Type 'X' cannot be used as an index type

Option 1: Narrow the type to a valid index type

function getValue(obj: Record<string, number>, key: string | null) {
  if (key === null) return undefined;
  return obj[key]; // OK — narrowed to string
}

Option 2: Cast to string/number before indexing

const index = someValue as string;
obj[index]; // OK

Option 3: Use Map for object-keyed lookups

const map = new Map<{ id: number }, string>();
map.set({ id: 1 }, "value"); // objects can be Map keys
map.get(key);

What does TS2538 mean?

TS2538 means you're using a value as an object index (obj[key]) where the key's type isn't a valid index type. Only string, number, and symbol are valid. Fix by narrowing or casting the key.

Why can't boolean be used as an index type?

JavaScript object property keys are always coerced to strings. Booleans become the strings 'true' or 'false'. TypeScript disallows boolean as an index type to make this explicit and prevent confusion.

TS2538 with string | null — how to fix?

Check for null before indexing: if (key !== null) { obj[key] }. TypeScript's control flow analysis narrows the type to string after the null check.

Can I use objects as Map keys in TypeScript?

Yes — use Map<KeyType, ValueType> instead of plain objects. Maps accept any value as a key including objects, and TypeScript types them correctly.

TS2538 with keyof — when does it help?

Using keyof ensures the key type is a valid property of the object: function get<T>(obj: T, key: keyof T). This prevents both TS2538 and TS2339 at once.