Object.keys using numbers in typescript

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 };

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

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