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For instance let's say we have

type AnObject = {
  a: string
  b: number
  c: string
  d: number
}

type ExtractKeysOfType<O extends {[K: string]: any}, T> = ///...

type StringKeys = ExtractKeysOfType<AnObject, string> // 'a' | 'c'

Is they're a way to achieve ExtractKeysOfType ?

zedryas
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1 Answers1

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You can use mapped types combined with conditional types here:

type ExtractKeysOfType<T, Target> = {
  [K in keyof T]: T[K] extends Target ? K : never
}[keyof T];

This essentially works by going over each key in type T. Does T[K] extend our Target type? if so, great, and the property value is simply that Key. If not, that key has type never.

This intermediary type, for your case, looks like: 

{
    a: "a";
    b: never;
    c: "c";
    d: never;
}

Then, that intermediary type is indexed again by keys of T. This will produce your desired union, since the never types are ignored by the compiler here.

Playground

kschaer
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