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The Rules of React

All libraries have subtle rules that you have to follow for them to work well. Often these are implied and undocumented rules that you have to learn as you go. This is an attempt to document the rules of React renders. Ideally a type system could enforce it.

What Functions Are "Pure"?

A number of methods in React are assumed to be "pure".

On classes that's the constructor, getDerivedStateFromProps, shouldComponentUpdate and render.

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var Bar1 = base => class extends base {
  componentWillMount(){
    super.componentWillMount();
    console.log('Bar1');
  }
};

var Bar2 = base => class extends base {
  componentWillMount(){
    super.componentWillMount();

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let cache = new Map();
let pending = new Map();

function fetchTextSync(url) {
  if (cache.has(url)) {
    return cache.get(url);
  }
  if (pending.has(url)) {
    throw pending.get(url);
  }

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import { Component } from "React";

export var Enhance = ComposedComponent => class extends Component {
  constructor() {
    this.state = { data: null };
  }
  componentDidMount() {
    this.setState({ data: 'Hello' });
  }
  render() {

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I heard some points of criticism to how React deals with reactivity and it's focus on "purity". It's interesting because there are really two approaches evolving. There's a mutable + change tracking approach and there's an immutability + referential equality testing approach. It's difficult to mix and match them when you build new features on top. So that's why React has been pushing a bit harder on immutability lately to be able to build on top of it. Both have various tradeoffs but others are doing good research in other areas, so we've decided to focus on this direction and see where it leads us.

I did want to address a few points that I didn't see get enough consideration around the tradeoffs. So here's a small brain dump.

"Compiled output results in smaller apps" - E.g. Svelte apps start smaller but the compiler output is 3-4x larger per component than the equivalent VDOM approach. This is mostly due to the code that is usually shared in the VDOM "VM" needs to be inlined into each component. The tr

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function asyncToReact(fn) {
  class PromiseComponent extends React.Component {
    state = { waiting: true, result: null };
    componentDidMount() {
      fn(...this.props.args).then(result => this.setState({ waiting: false, result }));
    }
    componentDidUpdate() {
      fn(...this.props.args).then(result => this.setState({ waiting: false, result }));
    }
    shouldComponentUpdate(newProps) {

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React (Virtual) DOM Terminology

In React's terminology, there are five core types that are important to distinguish:

• ReactElement / ReactElement Factory
• ReactNode
• ReactComponent / ReactComponent Class

React Elements

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Deprecating transferPropsTo

It's a common pattern in React to wrap a component in an abstraction. The outer component exposes a simple property to do something that might have more complex implementation details.

We used to have a helper function called transferPropsTo. We no longer support this method. Instead you're expected to use a generic object helper to merge props.

render() {
 return Component(Object.assign({}, this.props, { more: 'values' }));

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New React Element Factories and JSX

In React 0.12, we're making a core change to how React.createClass(...) and JSX works.

If you're using JSX in the typical way for all (and only) React components, then this transition will be seamless. Otherwise there are some minor breaking changes described below.

The Problem

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Lower Case JSX Convention

All lower case JSX tags will now be treated as HTML/SVG elements. They will no longer be treated as custom components in scope.

The React element produced by JSX can be either a React class that exists in the local scope or a global scope HTML/SVG element depending on a convention.

Previous Behavior

Currently, when you use React JSX to define a HTML element you can use any known HTML tag. E.g: