Make array and record lookups admit they can be undefined

Turn on noUncheckedIndexedAccess so indexing an array or a string-keyed record types as T | undefined, which is what it is at runtime.

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true
  }
}

const users: User[] = [];
const first = users[0];          // User | undefined
const byId: Record<string, User> = {};
const u = byId["missing"];       // User | undefined

Without the flag both are typed User, and users[0].name compiles then throws Cannot read properties of undefined on an empty array.

What changes in existing code

Every index read needs a check or a non-null assertion. The honest patterns:

const first = users[0];
if (!first) return;
first.name;

const last = users.at(-1) ?? fallback;

for (const user of users) user.name;   // iteration was always safe

Loops indexed by i are the noisy case. for...of, .map, and .entries() don’t index, so they need no change.

Gotchas

• Tuples and known keys are unaffected. const [a, b] = pair and obj.knownKey stay precise; only computed and numeric index reads widen.
• strict doesn’t include this flag. It has to be set on its own.
• Turning it on late in a project produces hundreds of errors in one go. Enable it, fix the real ones, and reach for ! only where the index is provably in range, such as right after a length check the compiler can’t follow.
• Writes are unaffected. arr[i] = x still types x as T, not T | undefined.