RxJS Common Creation Operators

Angular: The Full Gamut Edition

Charlie Greenman
April 27, 2021
5 min read

The common RXJS creation operators are:

  1. from

  2. fromEvent

  3. of


A common scenario when using from, is that you would like to return an http request, and turn that into an observable. Razroo has a very opinionated architecture, and we follow through with that architecture throughout the book. We realize that every scenario may not be able to follow the ideal architecture. However, let's assume that you are using three very integral libraries:

  1. GraphQL

  2. Apollo Client

  3. ngrx/store

from will be used in a scenario wherein you are trying to convert an apollo query into an observable,

getPosts(): Observable<Post[]> {const posts$ = this.apollo.query({ query: GetPosts });return from(posts$).pipe(pluck('data', 'posts'));}

so that it can be passed into your ngrx/store:

@Effect() loadPosts$ = this.dataPersistence.fetch(
  PostsActionTypes.LoadPosts,{run: (action: LoadPosts, state: PostsPartialState) => {return this.postsService.getPosts().pipe(map((posts: Post[]) => new PostsLoaded(posts)));},onError: (action: LoadPosts, error) => {console.error('Error', error);return new PostsLoadError(error);}});


Whole fromEvent is a common operator in RxJS, I would argue that within an Angular setting, it is not a common operator. There is truly no scenario within Angular whereinfromEvent makes sense. Instead use the core Angular even handlers within your template:

<button (click)="submitForm()">
submitForm(userData: User) {this.userFormFacade.submitForm(userData);}


of while similar in purpose to of, will convert a value to an observable. However, the main difference between the two, can be seen here in this snipper of code:

// the subscribe here, will emit all of the data at once  // [1, 2, 3]
Observable.of([1, 2, 3]).subscribe(x => console.log(x));// the subscribe here, will iterate through data one at a time
Observable.from([1, 2, 3]).subscribe(x => console.log(x));// 1// 2// 3

from for the most part is preferabble, as data will usually be contained in an array, and from sidesteps the need of tapping into the array, doing something like this:


From my personal experience, of becomes invaluable when it comes to mocking data within your unit tests. For instance, doing something such as the following:

const postMock = {
  id: '123',
  title: 'test title',
  featureImage: 'sample feature image',
  created_at: '2019-08-05T07:49:11.405Z'}const postMock$ = of(postMock);

This is the most efficient solution is this scenario for mocking data, and comes up quite frequently within your app, and something to be aware of.

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