TypeScript: Reference subtype of type definition (interface)

Question

I am using the following type in my TypScript:

interface ExerciseData {
    id : number;
    name : string;
    vocabulary : {
        from : string;
        to : string;
    }[];
}

Now I'd like to create a variable that is of the same type as the attribute vocabulary, trying the following:

var vocabs : ExerciseData.vocabulary[];

But that is not working. Is it possible to reference to a subtype somehow? Or would I have to do something like this?

interface ExerciseData {
    id : number;
    name : string;
    vocabulary : Vocabulary[];
}

interface Vocabulary {
        from : string;
        to : string;
}

var vocabs : Vocabulary[];

Thanks a lot for hints.

Answer 1

Since TypeScript 2.1 you can do the following using lookup types:

type vocabulary = ExerciseData['vocabulary'][]; // Array<{from: string, to: string}>

These lookup types can also be chained. For example, to get the type of the from field you can do the following:

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

For complex scenario's it's also possible to base the lookup type on another type as follows:

type fieldKey = 'id' | 'name';
type fieldTypes = ExerciseData[fieldKey]; // number | string