How to get the type of an interface property in TypeScript?

Question

So I have a code like:

interface MyInterface{
  a:any,
  b:string,
  c:boolean,
  d:number,
  readonly valueChanges:Subject<{key: keyof MyInterface, value: ???}>
}

where I dont know how to write the type of the value under the correct 'key'. I have tried the typeof MyInterface[key] but probably this is not how to solve it. :(

Thanks for your time in advance!

Answer 1

You can make use of the AllValues utility type posted by Titian Cernicova-Dragomir . Note that you do not need the extends Record<PropertyKey, PropertyKey> as that is specific to the problem of inverting.

type AllValues<T> = {
    [P in keyof T]: { key: P, value: T[P] }
}[keyof T]

You then apply this type to your interface, but make sure that {key: valueChanges...} isn't one of your options.

type KeyValueObject = AllValues<Omit<MyInterface, "valueChanges">>

interface MyInterface{
  a: any;
  b: string;
  c: boolean;
  d: number;
  readonly valueChanges: Subject<AllValues<Omit<MyInterface, "valueChanges">>>;
}

I don't like the circular referencing here, so personally I would break it into composable pieces. MyInterface would end up as a type rather than an interface , but there is very little practical difference.

interface BaseInterface {
  a: any;
  b: string;
  c: boolean;
  d: number;
}

type WithValueChanges<T> = T & {
    readonly valueChanges: Subject<AllValues<T>>
}

type MyInterface = WithValueChanges<BaseInterface>