How do I create a string array type that doesn't allow names of keys of another type?

Question

To illustrate what I'm looking for:

interface Thing {
  id: string
  name: string
  amount: number
}

type OtherProps = string[] // What goes in here instead of `string[]`?

function processOtherProps(props: OtherProps) {
  throw new Error("unimplemented")
}

processOtherProps(["someOtherProp"]) // Ok
processOtherProps(["name"]) // Compiler error because "name" is a key of Thing
processOtherProps(["someOtherProp", "amount"]) // Compiler error because "amount" is a key of Thing

How do I declare OtherProps in a way that produces the expected outputs in the last three lines?

There is no specific type that works how you want; negated types do not exist in TypeScript. You’ll need to make processOtherProps generic. On mobile now so if I don’t get to it I’m sure someone else will explain. Good luck! — jcalz
– jcalz, Commented Mar 12, 2022 at 16:34
Is it no possible through a combination of the conditional operator ` ? : ` and the type never? I tried this for a while but couldn't figure out myself — Felipe Müller
– Felipe Müller, Commented Mar 12, 2022 at 16:43
No, there is no specific type in TypeScript like "all string except keyof Thing". Conditional types do not give you this. Had negated types as in ms/TS#29317 been implemented, you could have written string & not keyof Thing. You can use generic types, but it would require something like type OtherProps<K extends string> = Exclude<K, keyof Thing>[] and then you carry K around everywhere, like this. If you want me to write that up as an answer, let me know (although the existing answer is similar) — jcalz
– jcalz, Commented Mar 12, 2022 at 22:39

sno2 · Accepted Answer · 2022-03-12 17:01:16Z

1

You can do this via a generic function as @jcalz stated. Basically, you can take in a generic array that must be readonly (via as const) then make sure none of the values in the array are keys in Thing.

interface Thing {
  id: string
  name: string
  amount: number
}

function processOtherProps<T extends readonly string[]>(props: T[number] extends Exclude<T[number], keyof Thing> ? T : never) {
  throw new Error("unimplemented")
}

processOtherProps(["someOtherProp"] as const) // Ok
processOtherProps(["name"] as const) // Compiler error because "name" is a key of Thing
processOtherProps(["someOtherProp", "amount"] as const) // Compiler error because "amount" is a key of Thing

TypeScript Playground Link

answered Mar 12, 2022 at 17:01

sno2

4,3832 gold badges21 silver badges42 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How do I create a string array type that doesn't allow names of keys of another type?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related