javascript - Chain Observable emissions? -

i have 2 independent observables i'd couple in such manner every time first emits value, causes second emit value.

in scenario, first observable infinite sequence based on async node.js-style method variable callback timing (no usage of delay or timer allowed). second observable finite sequence of independent values.

perhaps diagram can express i'm looking for:

i: ---a--d-----q-g---b-> (i infinite) f: -3512-|->             (f finite) o: ---3--5-----1-2-|->   (o finite) 

the output sequence (o) finite values (f), based on timing of (i).

here's (i):

// i'm stuck this, cannot change... function asyncoperation(callback) {   const delaymsec = math.floor(math.random() * 1000);   timers.settimeout(callback, delaymsec, null, delaymsec); }  const callbackobservable = rx.observable.bindnodecallback(asyncoperation);  const infinitecallbacksequence = rx.observable   .defer(callbackobservable)   .expand(x => callbackobservable()); 

and simplicity, here's (f):

const finitesequence = rx.observable.from([3, 5, 1, 2]); 

(this build upon previous question of mine here.)

i don't understand things enough make combine*, merge*, or *map work generate want -- assuming can. seems want take(1) on (i) each time (o) emits.

how behavior described?

would observable.zip work usecase?

const rx = require('rxjs'); const timers = require('timers');  const finite = rx.observable.from([3, 5, 1, 2]);  // i'm stuck this, cannot change... function asyncoperation(callback) {   const delaymsec = math.floor(math.random() * 1000);   timers.settimeout(callback, delaymsec, null, delaymsec); }  const callbackobservable = rx.observable.bindnodecallback(asyncoperation);  const infinitecallbacksequence = rx   .observable   .defer(callbackobservable)   .expand(x => callbackobservable());  const combined = rx.observable.zip(   finite,   infinitecallbacksequence ).foreach(v => console.log(v)); 

running on node 6:

➜ $ node test.js [ 3, 816 ] [ 5, 297 ] [ 1, 95 ] [ 2, 677 ] ➜ $