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Change type definition of Object.keys and loop #24243

Austaras opened this Issue May 18, 2018 · 2 comments


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Austaras commented May 18, 2018

Search Terms

Object.keys, index signature


Currently the definition of Object.keys in lib.d.ts is

keys(o: {}): string[];

why not change to

keys<T extends Object>(o: T): Array<keyof T>

and accordingly change for ... in loop

Use Cases

It is ridiculous that code like
const a = {}
Object.keys(a).forEach(key => a[key])
will raise a 'has no index signature' error


My suggestion meets these guidelines:

  • This wouldn't be a breaking change in existing TypeScript / JavaScript code
  • This wouldn't change the runtime behavior of existing JavaScript code
  • This could be implemented without emitting different JS based on the types of the expressions
  • This isn't a runtime feature (e.g. new expression-level syntax)

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MartinJohns commented May 18, 2018

When searching for object.keys I find:

#20853, #20503, #19765, #19448, #15627, #15570, #15295, #13254, #12870, #12314.

@Austaras Austaras closed this May 18, 2018


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ducin commented Jul 15, 2018

Yep, that works out of the box:

const getKeys = <T extends {}>(o: T): Array<keyof T> => <Array<keyof T>>Object.keys(o)
var keys = getKeys({ first: "John", last: "Lennon" }) // => ("first" | "last")[]
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