What is typeof operator?

The typeof returns a type of given value.

const h = "hello";

const w: typeof s = "world";

type TNumber = typeof 10;

This isn’t very useful for basic types, but combined with other type operators, you can use typeof to conveniently express many patterns.

const COLORS = {
    red: 'ff0000',
    green: '00ff00',
    blue: '0000ff',
} as const;

type TColors = keyof typeof COLORS;

For another example, let’s start by looking at the predefined type ReturnType<T>. It takes a function type and produces its return type:

type Predicate = (x: unknown) => boolean;

type K = ReturnType<Predicate>; // K: boolean