Convert an Rx4 Observable into an Rx5 Observable (in case you need both)

rx4torx5.js

/**
Convert an Rx4 Observable into an Rx5 Observable

Usage:
    // When referenced:

    var o4 = Rx4.Observable.interval(500).take(2).map('from 4');
    var o5 = Rx5.Observable.interval(500).take(2).mapTo('from 5');

    fromRx4(o4).concat(o5).subscribe(
        x => console.log('next', x),
        err => console.log('err', err),
        () => console.log('compl'));

    // Or when declared:

    var o4 = fromRx4(Rx4.Observable.interval(500).take(2).map('from 4'));
    var o5 = Rx5.Observable.interval(500).take(2).mapTo('from 5');

    o4.concat(o5).subscribe(
        x => console.log('next', x),
        err => console.log('err', err),
        () => console.log('compl'));

    // If disposed early the inner Rx4 will also dispose:

    var o4 = fromRx4(Rx4.Observable.interval(500)
        .do(
            x => console.log('Inner next', x),
            err => console.log('Inner err', err),
            () => console.log('Inner compl'))
        .finally(() => console.log('Inner disposed before completion.')));
    var o5 = Rx5.Observable.never();

    o5.merge(o4).take(3).subscribe(
        x => console.log('next', x),
        err => console.log('err', err),
        () => console.log('compl'));
**/
function fromRx4(o4) {
    // Note: adjust `Rx5` here for however you're importing Rx5.
    return Rx5.Observable.create(function(obs) {
        var sub4 = o4.subscribe(
            x => obs.next(x),
            err => obs.error(err),
            () => obs.complete());

        return () => sub4.dispose();
    });
}