Object.keys using numbers in typescript

Question

type Foo = { [key: number]: string }

const foo: Foo = { 100: 'foo', 200: 'bar' }
const sizes: number[] = Object.keys(foo)

Gives me:

Type 'string[]' is not assignable to type 'number[]

Why does Typescript assume string[]? And what is the correct way of creating an array of numbers here?

Answer 1

All keys are strings in JavaScript - just use map:

const sizes: number[] = Object.keys(foo).map(Number);

This'll only work for numeric strings - if it's a European string involving decimals, for instance, it'll be NaN, and that never ends well.

console.log(Number("10,7"));

Or change either of your types:

const sizes: string[] = Object.keys(foo);

Or:

type Foo = { [key: string]: string };

Answer 2

Object properties names are always string, if you want to convert property names to numbers on the fly, you can use map(Number):

const sizes: number[] = Object.keys(foo).map(Number);

Pay attention because, if your property name can't be converter to number, you'll get NaN in your sizes array