combinePrev

HTML

<pre id="output"></pre>
<script
 src="http://cdnjs.cloudflare.com/ajax/libs/rxjs/2.3.18/rx.all.js"></script>

JavaScript

function out() {
    var args = Array.prototype.slice.call(arguments, 0);
    document.getElementById('output').innerHTML += args.join(" ") + "\n";
}

(function() {
    var a = Rx.Observable.interval(700)
    .take(18)
    .map(function (i) { return 'a'+i;});
 
    b = Rx.Observable.interval(2500).take(4)
    .map(function(i){return 'b'+i;})
    .publish().refCount();
    
    // c combinePrev of a and b
    c = a.map(function (x) {
        return b.map(function (y) { return ''+x+'z'+y;}); 
    }).switch();

    a.subscribe(function (x) {
        out(x);
    });
    b.subscribe(function (x) { out(x); });
    c.subscribe(function (x) { out(x); });

})();