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 semantic of the elements you're using. It's worse to use a semantic element in a 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>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 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
altattribute 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>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>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>While the semicolon is technically a separator in CSS, always treat it as a terminator.
/* bad */
div {
color: red
}
/* good */
div {
color: red;
}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;
}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;
}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;
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 .infoAvoid 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;
}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 styles makes selectors and debugging harder. Avoid it when possible.
/* bad */
li {
visibility: hidden;
}
li:first-child {
visibility: visible;
}
/* good */
li + li {
visibility: hidden;
}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;
}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;
}Prefer English over math.
/* bad */
:nth-child(2n + 1) {
transform: rotate(360deg);
}
/* good */
:nth-child(odd) {
transform: rotate(1turn);
}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;
}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);
}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;
}If you need transparency, use rgba. Otherwise, always use the hexadecimal format.
/* bad */
div {
color: hsl(103, 54%, 43%);
}
/* good */
div {
color: #5a3;
}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;
}Don't use them.
/* bad */
div {
// position: relative;
transform: translateZ(0);
}
/* good */
div {
/* position: relative; */
will-change: transform;
}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);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" }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)
)();Embrace implicit coercion when it makes sense. Avoid it otherwise. Don't cargo-cult.
// bad
if (x === undefined || x === null) { ... }
// good
if (x == undefined) { ... }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]); // => 6If 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);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();Forget about apply(). Use the spread operator instead.
const greet = (first, last) => `Hi ${first} ${last}`;
const person = ["John", "Doe"];
// bad
greet.apply(null, person);
// good
greet(...person);Don't bind() when there's a more idiomatic approach.
// bad
["foo", "bar"].forEach(func.bind(this));
// good
["foo", "bar"].forEach(func, this);// bad
const person = {
first: "John",
last: "Doe",
greet() {
const full = function() {
return `${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()}`;
}
}Avoid nesting functions when you don't have to.
// bad
[1, 2, 3].map(num => String(num));
// good
[1, 2, 3].map(String);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); // => 12Cache 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"); // => falseFavor const over let and let over var.
// bad
var obj = {};
obj["foo" + "bar"] = "baz";
// good
const obj = {
["foo" + "bar"]: "baz"
};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";
})();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));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; // => 3Currying 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); // => 8Don'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);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);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"]);