How React Finds What Actually Changed Series: How React Works Under the Hood Part 1: Motivation Behind React Fiber: Time Slicing & Suspense Part 2: Why React Had to Build Its Own Execution Engine Prerequisites: Read Parts 1 and 2 first. Where We Left Off In Part 2 we understood the engine — Fiber gives React a custom call stack it controls, and the Scheduler decides what to run and when. Now the question is: what does React actually do with that engine? Every time state changes, React has a problem. It knows the old version of your UI and the new version. But it's working with a tree of potentially thousands of components. Re-creating or re-rendering all of them on every update would be catastrophically slow. React needs to answer one question as efficiently as possible: "Of everything that could have changed, what actually did?" The algorithm that answers that question is called the Reconciler . The Problem: Re-rendering Everything Is Too Slow Let's make the problem concrete.…