Extending a generic parameter in Typescript

Question

I want to create a utility function which creates a checklist by adding an isChecked knockout observable property to each item in an array. This function should look like this:

createCheckList<T>(allEntities: T[], selected: T[]) : CheckListItem<T> {
    ...
}

I am returning a CheckListItem<T> because this interface should extend T to add the isChecked property. However, typescript will not allow me to do this:

interface CheckListItem<T> extends T {
    isChecked: KnockoutObservable<boolean>;
}

Gives me the error:

An interface may only extend another class or interface.

Is there any way to do this?

Answer 1

Since TypeScript 1.6 you can use intersection types:

type CheckListItem<T> = T & {
  isChecked: KnockoutObservable<boolean>;
};

Answer 2

There isn't a way to express this in TypeScript.

You can obviously do this instead:

interface CheckListItem<T> {
    isChecked: KnockoutObservable<boolean>;
    item: T;
}

This has the advantage of not breaking the object when it happens to have its own isChecked property.