Angular 2 useExisting providers

Question

What are the usages for useExisting provider?

Is it useExistingOrThrowIfThereIsNone or useExistingOrCreateIfThereIsNone? Can o

Answer 1

When we write {provide: A, useClass: B}, Angular will create map between token A and class B.

When we write {provide: A, useExisting: B}, Angular will create map between token A and token B.

Difference between these maps:

• token A -> instance of class B
• token A -> token B -> instance of some class for token B