command pattern and how is it used?
• What is the Decorator pattern and how is it used?
• What is the definition of a higher-order function in JavaScript?
• What is the difference between == and === in JavaScript?
• What is the difference between event.preventDefault() and event.stopPropagation()?
• What is the difference between innerHTML and textContent?
• What is the difference between mouseenter and mouseover event in JavaScript and browsers?
• What is the difference between setTimeout(), setImmediate(), and process.nextTick()?
• What is the difference between a Map object and a plain object in JavaScript?
• What is the difference between a parameter and an argument?
• What is the difference between the Window object and the Document object?
• What is the DOM and how is it structured?
• What is the event loop in JavaScript runtimes?
• What is the Factory pattern and how is it used?
• What is the Module pattern and how does it help with encapsulation?
• What is the prototype chain and how does it work?
• What is the purpose of the break and continue statements?
• What is the purpose of the finally block?
• What is the purpose of the new keyword?
• What is the purpose of the switch statement?
• What is the spread operator and how is it used?
• What is the ternary operator and how is it used?
• What is the use of Promise.all()
• What language constructs do you use for iterating over object properties and array items in JavaScript?
• What tools and techniques do you use for debugging JavaScript code?
• What's a typical use case for anonymous functions in JavaScript?
• What's the difference between .call and .apply in JavaScript?
• What's the difference between a JavaScript variable that is: null, undefined or undeclared?
• What's the difference between an "attribute" and a "property" in the DOM?
• When would you use document.write()?
• Why is extending built-in JavaScript objects not a good idea?
• Why is it, in general, a good idea to leave the global JavaScript scope of a website as-is and never touch it?
• Why might you want to create static class members in JavaScript?