1. Operators
  2. Combining
  3. Join

Join

combine items emitted by two Observables whenever an item from one Observable is emitted during a time window defined according to an item emitted by the other Observable

Join

The Join operator combines the items emitted by two Observables, and selects which items to combine based on duration-windows that you define on a per-item basis. You implement these windows as Observables whose lifespans begin with each item emitted by either Observable. When such a window-defining Observable either emits an item or completes, the window for the item it is associated with closes. So long as an item’s window is open, it will combine with any item emitted by the other Observable. You define the function by which the items combine.

GroupJoin

Most ReactiveX implementations that have a Join operator also have a GroupJoin operator that is similar, except that the function you define to combine items emitted by the two Observables pairs individual items emitted by the source Observable not with an item from the second Observable, but with an Observable that emits items from the second Observable that fall in the same window.

See Also

Language-Specific Information:

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join

The join operator takes four parameters:

  1. the second Observable to combine with the source Observable
  2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable
  3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable
  4. a function that accepts an item from the first Observable and an item from the second Observable and returns an item to be emitted by the Observable returned from join

join does not by default operate on any particular Scheduler.

join

The groupJoin operator takes four parameters:

  1. the second Observable to combine with the source Observable
  2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable
  3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable
  4. a function that accepts an item from the first Observable and an Observable that emits items from the second Observable and returns an item to be emitted by the Observable returned from groupJoin

groupJoin does not by default operate on any particular Scheduler.

join

Note that there is also a join operator in the optional StringObservable class. It converts an Observable that emits a sequence of strings into an Observable that emits a single string that concatenates them all, separating them by a specified string delimiter.

join

The join operator takes four parameters:

  1. the second Observable to combine with the source Observable
  2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable
  3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable
  4. a function that accepts an item from the first Observable and an item from the second Observable and returns an item to be emitted by the Observable returned from join

join does not by default operate on any particular Scheduler.

join

The groupJoin operator takes four parameters:

  1. the second Observable to combine with the source Observable
  2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable
  3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable
  4. a function that accepts an item from the first Observable and an Observable that emits items from the second Observable and returns an item to be emitted by the Observable returned from groupJoin

groupJoin does not by default operate on any particular Scheduler.

join

Note that there is also a join operator in the optional StringObservable class. It converts an Observable that emits a sequence of strings into an Observable that emits a single string that concatenates them all, separating them by a specified string delimiter.

join

The join operator takes four parameters:

  1. the second Observable to combine with the source Observable
  2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable
  3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable
  4. a function that accepts an item from the first Observable and an item from the second Observable and returns an item to be emitted by the Observable returned from join

Sample Code

var xs = Rx.Observable.interval(100)
    .map(function (x) { return 'first' + x; });

var ys = Rx.Observable.interval(100)
    .map(function (x) { return 'second' + x; });

var source = xs
    .join(
        ys,
        function () { return Rx.Observable.timer(0); },
        function () { return Rx.Observable.timer(0); },
        function (x, y) { return x + y; }
    )
    .take(5);

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Next: first0second0
Next: first1second1
Next: first2second2
Next: first3second3
Next: first4second4
Completed
join

The groupJoin operator takes four parameters:

  1. the second Observable to combine with the source Observable
  2. a function that accepts an item from the source Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the second Observable
  3. a function that accepts an item from the second Observable and returns an Observable whose lifespan governs the duration during which that item will combine with items from the first Observable
  4. a function that accepts an item from the first Observable and an Observable that emits items from the second Observable and returns an item to be emitted by the Observable returned from groupJoin

Sample Code

ar xs = Rx.Observable.interval(100)
    .map(function (x) { return 'first' + x; });

var ys = Rx.Observable.interval(100)
    .map(function (x) { return 'second' + x; });

var source = xs.groupJoin(
    ys,
    function () { return Rx.Observable.timer(0); },
    function () { return Rx.Observable.timer(0); },
    function (x, yy) {
        return yy.select(function (y) {
            return x + y;
        })
    }).mergeAll().take(5);

var subscription = source.subscribe(
    function (x) { console.log('Next: ' + x); },
    function (err) { console.log('Error: ' + err); },
    function () { console.log('Completed'); });
Next: first0second0
Next: first1second1
Next: first2second2
Next: first3second3
Next: first4second4
Completed

join and groupJoin are found in each of the following distributions:

  • rx.all.js
  • rx.all.compat.js
  • rx.coincidence.js

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