What is C? (TypeScript interview question)

const colors = { red: "#f00", blue: "#00f" } as const;
type C = keyof typeof colors;

"red" | "blue"

The answer is "red" | "blue". typeof colors gives the object type, and keyof takes its keys.

"#f00" | "#00f" is the tempting wrong answer, because those are the values, not the keys. To get the values, you would write (typeof colors)[keyof typeof colors].

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