#Front-end Job Interview Questions

This file contains a number of front-end interview questions that can be used when vetting potential candidates. It is by no means recommended to use every single question here on the same candidate (that would take hours). Choosing a few items from this list should help you vet the intended skills you require.

Note: Keep in mind that many of these questions are open-ended and could lead to interesting discussions that tell you more about the person's capabilities than a straight answer would.

Table of Contents

1. General Questions
2. HTML Questions
3. CSS Questions
4. JS Questions
5. jQuery Questions
6. Coding Questions
7. Fun Questions

Getting Involved

1. Contributors
2. How to Contribute
3. License

General Questions:

• What did you learn yesterday/this week?
• What excites or interests you about coding?
• What is a recent technical challenge you experienced and how did you solve it?
• What UI, Security, Performance, SEO, Maintainability or Technology considerations do you make while building a web application or site?
• Talk about your preferred development environment. (OS, Editor or IDE, Browsers, Tools, etc.)
• Which version control systems are you familiar with?
• Can you describe your workflow when you create a web page?
• If you have 5 different stylesheets, how would you best integrate them into the site?
• Can you describe the difference between progressive enhancement and graceful degradation?
• How would you optimize a website's assets/resources?
• Traditionally, why has it been better to serve site assets from multiple domains?
• How many resources will a browser download from a given domain at a time?
  • What are the exceptions?
• Name 3 ways to decrease page load (perceived or actual load time).
• If you jumped on a project and they used tabs and you used spaces, what would you do?
• Write a simple slideshow page
• What tools do you use to test your code's performance?
• If you could master one technology this year, what would it be?
• What are the differences between Long-Polling, Websockets and SSE?
• Explain the importance of standards and standards bodies.
• What is FOUC? How do you avoid FOUC?
• Do your best to describe the process from the time you type in a website's URL to it finishing loading on your screen.
• Explain what ARIA and screenreaders are, and how to make a website accessible.
• Explain some of the pros and cons for CSS animations versus JavaScript animations
• Explain the following request and response headers:
  • Diff. between Expires, Date, Age and If-Modified-...
  • DNT
  • Cache-Control
  • Transfer-Encoding
  • ETag
  • X-Frame-Options

HTML Questions:

• What's a doctype do?
• What's the difference between standards mode and quirks mode?
• What are the limitations when serving XHTML pages?
• Are there any problems with serving pages as application/xhtml+xml?
• How do you serve a page with content in multiple languages?
• What kind of things must you be wary of when design or developing for multilingual sites?
• What are data- attributes good for?
• Consider HTML5 as an open web platform. What are the building blocks of HTML5?
• Describe the difference between cookies, sessionStorage and localStorage.
• Can you explain the difference between GET and POST?
• Describe the difference between <script>, <script async> & <script defer>.

CSS Questions:

• What is the difference between classes and ID's in CSS?
• Describe what a "reset" CSS file does and how it's useful.
• Describe Floats and how they work.
• Describe z-index and how stacking context is formed.
• What are the various clearing techniques and which is appropriate for what context?
• Explain CSS sprites, and how you would implement them on a page or site.
• What are your favourite image replacement techniques and which do you use when?
• CSS property hacks, conditionally included .css files, or... something else?
• How do you serve your pages for feature-constrained browsers?
  • What techniques/processes do you use?
• What are the different ways to visually hide content (and make it available only for screen readers)?
• Have you ever used a grid system, and if so, what do you prefer?
• Have you used or implemented media queries or mobile specific layouts/CSS?
• Any familiarity with styling SVG?
• How do you optimize your webpages for print?
• What are some of the "gotchas" for writing efficient CSS?
• What are the advantages/disadvantages of using CSS preprocessors? (Sass, Compass, Stylus, LESS)
  • If so, describe what you like and dislike about the CSS preprocessors you have used.
• How would you implement a web design comp that uses non-standard fonts?
• Explain how a browser determines what elements match a CSS selector.
• Explain your understanding of the box model and how you would tell the browser in CSS to render your layout in different box models.
• What does * { box-sizing: border-box; } do? What are its advantages?
• List as many values for the display property that you can remember.
• What's the difference between inline and inline-block?
• What's the difference between a relative, fixed, absolute and statically positioned element?
• The 'C' in CSS stands for Cascading. How is priority determined in assigning styles (a few examples)? How can you use this system to your advantage?
• What existing CSS frameworks have you used locally, or in production? (Bootstrap, PureCSS, Foundation etc.)
  • If so, which ones? If you could, how would you change/improve them?
• Have you played around with the new CSS Flexbox or Grid specs?
• How is responsive design different from adaptive design?
• Have you ever worked with retina graphics? If so, when and what techniques did you use?

JS Questions:

• Explain event delegation
• Explain how this works in JavaScript
• Explain how prototypal inheritance works
• How do you go about testing your JavaScript?
• AMD vs. CommonJS?
• Explain why the following doesn't work as an IIFE: function foo(){ }();.
  • What needs to be changed to properly make it an IIFE?
• What's the difference between a variable that is: null, undefined or undeclared?
  • How would you go about checking for any of these states?
• What is a closure, and how/why would you use one?
• What's a typical use case for anonymous functions?
• How do you organize your code? (module pattern, classical inheritance?)
• What's the difference between host objects and native objects?
• Difference between: function Person(){}, var person = Person(), and var person = new Person()?
• What's the difference between .call and .apply?
• Explain Function.prototype.bind.
• When do you optimize your code?
• When would you use document.write()?
• What's the difference between feature detection, feature inference, and using the UA string?
• Explain AJAX in as much detail as possible.
• Explain how JSONP works (and how it's not really AJAX).
• Have you ever used JavaScript templating?
  • If so, what libraries have you used? (Mustache.js, Handlebars etc.)
• Explain "hoisting".
• Describe event bubbling.
• What's the difference between an "attribute" and a "property"?
• Why is extending built in JavaScript objects not a good idea?
• Difference between document load event and document ready event?
• What is the difference between == and ===?
• Explain the same-origin policy with regards to JavaScript.
• Make this work:

duplicate([1,2,3,4,5]); // [1,2,3,4,5,1,2,3,4,5]

• Why is it called a Ternary expression, what does the word "Ternary" indicate?
• What is "use strict";? what are the advantages and disadvantages to using it?
• Create a for loop that iterates up to 100 while outputting "fizz" at multiples of 3, "buzz" at multiples of 5 and "fizzbuzz" at multiples of 3 and 5
• Why is it, in general, a good idea to leave the global scope of a website as-is and never touch it?

jQuery Questions:

• Explain "chaining".
• Explain "deferreds".
• What are some jQuery specific optimizations you can implement?
• What does .end() do?
• Name 4 different values you can pass to the jQuery method.
  • Selector (string), HTML (string), Callback (function), HTML element, object, array, element array, jQuery Object, etc.
• What is the difference between .get(), [], and .eq()?

Coding Questions:

Question: How would you make this work?

add(2, 5); // 7
add(2)(5); // 7

Question: What value is returned from the following statement?

"i'm a lasagna hog".split("").reverse().join("");

Question: What is the value of window.foo?

( window.foo || ( window.foo = "bar" ) );

Question: What is the outcome of the two alerts below?

var foo = "Hello"; 
(function() { 
  var bar = " World"; 
  alert(foo + bar); 
})(); 
alert(foo + bar);

Question: What is the value of foo.length?

var foo = [];
foo.push(1);
foo.push(2);

Fun Questions:

• What's a cool project that you've recently worked on?
• What are some things you like about the developer tools you use?
• Do you have any pet projects? What kind?
• What's your favorite feature of Internet Explorer?
• How do you like your coffee?

Contributors:

This document started in 2009 as a collaboration of @paul_irish @bentruyman @cowboy @ajpiano @SlexAxton @boazsender @miketaylr @vladikoff @gf3 @jon_neal @sambreed and @iansym.

It has since received contributions from 100 developers.