如何获取 Typescript 中 keyof 变量的类型？

Question

I have the following interface:

interface MyInterface { 
   a: string 
   b: number 
}

and the following Function:

function myFunction(key: keyof MyInterface, someItem: MyInterface) {
  if (key === "a") {
    throw new Error("Key is not corresponding to a number")
  }
  
  return someItem[key]
}

It's working, but I have a much bigger interface and I'd like to avoid hardcoding all the keys, that don't correspond to a number. Is there an easy way to do this?

Answer 1

We can get all the number keys with a type like this:

type NumberKeys<T> = {
    [K in keyof T]: T[K] extends number ? K : never;
}[keyof T];

Mapping over each key, we test if each key's value is a number. If it is, we map it to itself, otherwise, it's never .

Then finally, we get all of the results as a union with [keyof T] . Because T | never T | never is always T , we only get the keys that are numbers left.

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