This content originally appeared on DEV Community and was authored by Turkan İsayeva
Why React Cares About Immutability: A Deep Dive into Reference, Reconciliation, and Rendering.
When you first hear the term “immutability” in React, it might sound like just another fancy concept you’re expected to remember. But this one matters — a lot. Behind the scenes, it’s powering how your UI knows when to update, or when to stay quiet.
Let’s break it down step by step, with clarity and a little curiosity.
The Cycle Behind Every React Update
Whenever a React component updates, it’s because something triggered a change in its state or props. But React doesn’t guess that something has changed, it uses a smart process:
- You call
setStateor pass newprops - React compares the current Virtual DOM with the previous one
- It finds what changed and updates the real DOM only where needed
This process is called reconciliation, and the way React decides what changed is surprisingly simple:
It checks whether the reference of the new state/props is different from the previous one.
Why Reference?
In JavaScript, reference comparison is fast and cheap.
const a = { name: "Turkan" };
const b = { name: "Turkan" };
console.log(a === b); // false (different references)
Even though the content is the same, these are two separate objects in memory.
React takes advantage of this:
If the reference changes → React re-renders.
If the reference stays the same → React assumes nothing changed.
The Mutation Trap
Here’s where many beginners (and even experienced devs!) fall into a subtle trap:
const [user, setUser] = useState({ name: "Turkan" });
const updateUser = () => {
user.name = "Daniel"; // ❌ mutated directly
setUser(user); // ❌ same reference
};
Even though you changed the value inside user, the reference didn’t change.
React compares old and new state like this:
Object.is(prevState, newState); // true → no update
Result? No re-render. No UI update. Confused developer.
Immutability to the Rescue
Let’s fix it the React way:
const updateUser = () => {
setUser({ ...user, name: "Daniel" }); // ✅new object, new reference
};
Now React sees:
Object.is(prevState, newState); // false → trigger re-render
This is the heart of immutability in React:
Don’t modify the original, create a new version instead.
It’s Not Just Theory. It’s Performance
Immutability helps React:
- Quickly know what changed
- Skip unnecessary updates
- Keep your UI fast and snappy
It’s not about being “functional” or “fancy”.It’s about making the diffing algorithm efficient.
Finally:
If you’re updating arrays or objects in React, always ask yourself:
Am I mutating the old data, or creating a new version?
Because React doesn’t care what’s inside ; it only cares whether the reference changed.
Summary:
- React uses reference checks to detect changes
- Immutability ensures new references are created
- This powers reconciliation and efficient rendering
- Mutating state directly breaks this chain → no updates
- Always return new objects or arrays in
setState
This content originally appeared on DEV Community and was authored by Turkan İsayeva
Turkan İsayeva | Sciencx (2025-08-03T17:19:02+00:00) Why React Cares About Immutability: A Deep Dive into Reference, Reconciliation, and Rendering. Retrieved from https://www.scien.cx/2025/08/03/why-react-cares-about-immutability-a-deep-dive-into-reference-reconciliation-and-rendering/
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