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repolm.com/app — facebook/react

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Reconciler

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Event System

React — Architectural Overview

Expert level · 47 modules analyzed

// React's Fiber reconciler — core scheduling loop
function workLoopConcurrent() {
  while (workInProgress !== null && !shouldYield()) {
    performUnitOfWork(workInProgress);
  }
}

// Each fiber node represents a component
const fiber = {
  tag: FunctionComponent,
  stateNode: 'App',
  memoizedState: hook,
};

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Event System

Component Lifecycle

Slide 3 / 12

Fiber Architecture

How React schedules and reconciles updates

App

Header

Router

Home

Profile

Footer

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Episode: React Internals — Fiber & Reconciliation

12:34/ 28:15

Alex: So the key insight about React's Fiber architecture is that it made rendering interruptible—

Sam: Right, before Fiber it was all synchronous. One giant call stack.

Alex: Exactly. Now each unit of work is a fiber node, and React can pause mid-tree to handle higher priority updates.

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How does React's reconciler decide when to yield?

The concurrent reconciler uses a time-slicing approach. In workLoopConcurrent(), it checks shouldYield() after each unit of work — if the browser needs the thread back for user input, React pauses and resumes on the next frame.

Where's that implemented?

packages/react-reconciler/src/ReactFiberWorkLoop.js — line 1847. The shouldYield function delegates to Scheduler's shouldYieldToHost().

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