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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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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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setProps - depends on reading the last reconciled props from the current reconciled state of the app, at the time of the call. It also depends on an object that doesn't necessarily need to be there outside reconciliation. This is unlike setState, which is state that needs to be there. setState is queued up and merged at the time of reconciliation. Not at the time of the call. setState has a side-effect but is not a stateful nor mutative API.

isMounted - reads the current state of the tree, which may be stale if you're in a batch or reconciliation.

getDOMNode/findDOMNode - Reads the currently flushed node. This currently relies on the state of the system and that everything has flushed at this time. We could potentially do a forced render but that would still rely on the state of the system allowing us to synchronously being able to force a rerender of the system. Note: in 0.14, refs directly to DOM node will resolve to the DOM node. This allow you to get access to a node at the time of its choos

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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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/** @jsx React.DOM */

var Graphic = React.createClass({ 

  componentDidMount: function() {
    var context = this.getDOMNode().getContext('2d');
    this.paint(context);
  },

  componentDidUpdate: function() {

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JSX Spread Attributes

If you know all the properties that you want to place on a component a head of time, it is easy to use JSX:

  var component = <Component foo={x} bar={y} />;

Mutating Props is Bad, mkay

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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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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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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:

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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' }));