Variable that have one of the types of an interface?

Question

I was wondering; is something like this possible ?

interface User {
  name: string;
  age: number;
}

// The syntax User['*'] doesn't exist, this is just an example
const user: User['*'] = 'Bob'; // No error
const user: User['*'] = 32; // No error
const user: User['*'] = true; // Error

I know about the "or" (|), but this can become really redondent if the User interface has a lot of typed properties.

Are you looking for `keyof`? – jonrsharpe Jul 04 '19 at 13:10 — jonrsharpe, Jul 04 '19 at 13:10

score 2 · Accepted Answer · answered Jul 04 '19 at 13:10

2

You can use an indexed type query with keyof T to get a union of all possible values in an interface:

interface User {
   name: string;
   age: number;
}

const userValue1: User[keyof User] = 'Bob'; // No error
const userValue2: User[keyof User] = 32; // No error
const userValue3: User[keyof User] = true; // Error

If you use this a lot, a helper type might help:

type ValueOf<T> = T[keyof T]

answered Jul 04 '19 at 13:10

Titian Cernicova-Dragomir

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There must be a dupe for this by now, no? – jonrsharpe Jul 04 '19 at 13:11
@jonrsharpe yup jcalz had an answer for this one .. – Titian Cernicova-Dragomir Jul 04 '19 at 13:14
@jonrsharpe Yes it apparently is. Titian thanks your for the answer, and sorry for the duplicate. – Anatole Lucet Jul 04 '19 at 13:16
@T.J.Crowder there .. i killed the joy – Titian Cernicova-Dragomir Jul 04 '19 at 13:47

Variable that have one of the types of an interface?

1 Answers1