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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.

View Infrastructure.js
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);
View SubscriptionIsHard.js
class SubscriptionDoneRight extends Component {
state = {
getDerivedStateFromProps(props, state) {
return {
View propertynames.txt
View API.js
type FrameScope = {
[key:string]: mixed
type StackFrame = {
name?: string,
fileName?: string,
lineNumber?: number,
columnNumber?: number,
scope?: FrameScope
View shortout.js
(function() {
/** Used to detect hot functions by number of calls within a span of milliseconds. */
var HOT_COUNT = 800,
HOT_SPAN = 16;
/* Built-in method references for those with the same name as other `lodash` methods. */
var nativeNow =;

Tagged Objects

The goal is to implement a form of pattern matching that works well in the existing dynamic environment of ECMAScript.

The goal is to find a way to do efficient pattern matching such as using an object tag. JS VMs already have a field for this that is used to tag various kinds of built-in objects.

This tag could be extended to also include a user space range.

The Mechanism

View asyncToReact.js
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) {
* React v16.0.0-alpha
* Copyright 2013-present, Facebook, Inc.
* All rights reserved.
* This source code is licensed under the BSD-style license found in the
* LICENSE file in the root directory of this source tree. An additional grant
* of patent rights can be found in the PATENTS file in the same directory.
View shallowCompare.js
function shallowCompare(a, b) {
if (a === b) {
return true;
if (typeof a === 'object' && typeof b === 'object' && hiddenClass(a) === hiddenClass(b)) {
return !memcmp(a, b, sizeOfClass(a));
if (typeof a === 'function' && typeof b === 'function') {
if (sourceLocation(a) === sourceLocation(b)) {
return shallowCompare(getClosureContextOf(a), getClosureContextOf(b));