noUncheckedIndexedAccess Catches Missing Array Bounds Checks

Accessing an array or object index returns type 'T' even if the index is out of bounds, hiding potential undefined values. 'arr[0].name' compiles fine but crashes when arr is empty.

Enable 'noUncheckedIndexedAccess: true' in tsconfig. Index access now returns 'T | undefined', forcing explicit checks.

// tsconfig.json
{ "compilerOptions": { "noUncheckedIndexedAccess": true } }

const users: User[] = [];
const first = users[0]; // type: User | undefined

// Must handle undefined:
if (first !== undefined) {
  console.log(first.name); // OK
}

// Or use optional chaining:
console.log(users[0]?.name);

// Record indexing also affected:
const map: Record<string, number> = {};
const val = map['key']; // number | undefined

JavaScript array indexing never throws on out-of-bounds access; it returns undefined silently. TypeScript historically typed this as 'T' for convenience, hiding a class of real bugs.