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What forces layout / reflow

All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck.

Generally, all APIs that synchronously provide layout metrics will trigger forced reflow / layout. Read on for additional cases and details.

Element APIs

Getting box metrics

elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent

Looking for support tables for HTML/CSS/JavaScript features or details on their implementation status in major browsers?

Chrome Platform Status (covers Chromium and thus includes Opera)
Firefox Platform Status
Microsoft Edge Platform Status
WebKit Feature Status
API Catalog

And then there’s Can I use…?.

This document has moved!

It's now here, in The Programmer's Compendium. The content is the same as before, but being part of the compendium means that it's actively maintained.

Logic composability problems of lifecycle hooks in React

Suppose I have these components in my project:

class MessageHeader extends React.Component { /* ... */ }

class NiceButton extends React.Component { /* ... */ }

class FridgeContents extends React.Component { /* ... */ }

	function rfc3339(d) {

	function pad(n) {
	return n < 10 ? "0" + n : n;
	}

	function timezoneOffset(offset) {
	var sign;
	if (offset === 0) {
	return "Z";

	import { createStore } from 'redux';
	import combineReducers from './combineReducers';

	let Store = createStore(combineReducers({

	reducerA(state = 0, action, waitFor){
	if ( action.type === 'TEST'){
	waitState = waitFor(waitFor.reducerB)
	if ( waitState.reducerB === 5 )
	state = 10

	function mapValues(obj, fn) {
	return Object.keys(obj).reduce((result, key) => {
	result[key] = fn(obj[key], key);
	return result;
	}, {});
	}

	function pick(obj, fn) {
	return Object.keys(obj).reduce((result, key) => {
	if (fn(obj[key])) {

	var str = 'class ಠ_ಠ extends Array {constructor(j = "a", ...c) {const q = (({u: e}) => {return { [`s${c}`]: Symbol(j) };})({});super(j, q, ...c);}}' +
	'new Promise((f) => {const a = function* (){return "\u{20BB7}".match(/./u)[0].length === 2 \|\| true;};for (let vre of a()) {' +
	'const [uw, as, he, re] = [new Set(), new WeakSet(), new Map(), new WeakMap()];break;}f(new Proxy({}, {get: (han, h) => h in han ? han[h] ' +
	': "42".repeat(0o10)}));}).then(bi => new ಠ_ಠ(bi.rd));';

	try {
	eval(str);
	} catch(e) {
	alert('Your browser does not support ES6!')
	}

	import hoistStatics from 'hoist-non-react-statics';
	import React from 'react';

	/**
	* Allows two animation frames to complete to allow other components to update
	* and re-render before mounting and rendering an expensive `WrappedComponent`.
	*/
	export default function deferComponentRender(WrappedComponent) {
	class DeferredRenderWrapper extends React.Component {
	constructor(props, context) {

	// UPDATE: In 2023, you should probably stop using this! The narrow version of Safari that
	// does not support `nomodule` is probably not being used anywhere. The code below is left
	// for posterity.

	/**
	* Safari 10.1 supports modules, but does not support the `nomodule` attribute - it will
	* load <script nomodule> anyway. This snippet solve this problem, but only for script
	* tags that load external code, e.g.: <script nomodule src="nomodule.js"></script>
	*
	* Again: this will not prevent inline script, e.g.: