Use map function from the Observable's emitted value's own property in RxJs

Question

Is there any way to use a map function in RxJs from the observable's emitted value's property?

Let's say I have an Observable of type: Observable<Something>, and Something has a property called mapFunction with the value: mapTo({ isTicking: true }). Is there anyway I can use this map function on that observable? Like this:

private composingObs$ = this.streamOfSomethings$.pipe( // I would like to use the instance function: something.mapFunction as map here.

Are you trying to map a function over the observables from streamOfSomethings? — wentjun
– wentjun, Commented Jan 2, 2020 at 3:02
Yes, the problem is that the mapping function is in a property of the "something" instance, which is not available until after streamOfSomethings$ been mapped. — user10103655
– user10103655, Commented Jan 2, 2020 at 3:26
Could you post your code snippet for a clear understanding? @user10103655 — Raj Kumar
– Raj Kumar, Commented Jan 4, 2020 at 10:57

Fan Cheung · Accepted Answer · 2020-01-02 07:11:28Z

1

This will probably work. It fetch the function once, then subsequent emission will be handled by something.someFn which according to you is an operator mapTo

this.streamOfSomethings$.pipe(
take(1),
mergeMap(something=>this.streamOfSomethings$.pipe(something.someFn))
)

Although i think you should use general function or observable as property other than pipable functions, that will give you more flexibilities.

you can setup somethingObject as {data:..., fn:...}

somethingObject.pipe(map(({data,fn)=>fn(data)));

answered Jan 2, 2020 at 7:11

Fan Cheung

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