React Code Style Guide

Our React projects' best practices

Introduction

This is meant to be a guide to help new developers understand the React code style and best practices we adopt here at Pagar.me.

As this guide is an extension of our JavaScript style guide, we highly recommend reading it before you continue.

Installing

The rules described in this repository are also available as a NPM package. To install the package and its dependencies:

$ npm install --save-dev eslint@4.19.1 \
                         eslint-config-pagarme-react \
                         stylelint@8.0.0 \
                         stylelint-config-pagarme-react \

The peer dependencies specified above have hardcoded versions. If you prefer, you can use the command npm info eslint-config-pagarme-react@latest peerDependencies to find the exact peer dependencies to install.

To include these rules into your project, create the following config files in your root folder:

.eslintrc

{
  "extends": ["pagarme-react"],
  "env": {
    "browser": true
  }
}

.stylelintrc

{
  "extends": ["stylelint-config-pagarme-react"]
}

Table of contents

• Component definition
• Project organization
  • Presentational Components
  • Container Components
  • Page Containers
• Formatting CSS
  • 80 columns, soft tabs of 2 spaces
  • Camel case instead of dash-case for class names
  • Never use ID and tag name as root selectors!
  • When using multiple selectors, give each selector its own line
  • Break lines in CSS function arguments
  • When writing rules, be sure to
• Design Patterns
  • The parent constrains the child
  • The parent doesn't assume child structure
  • Components never leak margin
  • The parent spaces the children
  • Nested classes aren't for providing scope
  • Variables, lots of variables!

Component definition

All components (presentation, containers or pages) should always be defined as a directory, named with pascal casing. The main component file should be index.js, main stylesheet style.css. CSS custom properties can be kept in properties.css:

AwesomeCard/
├── index.js
├── properties.css
└── style.css

• Styles should always be defined in a separate CSS file
• Avoid prefixing or suffixing component names
  • E.g.: lib/pages/UserPage or lib/container/UserContainer
• On conflict rename on import time
  • import UserContainer from '...'  - import { User as UserContainer } from '...'

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Project organization

Your project components should be separated in at least three directories:

awesome-react-project/
└── src/
   ├── components/
   ├── containers/
   └── pages/

Each of these directories have special types of components:

components/

Stateless components. Shouldn't store state. Most components in this directory will be function-based components. Stuff like buttons, inputs, labels and all presentational components goes here. This components can also accept functions as props and dispatch events, but no state should be held inside.

containers/

Container components can store state. Containers are built mostly from the composition of presentational components with some styles to layout them together. Containers can also store internal state and access refs to provide additional logic, but all actions should be accepted as component callbacks.

pages/

Page components can store state, receive route parameters and dispatch Redux actions when applicable. Pages are the highest level of application's components. They represent the application routes and most times are displayed by a router. Pages are also responsible for handling container components callbacks and flowing data into children containers.

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Code standards

Destruct your props

More than 2 props from an object been used in the same place should be destructed

Code style

Line length should not exceed 80 characters.

Use explanatory variables

Bad

const onlyNumbersRegex = /^\d+$/
const validateNumber = message => value => !onlyNumbersRegex.test(value) && message

validateNumber('error message')(1)

Good

const onlyNumbersRegex = /^\d+$/
const getNumberValidation = message => value => !onlyNumbersRegex.test(value) && message

const isNumber = getNumberValidation('error message')

isNumber(1)

Use searchable names

Bad

setTimeout(doSomething, 86400000)

Good

const DAY_IN_MILLISECONDS = 86400000

setTimeout(doSomething, DAY_IN_MILLISECONDS)

Naming

Test files must start with the class which will be tested followed by .test.

Class and components folders names must start with capital letter.

React peculiarities

Never "promisify" the setState

It's a small anti-pattern which can cause some problems in the component lifecicle. You can found more arguments about this in this issue

Mixins

• Do not use mixins

Why? Mixins introduce implicit dependencies, cause name clashes, and cause snowballing complexity. Most use cases for mixins can be accomplished in better ways via components, higher-order components, or utility modules.

Components

One line props when are more than 2 or big props

Bad

<button type="submit" disabled onClick={() => null} className="aLongSpecificClassName">
  Click here
</button>

<button type="submit" className="aLongSpecificClassName">
  Click here
</button>

<button className="aLongSpecificClassName">Click here</button>

Good

<button
  className="aLongSpecificClassNameWithLasers"
  disabled={loading}
  onClick={() => null}
  type="submit"
>
  Click here
</button>

One line component

Bad

<div className="example"><span class="highlight">Bad</span> example</div>

Good

<div className="example">
  <span className="highlight">Bad</span>
  example
</div>

CSS are modules!

We use CSS modules everywhere. CSS modules are great because they provide scope to CSS and allow us to create compartmentalized styles that don't leak to global scope. Here are our good practices of doing CSS modules:

Formatting CSS

80 columns, soft tabs of 2 spaces

Keep your code lines under 80 columns wide. This helps when opening multiple splits. Use soft tabs of 2 spaces to save some space! 😛

Camel case instead of dash-case for class names

With CSS modules, camel case makes much more sense:

import style from './style.css'

const Item = ({ children }) =>
  <li className={style.circleBullet}>
    {children}
  </li>

export default Item

.circleBullet {
  list-style-type: disc;
}

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Never use ID and tag name as root selectors!

Using ID and tag name at the selector's root makes the rule to be applied globally.

import style from './style.css'

const Item = ({ title, thumbnail }) =>
  <div className={style.container}>
    <img src={thumbnail} alt={title} />
  </div>

export default Item

.container > img {
  background-color: #CCCCCC;
}

import style from './style.css'

const Item = ({ title, thumbnail }) =>
  <div className={style.container}>
    <img src={thumbnail} alt={title} />
  </div>

export default Item

img {
  background-color: #CCCCCC;
}

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When using multiple selectors, give each selector its own line

Organize one selector per line, even when placing all of them at the same line doesn't exceed 80 columns.

.container > img,
.container > div,
.container > section {
  background-color: #CCCCCC;
}

.container > img, .container > div, .container > section {
  background-color: #CCCCCC;
}

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Break lines in CSS function arguments

Sometimes, not to exceed the 80 columns limit, you need to break lines. While at it, be sure to do it right after the colon, and keep at one argument per line.

.container {
  background-color:
    linear-gradient(
      0deg,
      var(--color-light-yellow-12),
      var(--color-light-yellow-10),
    );
}

.container {
  background-color: linear-gradient(0deg, --color-light...
}

.container {
  background-color: linear-gradient(
    0deg, var(--color-light-yellow-12), var(--color-lig...
}

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When writing rules, be sure to

• Put a space before the opening brace {
• In properties put a space after (but not before) the : character
• Put closing braces } of rule declarations on a new line
• Leave ONE blank line in between rule declarations

.container {
  font-size: 12pt;
}

.thumbnail {
  width: 160px;
  height: 90px;
}

.container{
  font-size:12pt;}
.thumbnail{
  width:160px;
  height:90px;}

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CSS Design Patterns

The parent constrains the child

Leaf components shouldn't constrain width or height (unless it makes sense). That said, most components should default to fill the parent:

import style from './style.css'

const Input = ({ children }) =>
  <input className={style.input}>
    {children}
  </input>

export default Input

.input {
  box-sizing: border-box;
  padding: 10px;
  width: 100%;
}

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The parent doesn't assume the child's structure

Sometimes we don't want to fill the whole width by default. An example is the button component, which we want to resize itself based on title width.

In this cases, we should allow the parent component to inject styles into the child component's container. The child is responsible for choosing where parent styles are injected.

For merging styles, always use classnames package. The rightmost arguments overrides the leftmost ones.

import classNames from 'classnames'
import style from './style.css'

const Button = ({ children, className }) =>
  <button className={classNames(style.button, className)}>
    {children}
  </button>

export default Button

.button {
  box-sizing: border-box;
  padding: 10px;
  width: 100%;
}

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Components never leak margin

All components are self-contained and their final size should never suffer margin leakage! This allows the components to be much more reusable!

|--|-content size-|--| margin
 ____________________
|   ______________   | | margin
|  |              |  |
|  |              |  |
|  |              |  |
|  |______________|  |
|____________________| | margin

|---container size---|

   |-content size-|
    ______________
   |              |
   |              |
   |              |
   |______________|

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The parent spaces the children

When building lists or grids:

• Build list/grid items as separate components
• Use the the list/grid container to space children
• To space them horizontally, use margin-left
• To space them vertically, use margin-top
• Select the first-child to reset margins

import style from './style.css'

const Reviews = ({ items }) =>
  <div className={style.container}>
    {items.map(item =>
      <img src={item.image} alt={item.title} />
    )}
  </div>

export default Reviews

.container > img {
  margin-left: 10px;
}

.container > img:first-child {
  margin-left: unset;
}

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Nested classes aren't for providing scope

CSS modules already provides us scope. We don't need to use nested classes for providing scope isolation. Use nested class selectors for modifying children based on parent class. A use case is when a component is in error or success state:

import style from './style.css'

const Button = ({ children }) =>
  <button className={style.button}>
    <img className={style.icon} />
    {children}
  </button>

export default Button

.button {
  box-sizing: border-box;
  padding: 10px;
  width: 100%;
}

.button .icon {
  width: 22px;
  height: 22px;
}

import style from './style.css'

const Input = ({ value, onChange, error }) =>
  <div className={classNames({ [style.error]: error })}>
    <input onChange={onChange} />
    <p>{error}</p>
  </div>

export default Input

.error p {
  color: red;
  display: unset;
}

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Variables, lots of variables!

We encourage the "variabilification". Always define variables to increase reuse and make styles more consistent. The CSS4 specification defines a way to declare native variables. We adopted them as the standard.

To define a variable accessible globally:

:root {
  --color-green-1: #6CCFAE;
  --color-green-2: #6B66B5;
  --color-green-3: #AAC257;
  --color-green-4: #68B5C1;
}

.container {
  background-color:
    linear-gradient(
      0deg,
      var(--color-green-1),
      var(--color-green-2)
    );
}

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