What's difference between “reduce” and “scan”

Question

I\'m studying RXJS and stuck with the problem: the same code with operators \"reduce\" and \"scan\" works in different ways, but I think that must return equal result. Example b

Answer 1

From RxJS documentation,

Scan

apply a function to each item emitted by an Observable, sequentially, and emit each successive value

Reduce

apply a function to each item emitted by an Observable, sequentially, and emit the final value

Example codes

Scan

var source = Rx.Observable.range(1, 3)
.scan(
    function (acc, x) {
        return acc + x;
    });

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

For each value emitted by the Observable, scan emits corresponding output sequentially, So the Output will have 3 values for range 1 to 3, as follows

Output
Next: 1
Next: 3
Next: 6
Completed

Reduce

var source = Rx.Observable.range(1, 3)
    .reduce(function (acc, x) {
        return acc * x;
    }, 1)

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

Reduce function reduces the values from observables to a single value (final result) and emit. So the output will be as follows,

Next: 6
Completed

Answer 2

Rx.scan is continuous whereas reduce is not continuous.

Answer 3

there almost the same but scan emits each iteration. and reduce just emits a final results. i like scan when i want to actually see what reduce is doing. so sometimesi . use reduce and i cant visualize what reduce is actually doing. in this case i simple swap out reduce for scan since it will emit on each iteration. this way i can put in a log statement and see the results for each iteration.