Best way to import Observable from rxjs

Question

In my angular 2 app I have a service that uses the Observable class from the rxjs library.

import { Observable } from 'rxjs'; 

At the moment I am just using Observable so that I can use the toPromise() function.

I read in another StackOverflow question somewhere that importing in this way and also importing from rxjs/Rx will import a whole lot of unnecessary stuff from the rxjs library that will increase the page load times and/or the code base.

My question is, what is the best way to import Observable so I can use the toPromise() function without having to import everything else?

Answer 1

Rxjs v 6.*

It got simplified with newer version of rxjs .

1) Operators

import {map} from 'rxjs/operators'; 

2) Others

import {Observable,of, from } from 'rxjs'; 

Instead of chaining we need to pipe . For example

Old syntax :

source.map().switchMap().subscribe() 

New Syntax:

source.pipe(map(), switchMap()).subscribe() 

Note: Some operators have a name change due to name collisions with JavaScript reserved words! These include:

do -> tap,

catch -> catchError

switch -> switchAll

finally -> finalize

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Rxjs v 5.*

I am writing this answer partly to help myself as I keep checking docs everytime I need to import an operator . Let me know if something can be done better way.

1) import { Rx } from 'rxjs/Rx';

This imports the entire library. Then you don't need to worry about loading each operator . But you need to append Rx. I hope tree-shaking will optimize and pick only needed funcionts( need to verify ) As mentioned in comments , tree-shaking can not help. So this is not optimized way.

public cache = new Rx.BehaviorSubject(''); 

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Or you can import individual operators .

This will Optimize your app to use only those files :

2) import { _______ } from 'rxjs/_________';

This syntax usually used for main Object like Rx itself or Observable etc.,

Keywords which can be imported with this syntax

 Observable, Observer, BehaviorSubject, Subject, ReplaySubject 

3) import 'rxjs/add/observable/__________';

Update for Angular 5

With Angular 5, which uses rxjs 5.5.2+

import { empty } from 'rxjs/observable/empty'; import { concat} from 'rxjs/observable/concat'; 

These are usually accompanied with Observable directly. For example

Observable.from() Observable.of() 

Other such keywords which can be imported using this syntax:

concat, defer, empty, forkJoin, from, fromPromise, if, interval, merge, of,  range, throw, timer, using, zip 

4) import 'rxjs/add/operator/_________';

Update for Angular 5

With Angular 5, which uses rxjs 5.5.2+

import { filter } from 'rxjs/operators/filter'; import { map } from 'rxjs/operators/map'; 

These usually come in the stream after the Observable is created. Like flatMap in this code snippet:

Observable.of([1,2,3,4])           .flatMap(arr => Observable.from(arr)); 

Other such keywords using this syntax:

audit, buffer, catch, combineAll, combineLatest, concat, count, debounce, delay,  distinct, do, every, expand, filter, finally, find , first, groupBy, ignoreElements, isEmpty, last, let, map, max, merge, mergeMap, min, pluck,  publish, race, reduce, repeat, scan, skip, startWith, switch, switchMap, take,  takeUntil, throttle, timeout, toArray, toPromise, withLatestFrom, zip 

FlatMap: flatMap is alias to mergeMap so we need to import mergeMap to use flatMap.

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Note for /add imports :

We only need to import once in whole project. So its advised to do it at a single place. If they are included in multiple files, and one of them is deleted, the build will fail for wrong reasons.