Index Signatures: Typing Objects Whose Keys You Don't Know Yet

Sometimes you know the shape of the values in an object but not the names of the keys. Think of a phone book: every entry maps some person's name to a number, but you can't list every possible name ahead of time. For that, TypeScript gives you an index signature — a way to say "any key of this type maps to a value of that type."

The syntax lives inside an interface or type alias: { [k: string]: T }. The k is just a placeholder name (call it anything), the part before the colon is the key type, and T is the value type. Recall from earlier cards how a normal property fixes one specific key — here you're instead describing a whole family of keys at once.

interface PhoneBook {
  [name: string]: number;
}

const book: PhoneBook = {};
book["Alice"] = 5551234;
book["Bob"] = 5555678;   // any string key, number value — OK
// book["Carol"] = "nope"  ❌ Type 'string' not assignable to 'number'

Two important catches. First, index-signature keys can only be string, number, or symbol (JavaScript object keys are strings or symbols anyway). Second, and easy to trip over: reading a key gives you type T even when that key doesn't exist at runtime. TypeScript trusts your signature, so book["Zelda"] is typed as number but is actually undefined — turn on noUncheckedIndexedAccess to have the type include undefined and force you to check.