Why do keys matter in React, and how do they improve performance?

Source: Dev.to

Keys give React a stable identity for list elements, allowing the reconciliation algorithm to correctly match elements between renders. This minimizes DOM updates, preserves component state, and improves performance, especially in dynamic lists.

What problem do keys solve?

React uses a diffing (reconciliation) algorithm to compare the previous Virtual DOM with the next Virtual DOM.

Without keys, React:

• Compares elements by their index in the list.
• May assume elements have changed even when they haven’t, leading to unnecessary DOM replacements.

With keys, React:

• Tracks elements by a stable identity (the key).
• Reuses existing DOM nodes whenever possible, avoiding needless destruction and recreation.

How do keys improve performance?

• Prevent unnecessary DOM updates – DOM operations are expensive, and keys let React skip updates for unchanged items.
• Preserve component state – without keys, inputs can lose focus or value when the list changes; with keys, state is retained.
• Enable efficient re‑ordering – useful for drag‑and‑drop lists, sorting, filtering, etc. React can move existing nodes instead of destroying and recreating them.