[RxJS] Utility operator: do

We just saw map which is a transformation operator. There are a couple of categories of operators, such as filtering, combination, flattening, etc. One of these categories is the utility operators. The most important utility operator is

do

, useful for debugging.

var foo = Rx.Observable.interval(200).take(4);

/*
foo: ---0---1---2---3--...
do(x => console.log('before ' + x))
---0---1---2---3--...
map(x => x * 2)
---0---2---4---6--...
do(x => console.log('after ' + x))
---0---2---4---6--...
*/

var bar = foo
.do(x => console.log('before ' + x))
.map(x => x * 2)
.do(x => console.log('after ' + x));

bar.subscribe(
function (x) { console.log('next ' + x); },
function (err) { console.log('error ' + err); },
function () { console.log('done'); },
);