React/JSX Style Guide

A mostly reasonable approach to React and JSX

This style guide is mostly based on the standards that are currently prevalent in JavaScript, although some conventions (i.e async/await or static class fields) may still be included or prohibited on a case-by-case basis. Currently, anything prior to stage 3 is not included nor recommended in this guide.

Basic Rules

Only include one React component per file.
- However, multiple simple Functional sub-components are allowed per file. eslint: react/no-multi-comp.
Always use JSX syntax.
Do not use React.createElement unless you’re initializing the app from a file that is not JSX.
react/forbid-prop-types will allow arrays and objects only if it is explicitly noted what array and object contains, using arrayOf, objectOf, or shape.

Hooks vs Class vs `React.createClass` vs stateless

Prefer functional components with Hooks over class extends React.Component (when possible) or React.createClass. eslint: react/prefer-es6-class react/prefer-stateless-function

// bad
const Listing = React.createClass({
  // ...
  render() {
    return <div>{this.state.hello}</div>;
  }
});

// good
class Listing extends React.Component {
  // ...
  render() {
    return <div>{this.state.hello}</div>;
  }
}

And if you can accomplish the same with Hooks:

// bad
class Listing extends React.Component {
  render() {
    return <div>{this.props.hello}</div>;
  }
}

// good
const Listing = ({ hello }) => (
  <div>{hello}</div>
);

Mixins

Do not use mixins.

Why? It's a legacy feature not meant to be used in modern React applications.

Naming

Extensions: Use .jsx extension for React components. eslint: react/jsx-filename-extension
Filename: Use PascalCase for filenames. E.g., ReservationCard.jsx.

Reference Naming: Use PascalCase for React components and camelCase for their instances. eslint: react/jsx-pascal-case

// bad
import reservationCard from './ReservationCard';

// good
import ReservationCard from './ReservationCard';

// bad
const ReservationItem = <ReservationCard />;

// good
const reservationItem = <ReservationCard />;

Component Naming: Use the filename as the component name. For example, ReservationCard.jsx should have a reference name of ReservationCard. However, for root components of a directory, use index.jsx as the filename and use the directory name as the component name:
```
// bad
import Footer from './Footer/Footer';

// bad
import Footer from './Footer/index';

// good
import Footer from './Footer';
```

Higher-order Component Naming: Use a composite of the higher-order component’s name and the passed-in component’s name as the displayName on the generated component. For example, the higher-order component withFoo(), when passed a component Bar should produce a component with a displayName of withFoo(Bar).

Why? A component’s displayName may be used by developer tools or in error messages, and having a value that clearly expresses this relationship helps people understand what is happening.

// bad
export default function withFoo(WrappedComponent) {
  return function WithFoo(props) {
    return <WrappedComponent {...props} foo />;
  }
}

// good
export default function withFoo(WrappedComponent) {
  function WithFoo(props) {
    return <WrappedComponent {...props} foo />;
  }

  const wrappedComponentName = WrappedComponent.displayName
    || WrappedComponent.name
    || 'Component';

  WithFoo.displayName = `withFoo(${wrappedComponentName})`;
  return WithFoo;
}

Props Naming: Avoid using DOM component prop names for different purposes.

Why? People expect props like style and className to mean one specific thing. Varying this API for a subset of your app makes the code less readable and less maintainable, and may cause bugs.
```
// bad
<MyComponent style="fancy" />

// bad
<MyComponent className="fancy" />

// good
<MyComponent variant="fancy" />
```

Alignment

Follow these alignment styles for JSX syntax. eslint: react/jsx-closing-bracket-location react/jsx-closing-tag-location

// bad
<Foo superLongParam="bar"
     anotherSuperLongParam="baz" />

// good
<Foo
  superLongParam="bar"
  anotherSuperLongParam="baz"
/>

// if props fit in one line then keep it on the same line
<Foo bar="bar" />

// children get indented normally
<Foo
  superLongParam="bar"
  anotherSuperLongParam="baz"
>
  <Quux />
</Foo>

// bad
{showButton &&
  <Button />
}

// bad
{
  showButton &&
    <Button />
}

// good
{showButton && (
  <Button />
)}

// good
{showButton && <Button />}

Quotes

Always use double quotes (") for JSX attributes, but single quotes (') for all other JS. eslint: jsx-quotes

Why? Regular HTML attributes also typically use double quotes instead of single, so JSX attributes mirror this convention.
```
// bad
<Foo bar='bar' />

// good
<Foo bar="bar" />

// bad
<Foo style={{ left: "20px" }} />

// good
<Foo style={{ left: '20px' }} />
```

Spacing

Always include a single space in your self-closing tag. eslint: no-multi-spaces, react/jsx-tag-spacing

// bad
<Foo/>

// very bad
<Foo                 />

// bad
<Foo
 />

// good
<Foo />

Do not pad JSX curly braces with spaces. eslint: react/jsx-curly-spacing
```
// bad
<Foo bar={ baz } />

// good
<Foo bar={baz} />
```

Props

Always use camelCase for prop names.

// bad
<Foo
  UserName="hello"
  phone_number={12345678}
/>

// good
<Foo
  userName="hello"
  phoneNumber={12345678}
/>

Omit the value of the prop when it is explicitly true. eslint: react/jsx-boolean-value

// bad
<Foo
  hidden={true}
/>

// good
<Foo
  hidden
/>

// good
<Foo hidden />

Always include an alt prop on <img> tags. If the image is presentational, alt can be an empty string or the <img> must have role="presentation". eslint: jsx-a11y/alt-text

// bad
<img src="hello.jpg" />

// good
<img src="hello.jpg" alt="Me waving hello" />

// good
<img src="hello.jpg" alt="" />

// good
<img src="hello.jpg" role="presentation" />

Do not use words like "image", "photo", or "picture" in <img> alt props. eslint: jsx-a11y/img-redundant-alt

Why? Screenreaders already announce img elements as images, so there is no need to include this information in the alt text.
```
// bad
<img src="hello.jpg" alt="Picture of me waving hello" />

// good
<img src="hello.jpg" alt="Me waving hello" />
```

Use only valid, non-abstract ARIA roles. eslint: jsx-a11y/aria-role

// bad - not an ARIA role
<div role="datepicker" />

// bad - abstract ARIA role
<div role="range" />

// good
<div role="button" />

Do not use accessKey on elements. eslint: jsx-a11y/no-access-key

Why? Inconsistencies between keyboard shortcuts and keyboard commands used by people using screenreaders and keyboards complicate accessibility.

// bad
<div accessKey="h" />

// good
<div />

Avoid using an array index as key prop, prefer a stable ID. eslint: react/no-array-index-key

Why? Not using a stable ID is an anti-pattern because it can negatively impact performance and cause issues with component state.

We don’t recommend using indexes for keys if the order of items may change.

// bad
{todos.map((todo, index) =>
  <Todo
    {...todo}
    key={index}
  />
)}

// good
{todos.map(todo => (
  <Todo
    {...todo}
    key={todo.id}
  />
))}

Use spread props sparingly.

Why? Otherwise you’re more likely to pass unnecessary props down to components. And for React v15.6.1 and older, you could pass invalid HTML attributes to the DOM.

Exceptions:

HOCs that proxy down props and hoist propTypes

function HOC(WrappedComponent) {
  return class Proxy extends React.Component {
    Proxy.propTypes = {
      text: PropTypes.string,
      isLoading: PropTypes.bool
    };

    render() {
      return <WrappedComponent {...this.props} />
    }
  }
}

Spreading objects with known, explicit props. This can be particularly useful when testing React components with Mocha’s beforeEach construct.

export default function Foo {
  const props = {
    text: '',
    isPublished: false
  }

  return (<div {...props} />);
}

Notes for use: Filter out unnecessary props when possible. Also, use prop-types-exact to help prevent bugs.

// bad
render() {
  const { irrelevantProp, ...relevantProps  } = this.props;
  return <WrappedComponent {...this.props} />
}

// good
render() {
  const { irrelevantProp, ...relevantProps  } = this.props;
  return <WrappedComponent {...relevantProps} />
}

Refs

Always use ref callbacks, createRef or useRef hook. eslint: react/no-string-refs

// bad
<Foo
  ref="myRef"
/>

// good
<Foo
  ref={(ref) => { this.myRef = ref; }}
/>

Parentheses

Wrap JSX tags in parentheses when they span more than one line. eslint: react/jsx-wrap-multilines

// bad
render() {
  return <MyComponent variant="long body" foo="bar">
           <MyChild />
         </MyComponent>;
}

// good
render() {
  return (
    <MyComponent variant="long body" foo="bar">
      <MyChild />
    </MyComponent>
  );
}

// good, when single line
render() {
  const body = <div>hello</div>;
  return <MyComponent>{body}</MyComponent>;
}

Methods

Bind event handlers for the render method in the constructor. eslint: react/jsx-no-bind

Why? A bind call in the render path creates a brand new function on every single render. Do not use arrow functions in class fields, because it makes them challenging to test and debug, and can negatively impact performance, and because conceptually, class fields are for data, not logic.

// bad
class extends React.Component {
  handleClickDiv() {
    // do stuff
  }

  render() {
    return <div onClick={this.handleClickDiv.bind(this)} />;
  }
}

// bad
class extends React.Component {
  handleClickDiv = () => {
    // do stuff
  }

  render() {
    return <div onClick={this.handleClickDiv} />
  }
}

// good
class extends React.Component {
  constructor(props) {
    super(props);

    this.handleClickDiv = this.handleClickDiv.bind(this);
  }

  handleClickDiv() {
    // do stuff
  }

  render() {
    return <div onClick={this.handleClickDiv} />;
  }
}

Do not use underscore prefix for internal methods of a React component.

Why? Underscore prefixes are sometimes used as a convention in other languages to denote privacy. But, unlike those languages, there is no native support for privacy in JavaScript, everything is public. Regardless of your intentions, adding underscore prefixes to your properties does not actually make them private, and any property (underscore-prefixed or not) should be treated as being public.
```
// bad
React.createClass({
  _handleClickSubmit() {
    // do stuff
  },

  // other stuff
});

// good
class extends React.Component {
  handleClickSubmit() {
    // do stuff
  }

  // other stuff
}
```

Prefix event handlers with handle

Why? It helps differentiate between props and class methods.

// bad
class extends React.Component {
  onClickSubmit() {
    // do stuff
  }

  // other stuff
}

// good
class extends React.Component {
  handleClickSubmit() {
    // do stuff
  }

  // other stuff
}

Be sure to return a value in your render methods. eslint: react/require-render-return

// bad
render() {
  (<div />);
}

// good
render() {
  return (<div />);
}

Ordering

Ordering for class extends React.Component:

constructor
componentWillMount
componentDidMount
componentWillReceiveProps
shouldComponentUpdate
componentWillUpdate
componentDidUpdate
componentWillUnmount
getChildContext
getDefaultState
*getter and setter methods* like getSelectReason()orsetValue()`
clickHandlers or eventHandlers like handleClickSubmit() or handleChangeDescription()
optional render methods like renderNavigation() or renderProfilePicture()
render

How to define propTypes, defaultProps, contextTypes, etc...

import React from 'react';
import PropTypes from 'prop-types';

const propTypes = {
  id: PropTypes.number.isRequired,
  url: PropTypes.string.isRequired,
  text: PropTypes.string,
};

const defaultProps = {
  text: 'Hello World',
};

class Link extends React.Component {
  static methodsAreOk() {
    return true;
  }

  render() {
    return <a href={this.props.url} data-id={this.props.id}>{this.props.text}</a>;
  }
}

Link.propTypes = propTypes;
Link.defaultProps = defaultProps;

export default Link;

Hooks

Try to use functional components with Hooks whenever possible. Follow rules of hooks and be aware of the need to use useMemo and useCallback to avoid unnecessary rerenders.

Pay close attention to useEffect and try to define all of the variables inside of the hook body to avoid creating too many dependencies.

const Button = () => {
  const myFn = () => {...};
  // bad, myFn is now a dynamic dependency
  useEffect(() => myFn(), []);
}

const Button = () => {
  // good
  useEffect(() => {
    const myFn = () => {...};
    return myFn();
  }, []);
}

CSS Modules

Contrary to 'global css', CSS modules do not allow overriding styles inside components out of the box.

Use className prop to override styles of the root element.

// signup_button.js
import styles from "signup_button.module.scss"

<Button className={styles.button} />

// button.js
import styles from "button.module.scss"

const Button = ({ className: passedClassName }) => {
  const className = clsx(styles.button, passedClassName);
  return <div className={className}>Click me</button>;
};

Use theme prop to override multiple styles inside a component

// signup_button.js
import buttonStyles from "signup_button.module.scss"

<Button theme={buttonStyles} />

// button.js
import styles from "button.module.scss"

const Button = ({ theme }) => {
  const className = clsx(styles.button, theme.button);
  const iconClassName = clsx(styles.icon, theme.icon);
  return (
    <div className={className}>
      Click me
      <span className={iconClassName}>icon</span>
    </button>
  );
};

If you need to pass custom style names make sure they are mapped outside of the Component render call. That way the object remains the same between renders.

const Button = () => {
  // bad
  const theme = { ... };
  return <Button theme={buttonStyles} />;
}

// good
const theme = { ... };

const Button = () => {
  return <Button theme={buttonStyles} />;
}

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README.md

README.md

React/JSX Style Guide

Table of Contents

Basic Rules

Hooks vs Class vs `React.createClass` vs stateless

Mixins

Naming

Alignment

Quotes

Spacing

Props

Refs

Parentheses

Tags

Methods

Ordering

Hooks

CSS Modules

Files

README.md

Latest commit

History

README.md

File metadata and controls

React/JSX Style Guide

Table of Contents

Basic Rules

Hooks vs Class vs React.createClass vs stateless

Mixins

Naming

Alignment

Quotes

Spacing

Props

Refs

Parentheses

Tags

Methods

Ordering

Hooks

CSS Modules

Hooks vs Class vs `React.createClass` vs stateless