My answers to the H5BP Frontend Dev Interview Questions. Just for exercise.
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#Answers to Front-end Job Interview Questions

I decided to try and answer the H5BP Front-end Dev Interview Questions, as an exercise to revise some of the basics and out of curiosity to see where I stand. I tried to do as many as possible, naturally without using Google or books for help.

This is just my personal take and far from perfect. Some answers might be incorrect, not precise enough or just plain stupid - so don't use this as a reference ;)

Table of Contents

  1. HTML Questions
  2. CSS Questions
  3. JS Questions
  4. Coding Questions
  5. Fun Questions

HTML Questions:

  • What does a doctype do?

It sets the namespace and standard syntax rules for the browser to determine which rendering mode to use.

  • What's the difference between standards mode and quirks mode?

One follows a unified standard set by the W3C, the other relies on how different browsers interpret the markup, which can be unpredictable.

  • What's the difference between HTML and XHTML?

XHTML is HTML expressed in XML, can be "strict" or "transitional". Syntax rules are generally tighter, Doctype is required

  • Are there any problems with serving pages as application/xhtml+xml?

Might cause problems in old browsers (IE?) that interpret the MIME type wrong

  • How do you serve a page with content in multiple languages?

set a Content-Language header and a lang attribute on html element

  • What kind of things must you be wary of when design or developing for multilingual sites?
  • Character Encoding
  • Reading Direction ltr or rtl
  • Different Lengths of Headings / Captions and such
  • What are data- attributes good for?

Storing arbitrary data on HTML elements without affecting the semantics (too much).

  • Consider HTML5 as an open web platform. What are the building blocks of HTML5?

I don't really understand this one. Something like header, main, aside, footer? Or generally block-level and inline elements?

  • Describe the difference between a cookie, sessionStorage and localStorage.

I'm a little unsure here. All are client-side storage techniques. Cookies allow only strings while sS and lS also allow JavaScript data like objects. sessionStorage is only available in the session (duh), so is deleted when the browser closes.

  • Describe the difference between <script>, <script async> and <script defer>.

1: Script blocks rendering, 2: Script runs asnychronously, 3: Script loads after DOM is parsed.

  • Why is it generally a good idea to position CSS <link>s between <head></head> and JS <script>s just before </body>? Do you know any exceptions?

Performance reasons. You want the CSS to be available as soon as possible to correctly display the site, and scripts in the head would block rendering while they're being loaded, so you want them to be requested after the rest of the document. One exception would be a feature testing lib like 'Modernizr'.

  • What is progressive rendering?

Don't really know this one. I guess basically it's the idea of rendering chunks of content as soon as they're available, as opposed to waiting until everything's ready.

CSS Questions:

  • What is the difference between classes and ID's in CSS?

IDs must be unique and take higher priority in the cascade. Only Classes should be used for styling.

  • What's the difference between "resetting" and "normalizing" CSS? Which would you choose, and why?

Resetting overwrites the default browser behaviour (and usually sets everything to none or 0 ), while normalizing kind of 'evens it out' across browsers to make sure they all apply the same sensible defaults. I prefer normalizing.

  • Describe Floats and how they work.

Floats take an element out of the regular document flow and move it left or right.

  • Describe z-index and how stacking context is formed.

z-index sets the order of elements along the z-axis (from the screen outward), and defines which elements are displayed on top. Only applies to positioned elements. Context is inherited from the parent, so an element inside a parent with z-index can only be positioned within that context.

  • What are the various clearing techniques and which is appropriate for what context?

That depends. you could use a separate element with a clearfix class, or use pseudo-elements like in the micro-clearfix technique with display:table

  • What are your favourite image replacement techniques and which do you use when?

I try to avoid that alltogether, but if I had to, I'd use something like this to keep the element's content accessible:

text-indent: 100%;
white-space: nowrap;
overflow: hidden;
  • How would you approach fixing browser-specific styling issues?

I'd rather like to rely on feature detection and pin styles on a Modernizr class, i.e. no-rgba or something. Or use conditional comments on the html tag for older IE versions.

  • How do you serve your pages for feature-constrained browsers?

    • What techniques/processes do you use?

    Again, I like using feature detection (Modernizr) to leverage a progressive enhancement approach. When in doubt, build everything as if it was still 2005 and then layer new stuff on top.

  • What are the different ways to visually hide content (and make it available only for screen readers)?

position it off the screen, clip and hide it:

clip: rect(1px, 1px, 1px, 1px);
position: absolute !important;
height: 1px;
width: 1px;
overflow: hidden;
  • Have you ever used a grid system, and if so, what do you prefer?

I have tried different systems, like the bootstrap grid, susy, simple grids... I don't like it when the markup gets flooded with boilerplate classes end you end up with elements like <div class="col-sm-12 col-md-6 col-lg-4"> so I prefer an approach where I use SCSS mixins to style them directly.

  • Have you used or implemented media queries or mobile specific layouts/CSS?

Yes, I hardly ever build anything that's not responsive anymore. (And I don't think anyone should)

  • Any familiarity with styling SVG?

Just recently adopted a workflow for SVG Icons from an external spritemap, using <use xlink>, a grunt task called 'svgstore' and the 'svg4everybody' polyfill. Styling SVG in this case comes down to defining icon sizes and using CSS currentColor for the fill.

  • How do you optimize your webpages for print?

Print media queries, pretty much hiding anything but the essential content, disabling background images and other 'ink-intensive' stuff, setting body type color to #000, defining a few basic page-break-after rules, and so on.

  • What are some of the "gotchas" for writing efficient CSS?
  • Normalize
  • Honoring the cascade
  • Avoiding deep nested selectors (max 3 levels)
  • Good Naming conventions (BEM, SMACSS, etc)
  • What are the advantages/disadvantages of using CSS preprocessors?

They're great for a myriad of reasons. One disadvantage might be that if you're not careful / you don't know what you're doing, the CSS can get bloated.

  • Describe what you like and dislike about the CSS preprocessors you have used.

I use Sass with Grunt and I love it. Variables, Mixins, Extends, Auto-Prefixing ... the list of awesome goes on. My favourite feature might be just the fact that I can organize my code in small partials and have a flexible but meaningful file structure.

  • How would you implement a web design comp that uses non-standard fonts?

This is very much open to discussion, Filament group have some excellent pointers on how to do this best. Be aware of FOUT / FOIT but I guess the default approach would be to serve them via font-face and provide sensible fallbacks in a font stack.

  • 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.

Not going to explain the box model here. Simply put, it's how the browser calculates the dimensions of elements. I usually build sites on border-box. It just makes way more sense to me.

  • What does * { box-sizing: border-box; } do? What are its advantages?

Subtract padding and border values from a defined width rather than add to it. Advantages: it doesn't hurt my brain, and it makes things like percentage-based layouts with fixed paddings a lot easier.

  • List as many values for the display property that you can remember.
  • block
  • inline
  • inline-block
  • none
  • table, table-row, table-cell..
  • list-item
  • flex
  • What's the difference between inline and inline-block?

inline-block generally behaves like a block-level element, but does not force line breaks before or after

  • What's the difference between a relative, fixed, absolute and statically positioned element?
  • static: normal document flow
  • relative: position element in offset to normal document flow
  • absolute: position relative to parent element (or next higher element with positioning)
  • fixed: position relative to browser viewport
  • What existing CSS frameworks have you used locally, or in production? How would you change/improve them?

I use my own pattern library, just a loose collection of SCSS partials. Not a big fan of full-blown frameworks, but I have occasionally used bootstrap.

  • Have you played around with the new CSS Flexbox or Grid specs?

I did try to build some flex layouts, but I can't really say I know much about it.

  • How is responsive design different from adaptive design?

Fully responsive design relies on percentage-based layouts and is completely fluid, in contrast to a few "fixed" layouts triggered at different breakpoints in adaptive design.

  • Have you ever worked with retina graphics? If so, when and what techniques did you use?

I always use SVG if it all possible, I used dpi media query and "@2x" images for a while, but it's a pain.

  • Is there any reason you'd want to use translate() instead of absolute positioning, or vice-versa? And why?

translate for animations/transitions, or in combination with percentages to position an element relative to its own size.

JS Questions:

  • Explain event delegation

an event on a child element also fires on the parent due to bubbling, so it's possible to listen for events on the parent and then check for the original source.

  • Explain how this works in JavaScript

it depends on the context. this is usually the object that contains the function this is in.

  • Explain how prototypal inheritance works

any method that is part of an object, is also available in instances of objects that are based on it.

  • How do you go about testing your JavaScript?

I use JSHint for general Syntax errors, but I dont do TDD with Karma or something like that.

  • What do you think of AMD vs CommonJS?

Maybe I'm a bit behind here, but I never used either.

  • Explain why the following doesn't work as an IIFE: function foo(){ }();.

parser treats this a function declaration, with an unrelated () after. Throws Error

  • What needs to be changed to properly make it an IIFE?
( function foo(){ }() );
  • What's the difference between a variable that is: null, undefined or undeclared?
  • null is a value of nothing
  • undefined is a value of a variable that has not (yet) been set, or is not visible in the current scope
  • undeclared I'm not sure about...
  • How would you go about checking for any of these states?
  • x === null
  • typeof x === 'undefined'
  • ?
  • How do you organize your code? (module pattern, classical inheritance?)

Usually write it as an object literal with different modules

  • What's the difference between host objects and native objects?

Don't really know. Maybe host objects are anything not pre-baked into JS, like Array, Math etc.?

  • Difference between: function Person(){}, var person = Person(), and var person = new Person()?
  • class declaration
  • return value of function assigned to variable
  • new instance of a class
  • What's the difference between .call and .apply?

I think .call takes multiple individual arguments, .apply only one (array)

  • When would you use document.write()?

Never? or maybe as fallback for a CDN jQuery, to write a local script if CDN is not available.

  • What's the difference between feature detection, feature inference, and using the UA string?
  • Feature Detection runs a series of small tests to determine support,
  • Feature inference (guessing by the name) assumes support because of related supported features
  • the UA string is just plain evil. browser sniffing is not an option in 2015 anymore IMHO
  • Explain AJAX in as much detail as possible.

stands for Asynchronous JavaScript and XML. Enables the transfer of data asynchronously from the server via the XMLHttpRequest Object in JavaScript. I know this is part of the question but I really don't want to go into detail so I'm just gonna stop. There.

  • Explain how JSONP works (and how it's not really AJAX).

I can't really, sorry

  • Have you ever used JavaScript templating?

    • If so, what libraries have you used?

    I experimented with Angular and Meteor

  • Explain "hoisting".

A variable can be declared after it has been used.

x = 5; // Assign 5 to x

elem = document.getElementById("demo"); // Find an element 
elem.innerHTML = x;                     // Display x in the element

var x; // Declare x
  • Describe event bubbling.

Event bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. When you use event bubbling

               / \
---------------| |-----------------
| element1     | |                |
|   -----------| |-----------     |
|   |element2  | |          |     |
|   -------------------------     |
|        Event BUBBLING           |

the event handler of element2 fires first, the event handler of element1 fires last.

  • What's the difference between an "attribute" and a "property"?

attributes are written into the actual HTML tag, like <div class="foo"> while properties are set on the DOM node, after it is parsed.

  • Why is extending built in JavaScript objects not a good idea?

because everything is an object in JavaScript. Due to inheritance your customizations would almost certainly end up somewhere they shouldn't.

  • Difference between document load event and document ready event?

load fires after the entire document (+ assets) is done, ready fires after DOM parsing.

  • What is the difference between == and ===?

=== strict equals also compares type. generally preferred.

  • Explain the same-origin policy with regards to JavaScript.

I don't quite know what that is.

  • Make this work:
duplicate([1,2,3,4,5]); // [1,2,3,4,5,1,2,3,4,5]
function duplicate(a){
    return a.concat(a);
  • Why is it called a Ternary expression, what does the word "Ternary" indicate?

indicates a conditional expression, where the outcome depends on the parameter used. Can be used as shorthand for if/else statements, like:

return (myVar > 5) ? 'foo' : 'bar';
// returns 'foo' when myVar greater than 5, else 'bar'.
  • What is "use strict";? what are the advantages and disadvantages to using it?

"use strict" sets the JS parser to execute in "strict mode". This is good practice because it then throws errors for some of the weird JS quirks that would otherwise have unexpected results.

  • 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
for(var i = 1;i <= 100; i++){
    var out = '';
    if(i % 3 === 0) out += 'fizz';
    if(i % 5 === 0) out += 'buzz';
    out.length && console.log(out);
  • Why is it, in general, a good idea to leave the global scope of a website as-is and never touch it?

hmm- let's see..

  • to avoid naming collisions with other scripts
  • to keep "private" functions inaccessible from the console and variables only in the relevant scope
  • to keep the code clean and readable
  • Why would you use something like the load event? Does this event have disadvantages? Do you know any alternatives, and why would you use those?

I use load mostly for defering external JS (like analytics, the FB API etc) until after everything else is done. For the ususal tasks, the DOM ready event is a better pick, because scripts get to run sooner.

  • Explain what a single page app is and how to make one SEO-friendly.

single page apps dont follow the traditional request - fetch html page pattern, but dynamically insert pages via JS. history.pushState comes to mind to maintain back button and browser history support. But for SEO, things are complicated. I think I read somewhere that it's possible to pre-fetch your own site, parse the content and serve that to crawlers.

  • What is the extent of your experience with Promises and/or their polyfills?

Don't have much experience with promises. I used it recently in an Audio API but I'm not really confident with it yet. I know, I know. I'll get to it.

Coding Questions:

Question: What is the value of foo?

var foo = 10 + '20';


Question: How would you make this work?

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

first one:

function add(a,b){
  return a + b;

the second one uses currying, lodash has a handy curry method to transform the first into the second one. To learn a bit more about currying, see this video, or read this guide.

function add(a) {
  return function (b) {
    return a + b;

Question: What value is returned from the following statement?

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

"goh angasal a m'i"

Question: What is the value of

( || ( = "bar" ) );


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

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

"Hello World" and an Error since bar is out of scope there

Question: What is the value of foo.length?

var foo = [];


Fun Questions:

  • What's a cool project that you've recently worked on?

Did a site for an awesome backpackers hostel in jakarta.

  • What are some things you like about the developer tools you use?

I love Sublime Text for its speed and Grunt for the millions of packages that make my job easier.

  • Do you have any pet projects? What kind?

I like to design gig posters for fun.

  • What's your favorite feature of Internet Explorer?

It will die. One pale morning, it will draw its last frame, and the news will sound from every mountaintop. And the people will gather in silent admiration of the peace and serenity that will -for one divine moment- cover the earth, promising a brighter day tomorrow.

  • How do you like your coffee?

#000, thx.