Hey folks 👋,
So here I am again, sitting at my desk, coffee in hand, scratching my head over why one small UI update turned into a weird mess. You ever have those moments when you're like, "React, why are you re-rendering the whole world just because I changed one list item?" Yeah. That was me last week.
And that led me into finally understanding something I had always kind of ignored — Reconciliation and Diffing in React.
🧠 First Off, What Is Reconciliation?
Okay, picture this:
React is like a really smart artist who draws your app on a whiteboard.
Now, every time your app’s state or props change, React:
- Looks at the old sketch it made (old Virtual DOM).
- Looks at the new idea you gave it (new Virtual DOM).
- Figures out the difference.
- And updates just what’s changed on the actual webpage (the Real DOM).
That whole clever update process? That’s Reconciliation.
Definition
Reconciliation is the process React uses to determine how the UI should be updated when the component’s state or props change.
Here’s how it works:
- React creates a new Virtual DOM after every state/prop change.
- It compares this new Virtual DOM with the previous one.
- Only the changed parts are updated in the actual DOM (the browser's UI).
This approach allows React to minimize expensive DOM operations and ensure a smoother user experience.
🔍 Then What Is Diffing?
Diffing is the actual detective work React does during reconciliation.
It’s like React saying:
"Okay, what's different between what I had before and what I have now?"
And it does this super smartly:
- If the tag/component is the same, it just updates the content or attributes.
- If it’s different, React throws the old one out and makes a new one.
Definition
Diffing is the algorithm used during reconciliation to compare the new Virtual DOM with the previous one and identify what has changed.
Key points about the diffing process:
- Same component or element type: React updates only the changed attributes or content.
- Different component or element type: React unmounts the old node and mounts a new one.
-
For child elements in lists: React uses a
keyto track and identify individual elements efficiently
🧦 Real Life Analogy: My Messy Closet
Imagine your UI is a closet (mine usually is 😅).
Inside, you have 3 shirts:
👕 Green
👕 Red
👕 Blue
Now you want to change things:
- Move Blue to the top
- Keep Red in place
- Move Green to the bottom
React doesn’t go “Okay, let’s take everything out and start fresh.”
Instead, it just moves a couple shirts around.
Smart and efficient. That’s Diffing and Reconciliation in action.
🏷️ The Magic of Keys (Seriously, Don't Ignore These)
Now here’s where I used to mess up (and maybe you have too?): Keys.
React uses key props to track each item in a list.
If you use something like:
<ul>
<li key="1">Green</li>
<li key="2">Red</li>
<li key="3">Blue</li>
</ul>
React’s core strength lies in its ability to efficiently update and render components as data changes. Two key concepts that enable this performance are Reconciliation and Diffing. As a full-stack developer working with React (Next.js), understanding these internal processes helps in writing optimized, predictable UI code.
Why Keys Matter in Lists
In dynamic lists, using the correct key is crucial for performance and stability. The key prop helps React identify which items have changed, are added, or are removed.
✅ Correct usage:
<ul>
{items.map(item => (
<li key={item.id}>{item.name}</li>
))}
</ul>
React reuses existing DOM elements based on keys, avoiding unnecessary DOM manipulations.
❌ Inorrect usage:
<ul>
{items.map((item, index) => (
<li key={index}>{item.name}</li>
))}
</ul>
Using index as a key can cause:
- Unnecessary re-renders
- Lost input focus
- Animation glitches
Overview
In React, Reconciliation is the process used to update the user interface efficiently when the state or props change.
React keeps a Virtual DOM, which is an in-memory copy of the real DOM. Whenever something changes, React creates a new Virtual DOM and compares it with the previous one. This comparison step is called diffing.
During diffing, React checks which elements have changed. It figures out the minimum number of updates needed and then applies only those changes to the real DOM. This makes the UI updates faster and more efficient.
For example, if I have a list like Pen, Paper, Pencil and I change it to Pen, Paper, Whitener, React will detect that only "Pencil" changed to "Whitener" and will update just that item — it won’t touch the others.
Using unique keys with list items helps React identify elements correctly and avoid unnecessary re-renders. This is especially important when reordering or updating list items.
So in short:
Diffing is comparing Virtual DOMs
Reconciliation is updating the real DOM based on the diff
And keys help React do this more efficiently.
Top comments (4)
Do you use React at work ?
yes i do
Love the closet analogy, it nails how small mistakes with keys can really mess with state and UI. Have you ever had a tough-to-debug issue caused by a bad key prop?
Thanks! 😊 Yes, I’ve had issues where using index as a key caused inputs to lose focus or weird re-renders. Took me a while to realize it was the key prop all along! Keys really make or break list rendering in React. Have you run into something similar?