Limit number of requests at a time with RxJS

It's 2018, rxjs 5 is here and this is how I solved it

urls$
  .mergeMap((url) => request({ url }), null, 10)
  .subscribe()

mergeMap (aka flatMap) already takes the "max concurrency" as its 3rd parameter (see the docs)

Btw. I am using universal-rxjs-ajax (the request) for node compatibility, but it should work the same with Observable.ajax

RxJS v6 update

pipe through mergeMap with your parallel limit as 2nd parameter

const MAX_PARALLEL_QUERIES = 3;
let allResults = [];
let observables = [] // fill with observables
from(observables)
            .pipe(mergeMap(observable => observable, MAX_PARALLEL_QUERIES))
            .subscribe(
                partialResults => {
                    allResults = allResults.concat(partialResults);
                },
                err => {
                    // handle error
                },
                () => {
                    // get here when all obserable has returned
                    allResults.forEach(result=> {
                        // do what you want
                    });
                }
            );