React: Key Prop Best Practices

Core Principle

Use stable, unique IDs from your data. Never use array index for dynamic lists.

The key prop provides stable identity to list elements during React's reconciliation process.

When to Use What

Use Data IDs (Preferred)

Always use unique, stable identifiers directly from your data:

// ✅ Correct
{todos.map((todo) => (
  <li key={todo.id}>{todo.text}</li>
))}

Ideal keys are:

• Unique - No two items share the same key
• Stable - Never changes during component lifetime
• Predictable - Directly tied to the data item

Generate IDs on Data Load

When data lacks IDs, create them once when receiving data:

import { nanoid } from 'nanoid';

useEffect(() => {
  fetch('/api/items')
    .then(res => res.json())
    .then(data => {
      const itemsWithIds = data.map(item => ({
        ...item,
        _tempId: nanoid() // Stable ID generated once
      }));
      setItems(itemsWithIds);
    });
}, []);

When Index Is Acceptable (Rare)

Index as key is acceptable ONLY when ALL conditions are met:

• List is absolutely static
• Items never added/removed (except at the end)
• Order never changes
• Items have no internal state

Anti-Patterns to Avoid

Never Generate Keys During Render

// ❌ WRONG: Creates new key every render
{items.map(item => (
  <li key={Math.random()}>{item.name}</li>
))}

This forces React to destroy and recreate all components on every render.

Don't Use Index for Dynamic Lists

// ❌ WRONG for dynamic lists
{items.map((item, index) => (
  <li key={index}>{item.name}</li>
))}

Index fails when:

• Items are added/removed from beginning or middle
• List order changes (sorting, filtering)
• Items have internal state (like form inputs)

The bug: Index represents position, not data identity. When positions change but indexes stay the same, React incorrectly "mutates" existing components instead of creating new ones, causing state mismatch.

Don't Use useId() for List Keys

React's useId() hook is for accessibility (linking labels to inputs), not for generating list keys.

Quick Reference

DO

• Always use key when rendering lists
• Prefer unique, stable id from your data
• Generate IDs once at data load time (nanoid/uuid)

DON'T

• Never generate key during render (Math.random(), Date.now())
• Avoid index as key for dynamic lists
• Don't use useId() for list keys

References

• React Docs - Rendering Lists
• nanoid - Tiny ID generator