TypeScript syntax explanation, optional parameters

Question

I recently came across the following function declaration

subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void), error?: (error: any) => void, complete?: () => void): Subscription;

I understand that '?' means optional , but what does the rest mean, especially the : PartialObserver<T> | ((value: T) => void) part?

Possible duplicate of What does the pipe(|) mean in typescript? — Aleksey L.
– Aleksey L., Commented Nov 2, 2016 at 7:22

CodeNotFound · Accepted Answer · 2016-11-01 22:00:14Z

1

I understand that '?' means optional , but what does the rest mean, especially the : PartialObserver<T> | ((value: T) => void) part?

The char '|' is known as Union Type and is used here to tell that the observerOrNextparameter can be one of the foolowing type:

generic type PartialObserver<T>
or a callback function which signature must return nothing e.g. void and accept a parameter value of type T.

The TypeScript documentation for Advanced Types explains in a better way when to use Union Type.

answered Nov 1, 2016 at 21:54

CodeNotFound

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