Angular 2 useExisting providers

Question

What are the usages for useExisting provider?

Is it useExistingOrThrowIfThereIsNone or useExistingOrCreateIfThereIsNone? Can o

Answer 1

With this example

providers: [
    A, 
    {provide: B, useClass: A}, 
    {provide: C, useExisting: A}]

If you have

constructor(private a: A)

an instance for the first provider is created.

constructor(private b: B)

an instance for the 2nd provider is created

constructor(private c: C)

the instance of the first provider is injected.

If you start fresh with

constructor(private c: C)

an instance for the first provider is created and injected