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React style guide

Follow the normal JavaScript style guide - including the 80 character line limit. In addition, there are several React-specific rules.

In addition to these style rules, you may also be interested in React best practices.


Use ES2015 classes.

  1. Use static properties for defaultProps.

  2. Use an instance property for state.

  3. Use flow for props and state.

  4. Autobind event handlers and callbacks.


    import React, {Component} from 'react';
    class Foo extends Component {
        static defaultProps = {}
        state = {}
        state: {}
        props: {}
        handleClick = (e) => {
            // handle the click
  5. If state depends on props, define it in the constructor.


    class Bar extends Component {
        constructor(props) {
            super(props);   // must be called first
            this.state = {
                value: props.value,
  6. Use higher order components instead of mixins. ES6 style classes do not support mixins. See the mixins considered harmful blog post for details of how to convert mixins to higher order components.


Converting legacy components to use ES2015 classes? There's a codemod for that.

Convert one or more files with the following command:

$ tools/react-codemod.js class path/to/file path/to/other/file

Component method and property ordering

Ordering within a React component is strict. The following example illustrates the precise ordering of various component methods and properties:

class Foo extends Component {
    // Static properties
    static defaultProps = {}

    // The `constructor` method
    constructor() {

    // Instance properties
    state = { hi: 5}

    // Flow `state` annotation
    // NOTE: `state` is special. All other Flow annotations live below.
    state: { hi: number }

    // React lifecycle hooks.
    // They should follow their chronological ordering:
    // 1. componentWillMount
    // 2. componentDidMount
    // 3. componentWillReceiveProps
    // 4. shouldComponentUpdate
    // 5. componentWillUpdate
    // 6. componentDidUpdate
    // 7. componentWillUnmount
    componentDidMount() { ... }

    // All other Flow annotations
    props: { ... }
    someRandomData: string,

    // All other instance methods
    handleClick = (e) => { ... }

    // Finally, the render method
    render() { ... }

Name handlers handleEventName.


<Component onClick={this.handleClick} onLaunchMissiles={this.handleLaunchMissiles} />

Name handlers in props onEventName.

This is consistent with React's event naming: onClick, onDrag, onChange, etc.


<Component onLaunchMissiles={this.handleLaunchMissiles} />

Open elements on the same line.

The 80-character line limit is a bit tight, so we opt to conserve the extra 4.


return <div>


return (      // "div" is not on the same line as "return"

Align and sort HTML properties.

Fit them all on the same line if you can. If you can't, put each property on a line of its own, indented four spaces, in sorted order. The closing angle brace should be on a line of its own, indented the same as the opening angle brace. This makes it easy to see the props at a glance.


<div className="highlight" key="highlight-div">


<div className="highlight"      // property not on its own line
<div                            // closing brace not on its own line
<div                            // property is not sorted

Only export a single react class.

Every .jsx file should export a single React class, and nothing else. This is for testability; the fixture framework requires it to function.

Note that the file can still define multiple classes, it just can't export more than one.

Language features

Make "presentation" components pure.

It's useful to think of the React world as divided into "logic" components and "presentation" components.

"Logic" components have application logic, but do not emit HTML themselves.

"Presentation" components are typically reusable, and do emit HTML.

Logic components can have internal state, but presentation components never should.

Prefer props to state.

You almost always want to use props. By avoiding state when possible, you minimize redundancy, making it easier to reason about your application.

A common pattern — which matches the "logic" vs. "presentation" component distinction — is to create several stateless components that just render data, and have a stateful component above them in the hierarchy that passes its state to its children via props. The stateful component encapsulates all of the interaction logic, while the stateless components take care of rendering data in a declarative way.

Copying data from props to state can cause the UI to get out of sync and is especially bad.

Never store state in the DOM.

Do not use data- attributes or classes. All information should be stored in JavaScript, either in the React component itself, or in a React store if using a framework such as Redux.

Type-checking with Flow

Flow is a type-checker that runs at compile-time to catch issues and prevent bugs. It should be used on all new components.

For more information on how we use Flow, check the Javascript style guide

Use Flow instead of PropTypes

Props can now be validated with Flow instead of React's PropTypes. Flow provides a much more expressive way to set types for props, with the additional benefits of being able to annotate state (and any additional methods or data).

Types can be defined on the class itself:

class Foo extends Component {
    props: {
        name: string,
        uniqueId: number,
        complexData: {
            setOfThings: Array<string>,

If you're writing a Stateless Functional Component, or if you use lifecycle methods that reference props (eg. componentDidUpdate), you may find it helpful to define a Props type, and reference it:

type Props = {
    numbers: Array<number>,

class Foo extends Component {
    props: Props

    shouldComponentUpdate(nextProps: Props) {

const StatelessFoo = ({numbers}: Props) => {

Annotate children

children is something of a special prop in React. Most often, you'll want to pass a React element (or an array of React elements).

This can be annotated like so:

children: React$Element<any> | Array<React$Element<any>>

Note that this is only the most common use-case for children. Children can also be other types (like a string, or a function).

Server-side rendering

To make components safe to render server-side, they must adhere to a few more restrictions than regular components.

Props must be plain JSON

In order to render server-side, the props are serialized and deserialized from JSON. This means that e.g. dates must be passed as timestamps as opposed to JS Date objects.

Note that this only applies to the root component being rendered with server-side rendering. If the root component constructs more complex data structures from props, and passes those constructs down to child components, that won't cause problems.

Pure functions of props and state

Components must be pure functions of their props and state. This means the output of their render() function must not depend on either KA-specific globals, on browser-specific state, or browser-specific APIs.

Examples of KA-specific globals include anything attached to the KA global, e.g. KA.getUserId(), or data extracted from the DOM such as data- properties attached to other DOM nodes.

Examples of browser-specific state include the user agent, the screen resolution, the device pixel density etc.

An example of a browser-specific API is canvas.getContext().

The output must be deterministic. One way to get non-deterministic output is to generate random IDs in getInitialState(), and have the output of render depend on that ID. Don't do this.

Side effect free until componentDidMount

The parts of the React component lifecycle that are run to render on the server must be free from side effects.

Examples of side effects that must be avoided:

  • Sending an AJAX request
  • Mutating global JS state
  • Injecting elements into the DOM
  • Changing the page title
  • alert()

The lifecycle methods that run on the server are currently:

  • getInitialState()
  • getDefaultProps()
  • componentWillMount()
  • render()

If you need to execute any of the above listed side effects, you must do so in componentDidMount or later in the component lifecycle. These functions are not executed server-side.

React libraries and components

Do not use Backbone models.

Use redux actions, or fetch directly, instead.

We are trying to remove Backbone from our codebase entirely.

Minimize use of jQuery.

Never use jQuery for DOM manipulation.

Try to avoid using jQuery plugins. When necessary, wrap the jQuery plugin with a React component so you only have to touch the jQuery once.

Use the fetch API (accessible via khanFetch) instead of $.ajax.

For more information on khanFetch, see the javascript style guide.

Reuse standard components.

If possible, re-use existing components, especially low-level, pure components that emit HTML directly. If you write a new such one, and it finds a use in a different project, put it in a shared location such as the react.js package.

The standard shared location for useful components that have been open sourced is the react-components.js package in javascript-packages.json. This includes components such as these:

  • SetIntervalMixin - provides a setInterval method so something can be done every x milliseconds
  • $_ - the i18n wrapper to allow for translating text in React.
  • TimeAgo - “five minutes ago”, etc - this replaces $.timeago

Reusable components that have not (yet) been open sourced are in the (poorly named) react.js package. This include components such as these:

  • KUIButton - render a Khan Academy styled button.
  • Modal - create a modal dialog.

This list is far from complete.