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I am attempting to constrain an array's values to particular keys of an interface:

interface Foo {
  bar: string;
  baz: number;
  foo: string;
}

type ExtractArrayOfKeys<T, K extends keyof T> = Pick<T, K>[];

const keyArray: ExtractArrayOfKeys<Foo, "bar" | "baz"> = ["bar", "baz"]; // Error: Type 'string' is not assignable to type 'Pick<Foo, "bar" | "baz">'
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  • Array<keyof Foo> Commented Nov 26, 2020 at 14:41
  • @bugs I wish to pick particular keys... So in the above, how could I type the array to allow bar and baz as valid entries, but disallow foo? Commented Nov 26, 2020 at 14:44
  • Sorry, I misread the question... Commented Nov 26, 2020 at 14:47
  • No worries, all good :) Commented Nov 26, 2020 at 14:48

1 Answer 1

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This can be done relatively easily with the Extract utility type, something like

type AllFoo = keyof Foo
type BarBaz = Extract<keyof Foo, 'bar' | 'baz'>

const allFooOK: AllFoo[] = ['bar', 'baz', 'foo']
const allFooNotOK: AllFoo[] = ['bar', 'kek'] // Type '"kek"' is not assignable to type '"bar" | "baz" | "foo"'

const barBazOK: BarBaz[] = ['bar', 'baz']
const barBazNotOK: BarBaz[] = ['bar', 'foo'] // Type '"foo"' is not assignable to type '"bar" | "baz"'.

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