What is `K`?

type Dict = { [key: string]: number };
type K = keyof Dict;

string | number

The answer is string | number. With a string index signature, keyof includes number too. JavaScript converts numeric keys to strings, so dict[0] is also allowed.

string is the tempting wrong answer. Note that a number index signature alone would give just number.

Keyof type operator (https://www.typescriptlang.org/docs/handbook/2/keyof-types.html)typescriptlang.org