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

Answer 1

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.

Answer 2

Since TypeScript 1.6 you can use intersection types:

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