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React performance tricks

Published 10 Aug 2019 · 5min coffee icon5min coffee icon5min coffee icon5min coffee icon20 min read

This post requires a basic knowledge about Shapes and Inline Cache. If you didn’t read V8 function optimization, it might be difficult to follow this one.

Every time someones talk about “performance” in React it’s usually about virtual DOM. It is an important piece of react but we will focus on sth else today (still connected).


In 2017 on F8 conference React announced a major rewrite of their core algorithm. That’s where Fiber comes from. But what that exactly is?

A Fiber is work on a Component that needs to be done or was done. (another words “wrapper for processing”). There are different ways to create Fibers but all of them have the same data structure (that’s important for us). These are only some of them:


You can check how Fiber looks right here

We’re not going to go deep into Fiber internals (maybe next time). What is important is that React uses those containers instead of operating on components directly.

Now let’s see that react does when we return JSX:

return <Layout>{children}</Layout>;

that transforms it into:

React.createElement(Layout, props, children);

and creates ReactElement which is just another kind of container.

Simplify React internals

I’m going to simplify React’s internal structure a little bit, to make it easier to understand what it does. Besides that, I will use a class instead of function for FiberNode:

class Component {
  constructor() { = 'Component';

  render() {}

class MyComponent1 extends Component {
  render() {
    return '<span>Test 1</span>';

class MyComponent2 extends Component {
  render() {
    return '<span>Test 2</span>';

class MyComponent3 extends Component {
  render() {
    return '<span>Test 3</span>';

function createElement(element, {
  return {
    type: element,

That’s our component classes which should look familiar and createElement which returns Element like structure (oversimplified).

const ReactWorkTags = {
  ClassComponent: 1,
  HostComponent: 5,

class FiberNode {
  constructor(tag, {
    this.tag = tag;
    // Other important staff...

const createFiberFromElement = function(element, {
  // another simplification, there are different WorkTags
  const fiberTag =
    typeof element.type === 'string'
      ? ReactWorkTags.HostComponent
      : ReactWorkTags.ClassComponent;
  const fiber = new FiberNode(fiberTag,;
  fiber.type = element.type;

  return fiber;

Here we have a definition of our FiberNode and one way we could create it. It’s important to know that when passing our element, FiberNode created from that element has the same shape as any other FiberNode (even if we’re using different component types).

const comp1 = createElement(MyComponent1);
const comp2 = createElement(MyComponent2);
const comp3 = createElement(MyComponent3);
const comp4 = createElement('div');

const fiber1 = createFiberFromElement(comp1);
const fiber2 = createFiberFromElement(comp2);
const fiber3 = createFiberFromElement(comp3);
const fiber4 = createFiberFromElement(comp4);

console.log(%HaveSameMap(fiber1, fiber2)); // true
console.log(%HaveSameMap(fiber1, fiber3)); // true
console.log(%HaveSameMap(fiber1, fiber4)); // true
console.log(%HaveSameMap(fiber2, fiber3)); // true
console.log(%HaveSameMap(fiber2, fiber4)); // true
console.log(%HaveSameMap(fiber3, fiber4)); // true

What would happen if you just generate those components manually and assign additional properties to it?

const comp1 = new MyComponent1();
const comp2 = new MyComponent2();

console.log(%HaveSameMap(comp1, comp2)); // false

How that helps us run code faster

As you might know, every time there is a change in React, it has to go through the reconciliation process. Reconciliation is using fibers to figure out the next tree. How does it works?

Inside ReactFiberWorkLoop everything goes into workLoopSync (or workLoop). After that performUnitOfWork is called with “top” Fiber. performUnitOfWork is doing one of three things:

  • nothing
  • calls beginWork with current unitOfWork (Fiber)
  • calls completeUnitOfWork with current unitOfWork (also Fiber)

In the end, it returns null or results of one of those two functions (surprise… it’s Fiber).

From this point, reconciler is just looping over all Fibers using performUnitOfWork and waits until workInProgress is empty.

What is important is that React not using recursion to go through the tree. They are using simple while loops to avoid having large stacks (maybe if we could get PTC into JS that would be different).

while(sthToDo !== null) {
    sthToDo = doSomeWork(sthToDo)


React has extra optimization for simple “Text” nodes to avoid creating extra elements

That’s where we could spot optimization in React structure.

Because all of it is complicated enough, let’s check how functions behave when they are called thousands of times with our simplified structure:

function doSomeWork(unitOfWork) {
	let result = '';
    // I realize that is stupid but just to make a point of doing sth
	for(let i=0; i<1000; i += 1) {
		result +=;
	return unitOfWork.nextFiber;

I know we’re not referring to returned unitOfWork, but that’s just to run doSomeWork many times.

const N = 100000;

const startComp =;
for(let i=0; i<N; i += 1) {
console.log("test with components:", - startComp, "ms.");
const N = 100000;

const startFiber =;
for(let i=0; i<N; i += 1) {
console.log("test with fibers:", - startFiber, "ms.");
test with components: 4186 ms.
test with fibers: 2431 ms.

Why the results are different? Mainly because of FiberNode wrapper. Every time function is called V8 creates sth called Inline Cache (IC). ICs are used to optimize function execution when object with the same Shape is passed into it. As you might remember our fibers have the same map:

console.log(%HaveSameMap(fiber1, fiber2)); // true

and our components haven’t

console.log(%HaveSameMap(comp1, comp2)); // false

Because of that simple thing, V8 cannot optimize doSomeWork for components


but it can for fibers


If you look on screenshots above, you can notice that first time V8 goes through premonomorphic (.) -> monomorphic (1) -> polymorphic (P)-> megamorphic (N) states but second one stays in premonomorphic (.) -> monomorphic (1) state. In my previous post I’ve described how optimization works in V8. Here we have to deal with Megamorphic function and that means it won’t be optimized by the engine, on the other hand we have Monomorphic function which is optimized for a given Shape (Fiber’s Shape). So even with an extra layer (more complicated object), function might execute faster.


This is just one of many optimizations done inside React codebase. Reason for describing this one is because it’s useful outside React. Angular is using the same approach with sth called View Nodes. You might even want to implement this kind of structure inside our own application.

I have to apologize for oversimplifying React structure. I really encourage you to check it out and see how workLoop actually works under the hood. My point was to show that even if you have to process a lot of different components, there is a way to speed up function execution by creating “special” containers instead of passing components directly.

If you want to run that code on your machine check out this gist. Feel free to modify components or FiberNode implementation and see what happens.

And ofc have a nice rest of the day :)


Kemal Erdem, (Aug 2019). "React performance tricks".
    title   = "React performance tricks",
    author  = "Kemal Erdem",
    journal = "",
    year    = "2019",
    month   = "Aug",
    url     = ""