TypeScript: Reference subtype of type definition (interface)

You can reference interface subtypes using lookup types, added in TypeScript 2.1:

interface ExerciseData {
    id: number;
    name: string;
    vocabulary: Array<{
        from: string;
        to: string;
    }>;
}

type Name = ExerciseData['name']; // string

These lookup types can also be chained. So to get the type of a vocabulary item you can do this:

type Vocabulary = ExerciseData['vocabulary'][number]; // { from: string; to: string; }

Or with even more chaining, the from field:

type From = ExerciseData['vocabulary'][number]['from']; // string

For complex scenarios it's also possible to base the lookup type on another type. For example, on a string literal union type:

type Key = 'id' | 'name';
type FieldTypes = ExerciseData[Key]; // number | string

Not exactly what you want but you can hack around this with the typof keyword but only if you have a var that is declared as your interface type like below. Note that I think what you did in your last codeblock is a lot better :)

interface ExerciseData {
    id : number;
    name : string;
    vocabulary : {
        from : string;
        to : string;
    }[];
}
var x: ExerciseData;
var vocabs : typeof x.vocabulary[];

I found it's working in the next way these days:

interface User {
  avatar: string;
}

interface UserData {
  someAvatar: User['avatar'];
}

very useful if you don't want to export all the things.