qcast-front/.agents/skills/vercel-react-best-practices/rules/js-tosorted-immutable.md

title	impact	impactDescription	tags
Use toSorted() Instead of sort() for Immutability	MEDIUM-HIGH	prevents mutation bugs in React state	javascript, arrays, immutability, react, state, mutation

Use toSorted() Instead of sort() for Immutability

.sort() mutates the array in place, which can cause bugs with React state and props. Use .toSorted() to create a new sorted array without mutation.

Incorrect (mutates original array):

function UserList({ users }: { users: User[] }) {
  // Mutates the users prop array!
  const sorted = useMemo(
    () => users.sort((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
}

Correct (creates new array):

function UserList({ users }: { users: User[] }) {
  // Creates new sorted array, original unchanged
  const sorted = useMemo(
    () => users.toSorted((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
}

Why this matters in React:

1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only
2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior

Browser support (fallback for older browsers):

.toSorted() is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:

// Fallback for older browsers
const sorted = [...items].sort((a, b) => a.value - b.value)

Other immutable array methods:

• .toSorted() - immutable sort
• .toReversed() - immutable reverse
• .toSpliced() - immutable splice
• .with() - immutable element replacement