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Quick note on React internal

web dev2 min read

Given this block of React codebase

const element = <h1 title="foo">Hello</h1>
const container = document.getElementById("root")
ReactDOM.render(element, container)

Step 1. Convert JSX to an object (using Babel and a method called React.creatElement).

The result is an object that looks like this

const element = {
type: "h1",
props: {
title: "foo",
children: "Hello",

Step 2. The element object will then be passed to render.

Step 3. The render method recursively creates DOM nodes for the element and its children.

  1. The whole process will be done in multiple batches to avoid hogging the main thread. (aka Concurrent mode)
  2. The data structure behind the scene that stores all the "batches" is called a Fiber tree.

Step 4. Once the DOM tree is ready, it's committed to the real DOM. The Fiber tree will also be kept as a reference for the next step.

Step 5. On subsequent updates, steps 1 to 3 are repeated again, the new Fiber tree will then be compared with the existing Fiber (this process is called Reconciliation, the differences will be committed to the DOM.

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