Why does `Object.keys(config)` give me `string[]` instead of the keys I can see right there?

The recipe

type Keys<T extends object> = (keyof T)[]

const typedKeys = <T extends object>(value: T): Keys<T> =>
  Object.keys(value) as Keys<T>

const config = { retries: 3, verbose: true }

const keys = typedKeys(config)

The build compiles this and checks each result below.

How it works

1. 01

   type Keys<T extends object> = (keyof T)[]

   The shape you wish `Object.keys` had. Naming it separately keeps the cast in one place and readable.

2. 02

   Object.keys(value) as Keys<T>

   A cast, and an honest one: you are asserting the object has no keys beyond its type. For a literal you just built that is true; for something that arrived from elsewhere it may not be.

What you get

• typeof keys

  →("retries" | "verbose")[]

• Keys<{ a: 1; b: 2 }>

  →("a" | "b")[]

• (typeof keys)[number]

  →"retries" | "verbose"

  And indexing by `number` recovers the union, which is usually what you wanted to iterate over.

Where it goes wrong

The cast is a lie whenever the object came from outside your program. Extra properties are legal — a value typed `{ a: 1 }` may carry `b` at run time — and this helper will hand you a key list that does not include it while claiming to be complete.