typeof

Two different typeofs — don't confuse them

typeof exists in both JavaScript and TypeScript, but they operate in different worlds:

• JS typeof (runtime) — returns a string like "number", used in real code to check values.
• TS typeof (type level) — takes a value and gives you its type, used in type positions.

// JS typeof — runtime narrowing
if (typeof x === "string") { /* ... */ }

// TS typeof — grab the type OF an existing value
const person = { name: "Ajay", age: 23 };
type Person = typeof person; // { name: string; age: number }

Why the type-level typeof is useful

It lets you derive a type from a value you already have, instead of writing the type out twice and keeping them in sync.

const config = {
  url: "https://api.example.com",
  retries: 3,
  debug: false,
};

type Config = typeof config; // { url: string; retries: number; debug: boolean }

Common combo: typeof + keyof

Pair them to get the union of a value's keys or values:

const person = { name: "Ajay", age: 23 };

type Keys = keyof typeof person;              // "name" | "age"
type Values = (typeof person)[keyof typeof person]; // string | number

Read it inside-out: typeof person → the type, then keyof → its keys, then index into it → its value types.

The type-level typeof is a bridge from the value world to the type world — "give me the type of this thing I already built," so your types stay derived and never drift.