What is pipe for in rxJS

Question

I think I have the base concept, but there are some obscurities

So in general this is how I use an observable:

observable.subscribe(x => {

})

If I want to filter data I can use this:

import { first, last, map, reduce, find, skipWhile } from 'rxjs/operators';
observable.pipe(
    map(x => {return x}),
    first()
    ).subscribe(x => {

})

I can also do this:

import 'rxjs/add/operator/map';
import 'rxjs/add/operator/first';

observable.map(x => {return x}).first().subscribe(x => {

})

So my questions are:

1. What is the difference?
2. If there is no difference, why the function pipe exists?
3. Why those functions need different imports?

Answer 1

The "pipable" (former "lettable") operators is the current and recommended way of using operators since RxJS 5.5.

I strongly recommend you to read the official documentation https://github.com/ReactiveX/rxjs/blob/master/doc/pipeable-operators.md

The main difference is that it's easier to make custom operators and that it's better treeshakable while not altering some global Observable object that could possible make collisions if two different parties wanted to create an operator of the same name.

Using separate import statement for each operator 'rxjs/add/operator/first' was a way to make smaller app bundles. By importing only operators you need instead of the entire RxJS library you can significantly reduce the total bundle size. However the compiler can't know if you imported 'rxjs/add/operator/first' because you really need it in you code or you just forgot to remove it when refactoring your code. That's one of the advantages of using pipable operators where unused imports are ignored automatically.

Answer 2

The pipe method

All this looks cool but its still very verbose. Well, thanks to RxJS 5.5 observables now have a pipe method available on the instances allowing you to clean up the code above by calling pipe with all our pure functions operators:

What does that mean?

That means that any operators you previously used on the instance of observable are available as pure functions under rxjs/operators. This makes building a composition of operators or re-using operators becomes really easy, without having to resort to all sorts of programming gymnastics where you have to create a custom observable extending Observable, then overwrite lift just to make your own custom thing.

const { Observable } = require('rxjs/Rx')
const { filter, map, reduce,  } = require('rxjs/operators')
const { pipe } = require('rxjs/Rx')

const filterOutEvens = filter(x => x % 2)
const doubleBy = x => map(value => value * x);
const sum = reduce((acc, next) => acc + next, 0);
const source$ = Observable.range(0, 10)

source$.pipe(
  filterOutEvens, 
  doubleBy(2), 
  sum)
  .subscribe(console.log); // 50