Frontend Guidelines

HTML

Semantics

HTML5 provides us with lots of semantic elements aimed to describe precisely the content. Make sure you benefit from its rich vocabulary.

<!-- bad -->
<div id="main">
  <div class="article">
    <div class="header">
      <h1>Blog post</h1>
      <p>Published: <span>21st Feb, 2015</span></p>
    </div>
    <p>…</p>
  </div>
</div>

<!-- good -->
<main>
  <article>
    <header>
      <h1>Blog post</h1>
      <p>Published: <time datetime="2015-02-21">21st Feb, 2015</time></p>
    </header>
    <p>…</p>
  </article>
</main>

Make sure you understand the semantics of the elements you're using. It's worse to use a semantic element in the wrong way than staying neutral.

<!-- bad -->
<h1>
  <figure>
    <img alt=Company src=logo.png>
  </figure>
</h1>

<!-- good -->
<h1>
  <img alt=Company src=logo.png>
</h1>

Brevity

Keep your code terse. Forget about your old XHTML habits.

<!-- bad -->
<!doctype html>
<html lang=en>
  <head>
    <meta http-equiv=Content-Type content="text/html; charset=utf-8" />
    <title>Contact</title>
    <link rel=stylesheet href=style.css type=text/css />
  </head>
  <body>
    <h1>Contact me</h1>
    <label>
      Email address:
      <input type=email placeholder=[email protected] required=required />
    </label>
    <script src=main.js type=text/javascript></script>
  </body>
</html>

<!-- good -->
<!doctype html>
<html lang=en>
  <meta charset=utf-8>
  <title>Contact</title>
  <link rel=stylesheet href=style.css>

  <h1>Contact me</h1>
  <label>
    Email address:
    <input type=email placeholder=[email protected] required>
  </label>
  <script src=main.js></script>
</html>

Accessibility

Accessibility shouldn't be an afterthought. You don't have to be a WCAG expert to improve your website, you can start immediately by fixing the little things that make a huge difference, such as:

learning to use the alt attribute properly
making sure your links and buttons are marked as such (no <div class=button> atrocities)
not relying exclusively on colors to communicate information
explicitly labelling form controls

<!-- bad -->
<h1><img alt="Logo" src="logo.png"></h1>

<!-- good -->
<h1><img alt="My Company, Inc." src="logo.png"></h1>

Language

While defining the language and character encoding is optional, it's recommended to always declare both at document level, even if they're specified in your HTTP headers. Favor UTF-8 over any other character encoding.

<!-- bad -->
<!doctype html>
<title>Hello, world.</title>

<!-- good -->
<!doctype html>
<html lang=en>
  <meta charset=utf-8>
  <title>Hello, world.</title>
</html>

Performance

Unless there's a valid reason for loading your scripts before your content, don't block the rendering of your page. If your style sheet is heavy, isolate the styles that are absolutely required initially and defer the loading of the secondary declarations in a separate style sheet. Two HTTP requests is significantly slower than one, but the perception of speed is the most important factor.

<!-- bad -->
<!doctype html>
<meta charset=utf-8>
<script src=analytics.js></script>
<title>Hello, world.</title>
<p>...</p>

<!-- good -->
<!doctype html>
<meta charset=utf-8>
<title>Hello, world.</title>
<p>...</p>
<script src=analytics.js></script>

CSS

Semicolons

While the semicolon is technically a separator in CSS, always treat it as a terminator.

/* bad */
div {
  color: red
}

/* good */
div {
  color: red;
}

Box model

The box model should ideally be the same for the entire document. A global * { box-sizing: border-box; } is fine, but don't change the default box model on specific elements if you can avoid it.

/* bad */
div {
  width: 100%;
  padding: 10px;
  box-sizing: border-box;
}

/* good */
div {
  padding: 10px;
}

Flow

Don't change the default behavior of an element if you can avoid it. Keep elements in the natural document flow as much as you can. For example, removing the white-space below an image shouldn't make you change its default display:

/* bad */
img {
  display: block;
}

/* good */
img {
  vertical-align: middle;
}

Similarly, don't take an element off the flow if you can avoid it.

/* bad */
div {
  width: 100px;
  position: absolute;
  right: 0;
}

/* good */
div {
  width: 100px;
  margin-left: auto;
}

Positioning

There are many ways to position elements in CSS but try to restrict yourself to the properties/values below. By order of preference:

display: block;
display: flex;
position: relative;
position: sticky;
position: absolute;
position: fixed;

Selectors

Minimize selectors tightly coupled to the DOM. Consider adding a class to the elements you want to match when your selector exceeds 3 structural pseudo-classes, descendant or sibling combinators.

/* bad */
div:first-of-type :last-child > p ~ *

/* good */
div:first-of-type .info

Avoid overloading your selectors when you don't need to.

/* bad */
img[src$=svg], ul > li:first-child {
  opacity: 0;
}

/* good */
[src$=svg], ul > :first-child {
  opacity: 0;
}

Specificity

Don't make values and selectors hard to override. Minimize the use of id's and avoid !important.

/* bad */
.bar {
  color: green !important;
}
.foo {
  color: red;
}

/* good */
.foo.bar {
  color: green;
}
.foo {
  color: red;
}

Overriding

Overriding styles makes selectors and debugging harder. Avoid it when possible.

/* bad */
li {
  visibility: hidden;
}
li:first-child {
  visibility: visible;
}

/* good */
li + li {
  visibility: hidden;
}

Inheritance

Don't duplicate style declarations that can be inherited.

/* bad */
div h1, div p {
  text-shadow: 0 1px 0 #fff;
}

/* good */
div {
  text-shadow: 0 1px 0 #fff;
}

Brevity

Keep your code terse. Use shorthand properties and avoid using multiple properties when it's not needed.

/* bad */
div {
  transition: all 1s;
  top: 50%;
  margin-top: -10px;
  padding-top: 5px;
  padding-right: 10px;
  padding-bottom: 20px;
  padding-left: 10px;
}

/* good */
div {
  transition: 1s;
  top: calc(50% - 10px);
  padding: 5px 10px 20px;
}

Language

Prefer English over math.

/* bad */
:nth-child(2n + 1) {
  transform: rotate(360deg);
}

/* good */
:nth-child(odd) {
  transform: rotate(1turn);
}

Vendor prefixes

Kill obsolete vendor prefixes aggressively. If you need to use them, insert them before the standard property.

/* bad */
div {
  transform: scale(2);
  -webkit-transform: scale(2);
  -moz-transform: scale(2);
  -ms-transform: scale(2);
  transition: 1s;
  -webkit-transition: 1s;
  -moz-transition: 1s;
  -ms-transition: 1s;
}

/* good */
div {
  -webkit-transform: scale(2);
  transform: scale(2);
  transition: 1s;
}

Animations

Favor transitions over animations. Avoid animating other properties than opacity and transform.

/* bad */
div:hover {
  animation: move 1s forwards;
}
@keyframes move {
  100% {
    margin-left: 100px;
  }
}

/* good */
div:hover {
  transition: 1s;
  transform: translateX(100px);
}

Units

Use unitless values when you can. Favor rem if you use relative units. Prefer seconds over milliseconds.

/* bad */
div {
  margin: 0px;
  font-size: .9em;
  line-height: 22px;
  transition: 500ms;
}

/* good */
div {
  margin: 0;
  font-size: .9rem;
  line-height: 1.5;
  transition: .5s;
}

Colors

If you need transparency, use rgba. Otherwise, always use the hexadecimal format.

/* bad */
div {
  color: hsl(103, 54%, 43%);
}

/* good */
div {
  color: #5a3;
}

Drawing

Avoid HTTP requests when the resources are easily replicable with CSS.

/* bad */
div::before {
  content: url(white-circle.svg);
}

/* good */
div::before {
  content: "";
  display: block;
  width: 20px;
  height: 20px;
  border-radius: 50%;
  background: #fff;
}

Hacks

Don't use them.

/* bad */
div {
  // position: relative;
  transform: translateZ(0);
}

/* good */
div {
  /* position: relative; */
  will-change: transform;
}

JavaScript

Performance

Favor readability, correctness and expressiveness over performance. JavaScript will basically never be your performance bottleneck. Optimize things like image compression, network access and DOM reflows instead. If you remember just one guideline from this document, choose this one.

// bad (albeit way faster)
const arr = [1, 2, 3, 4];
const len = arr.length;
var i = -1;
var result = [];
while (++i < len) {
  var n = arr[i];
  if (n % 2 > 0) continue;
  result.push(n * n);
}

// good
const arr = [1, 2, 3, 4];
const isEven = n => n % 2 == 0;
const square = n => n * n;

const result = arr.filter(isEven).map(square);

Statelessness

Try to keep your functions pure. All functions should ideally produce no side-effects, use no outside data and return new objects instead of mutating existing ones.

// bad
const merge = (target, ...sources) => Object.assign(target, ...sources);
merge({ foo: "foo" }, { bar: "bar" }); // => { foo: "foo", bar: "bar" }

// good
const merge = (...sources) => Object.assign({}, ...sources);
merge({ foo: "foo" }, { bar: "bar" }); // => { foo: "foo", bar: "bar" }

Natives

Rely on native methods as much as possible.

// bad
const toArray = obj => [].slice.call(obj);

// good
const toArray = (() =>
  Array.from ? Array.from : obj => [].slice.call(obj)
)();

Coercion

Embrace implicit coercion when it makes sense. Avoid it otherwise. Don't cargo-cult.

// bad
if (x === undefined || x === null) { ... }

// good
if (x == undefined) { ... }

Loops

Don't use loops as they force you to use mutable objects. Rely on array.prototype methods.

// bad
const sum = arr => {
  var sum = 0;
  var i = -1;
  for (;arr[++i];) {
    sum += arr[i];
  }
  return sum;
};

sum([1, 2, 3]); // => 6

// good
const sum = arr =>
  arr.reduce((x, y) => x + y);

sum([1, 2, 3]); // => 6

If you can't, or if using array.prototype methods is arguably abusive, use recursion.

// bad
const createDivs = howMany => {
  while (howMany--) {
    document.body.insertAdjacentHTML("beforeend", "<div></div>");
  }
};
createDivs(5);

// bad
const createDivs = howMany =>
  [...Array(howMany)].forEach(() =>
    document.body.insertAdjacentHTML("beforeend", "<div></div>")
  );
createDivs(5);

// good
const createDivs = howMany => {
  if (!howMany) return;
  document.body.insertAdjacentHTML("beforeend", "<div></div>");
  return createDivs(howMany - 1);
};
createDivs(5);

Arguments

Forget about the arguments object. The rest parameter is always a better option because:

it's named, so it gives you a better idea of the arguments the function is expecting
it's a real array, which makes it easier to use.

// bad
const sortNumbers = () =>
  Array.prototype.slice.call(arguments).sort();

// good
const sortNumbers = (...numbers) => numbers.sort();

Apply

Forget about apply(). Use the spread operator instead.

const greet = (first, last) => `Hi ${first} ${last}`;
const person = ["John", "Doe"];

// bad
greet.apply(null, person);

// good
greet(...person);

Bind

Don't bind() when there's a more idiomatic approach.

// bad
["foo", "bar"].forEach(func.bind(this));

// good
["foo", "bar"].forEach(func, this);

// bad
const person = {
  first: "John",
  last: "Doe",
  greet() {
    const full = function() {
      return `${this.first} ${this.last}`;
    }.bind(this);
    return `Hello ${full()}`;
  }
}

// good
const person = {
  first: "John",
  last: "Doe",
  greet() {
    const full = () => `${this.first} ${this.last}`;
    return `Hello ${full()}`;
  }
}

Higher-order functions

Avoid nesting functions when you don't have to.

// bad
[1, 2, 3].map(num => String(num));

// good
[1, 2, 3].map(String);

Composition

Avoid multiple nested function calls. Use composition instead.

const plus1 = a => a + 1;
const mult2 = a => a * 2;

// bad
mult2(plus1(5)); // => 12

// good
const pipeline = (...funcs) => val => funcs.reduce((a, b) => b(a), val);
const addThenMult = pipeline(plus1, mult2);
addThenMult(5); // => 12

Caching

Cache feature tests, large data structures and any expensive operation.

// bad
const contains = (arr, value) =>
  Array.prototype.includes
    ? arr.includes(value)
    : arr.some(el => el === value);
contains(["foo", "bar"], "baz"); // => false

// good
const contains = (() =>
  Array.prototype.includes
    ? (arr, value) => arr.includes(value)
    : (arr, value) => arr.some(el => el === value)
)();
contains(["foo", "bar"], "baz"); // => false

Variables

Favor const over let and let over var.

// bad
var obj = {};
obj["foo" + "bar"] = "baz";

// good
const obj = {
  ["foo" + "bar"]: "baz"
};

Conditions

Favor IIFE's and return statements over if, else if, else and switch statements.

// bad
var grade;
if (result < 50)
  grade = "bad";
else if (result < 90)
  grade = "good";
else
  grade = "excellent";

// good
const grade = (() => {
  if (result < 50)
    return "bad";
  if (result < 90)
    return "good";
  return "excellent";
})();

Object iteration

Avoid for...in when you can.

const shared = { foo: "foo" };
const obj = Object.create(shared, {
  bar: {
    value: "bar",
    enumerable: true
  }
});

// bad
for (var prop in obj) {
  if (obj.hasOwnProperty(prop))
    console.log(prop);
}

// good
Object.keys(obj).forEach(prop => console.log(prop));

Objects as Maps

While objects have legitimate use cases, maps are usually a better, more powerful choice. When in doubt, use a Map.

// bad
const me = {
  name: "Ben",
  age: 30
};
var meSize = Object.keys(me).length;
meSize; // => 2
me.country = "Belgium";
meSize++;
meSize; // => 3

// good
const me = Map();
me.set("name", "Ben");
me.set("age", 30);
me.size; // => 2
me.set("country", "Belgium");
me.size; // => 3

Curry

Currying might have its place in other languages, but avoid it in JavaScript. It makes your code harder to read by introducing a foreign paradigm while the appropriate use cases are extremely unusual.

// bad
const sum = a => b => a + b;
sum(5)(3); // => 8

// good
const sum = (a, b) => a + b;
sum(5, 3); // => 8

Readability

Don't obfuscate the intent of your code by using seemingly smart tricks.

// bad
foo || doSomething();

// good
if (!foo) doSomething();

// bad
void function() { /* IIFE */ }();

// good
(function() { /* IIFE */ }());

// bad
const n = ~~3.14;

// good
const n = Math.floor(3.14);

Code reuse

Don't be afraid of creating lots of small, highly composable and reusable functions.

// bad
arr[arr.length - 1];

// good
const first = arr => arr[0];
const last = arr => first(arr.slice(-1));
last(arr);

// bad
const product = (a, b) => a * b;
const triple = n => n * 3;

// good
const product = (a, b) => a * b;
const triple = product.bind(null, 3);

Dependencies

Minimize dependencies. Third-party is code you don't know. Don't load an entire library for just a couple of methods easily replicable:

// bad
var _ = require("underscore");
_.compact(["foo", 0]));
_.unique(["foo", "foo"]);
_.union(["foo"], ["bar"], ["foo"]);

// good
const compact = arr => arr.filter(el => el);
const unique = arr => [...Set(arr)];
const union = (...arr) => unique([].concat(...arr));

compact(["foo", 0]);
unique(["foo", "foo"]);
union(["foo"], ["bar"], ["foo"]);

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Frontend Guidelines

HTML

Semantics

Brevity

Accessibility

Language

Performance

CSS

Semicolons

Box model

Flow

Positioning

Selectors

Specificity

Overriding

Inheritance

Brevity

Language

Vendor prefixes

Animations

Units

Colors

Drawing

Hacks

JavaScript

Performance

Statelessness

Natives

Coercion

Loops

Arguments

Apply

Bind

Higher-order functions

Composition

Caching

Variables

Conditions

Object iteration

Objects as Maps

Curry

Readability

Code reuse

Dependencies

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