[Javascript + rxjs] Using the map method with Observable

Like an array, Observable has a map method that allows us to transform a sequence into a new Observable.

var Observable = Rx.Observable;

//Create click events by Observable
var clicks = Observable.fromEvent(button, 'click');
var points = clicks.map(function(e) {
    return {x: e.clientX, y: e.clientY};
});

//Then we are able to use forEach, concatAll, map, fliter function
//The function return an subscription object, which we can use to call dispose() method to remove listener
var subscription = points.forEach(function onNext(point) {
    console.log('clicked:' + JSON.stringify(point));
    //subscription.dispose();
}, function onError() {
    console.log('error');
}, function onCompleted() {
    console.log('complete');
});

 

We change forEach method on points:

var subscription = points.forEach(function onNext(point) {

So it print out each click's position.

 

var points = clicks.map(function(e) {
    return {x: e.clientX, y: e.clientY};
});

creates another Observable object, remember once you use observable object to generate another object by foreach, map or concatAll method, this object would be observable.

 

observable is lazy:

If we comment out this part of code:

/*var subscription = points.forEach(function onNext(point) {
    console.log('clicked:' + JSON.stringify(point));
    //subscription.dispose();
}, function onError() {
    console.log('error');
}, function onCompleted() {
    console.log('complete');
});*/

Then you click the button, nothing happend. 

 

As a matter of fact, addEventListener, under the hood, has not even been called by Observable.fromEvent.

The way Observable works is it waits until you call forEach to have any side effects, to carry out any side effects whatsoever. What we've done is we've really just built an Observable that promises that when you will call forEach on it, it will hook up an event listener.

When we map over that Observable, we've created another Observable that promises that when we forEach over it, it will forEach over the underlying data source, clicks, and then, as data arrives, will transform that data, using a projection function, into new data. Just simply creating Observables causes nothing to happen.

We have to forEach over the Observable in order for something to happen.