What does “keyof typeof” mean in TypeScript?

 What does "keyof typeof" mean in TypeScript? - Stack Overflow

keyof takes an object type and returns a type that accepts any of the object's keys.

type Point = { x: number; y: number };
type P = keyof Point; // type '"x" || "y"'

const coordinate: P = 'z' // Type '"z"' is not assignable to type '"x" | "y"'.

typeof with TypeScript types

typeof behaves differently when called on javascript objects, to when it is called on typescript types.

• TypeScript uses javascript's typeof when called on javascript values at runtime and returns one of "undefined", "object", "boolean", "number", "bigint", "string", "symbol", "function"
• TypeScript's typeof is called on type values, but can also be called on javascript values when in a type expression. It can also infer the type of javascript objects, returning a more detailed object type.

type Language = 'EN' | 'ES'; 
const userLanguage: Language = 'EN';
const preferences = { language: userLanguage, theme: 'light' };

console.log(typeof preferences); // "object"
type Preferences = typeof preferences; // type '{language: 'EN''; theme: string; }'

Because the second typeof preferences is in a type expression it is actually TypeScript's own typeof that get called, and not javascript's.

keyof typeof

Because keyof is a TypeScript concept we will be calling TypeScript's verion of typeof.

keyof typeof will infer the type of a javascript object and return a type that is the union of its keys. Because it can infer the exact value of the keys it can return a union of their literal types instead of just returning "string".

type PreferenceKeys = keyof typeof preferences; // type '"language" | "theme"'