Typescript doesn't infer value of generic object

Question

Consider this function, it iterates over the provided object's keys and sets each key to a number and then returns the object. For some reason, typescript doesn't infer that obj[key] value is a number, I'm getting the following error:

Type '1' is not assignable to type 'T[keyof T]'.ts(2322)

Does anyone know how to fix this? Parameter should always be be Record<string, number>.

function setObjKeysToOne<T extends Record<string, number>>(obj: T) {
  (Object.keys(obj) as Array<keyof typeof obj>).forEach((key) => {
    obj[key] = 1; // Type '1' is not assignable to type 'T[keyof T]'.ts(2322)
  });

  return obj;
}

Yes I do need it, this is not real-world code, I created it to illustrate the problem, In reality I need a generic there. — Umbrella
– Umbrella, Commented May 11, 2020 at 21:14
Without more of a use case I'd be wary of this sort of code, though. That signature doesn't need to be generic, and if T is narrower than Record<string, number>, such as {a: 123, b: 456} where the properties are numeric literal types then you want the compiler to complain about setting its properties to 1. — jcalz
– jcalz, Commented May 12, 2020 at 2:10
@jcalz I see the problem here, but I just want to make sure that values are numbers, I don't care whether they are literal or not. I guess typecasting is the only way here. In the real-life code I set the values to 0 when a certain condition is met if that makes more sense, this is a contrived example. — Umbrella
– Umbrella, Commented May 12, 2020 at 9:49

groetzi · Accepted Answer · 2020-05-11 22:58:53Z

1

you could cast the obj during the assignment e.g.

function setObjKeysToOne<T extends Record<string, number>>(obj: T) {
  Object.keys(obj).forEach((key) => {
    (obj as Record<string, number>)[key] = 1;
  });

  return obj;
}

there's a related issue on github with an explanation why typescript behaves that way.

answered May 11, 2020 at 22:58

groetzi

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