Why React Had to Build Its Own Execution Engine

Series: How React Works Under the Hood

Part 1: Motivation Behind React Fiber: Time Slicing & Suspense

Prerequisites: Read Part 1 first — this article picks up exactly where it left off.

Where We Left Off

In Part 1 we landed on one sentence that explains why React had to be completely rewritten:

React needs to be able to interrupt what it's doing and resume it later.

Time Slicing needs it — to yield within the browser's 16ms frame budget.

Suspense needs it — to pause and wait for async data.

Simple enough requirement. But here's the problem: JavaScript itself won't let you do it.

This article is about why that's true, what React did about it, and why that decision — building an entirely custom execution model — is what makes modern React possible.

The Problem: JavaScript Runs to Completion

When JavaScript starts executing a function, it finishes it.…