Angular tap vs map
Asked on November 19, 2018
What is the difference between tap and map in Angular?
Replied on November 19, 2018
tap and map both are RxJS operator.
Pipeable operators such as
nextOrObserver: A normal
error: Callback for errors in source
complete: Callback for completion of the source.
Find the sample example.We will get following output.
Suppose we have anNow we will map this string into array and return result into We can iterate
1. tap
RxJS
tap performs side effects for every value emitted by source Observable and returns an Observable identical to the source Observable until there is no error. tap is the pipeable operator and it is the changed name of the RxJS do operator. Angular 6 uses RxJS 6 that has been shipped with pipeable operators which can be used independent of an Observable. To avoid the conflict with JavaScript keyword, some RxJS operators name has been changed such as do changed to tap, catch changed to catchError, switch changed to switchAll and finally changed to finalize. These operators are imported from rxjs/operators. For example tap is imported as following.import { tap } from 'rxjs/operators';
tap, are used within pipe function of Observable. tap performs side effects only when the Observable returned by tap is subscribed. tap can be used to debug values emitted by Observable or to perform any side effect.tap is declared as following.public tap(nextOrObserver: Observer | function, error: function, complete: function): Observable
tap has three optional parameters. nextOrObserver: A normal
Observable object to perform side effect. error: Callback for errors in source
Observable. complete: Callback for completion of the source.
Find the sample example.
of(1, 2, 3, 4).pipe( tap(el => console.log("Process "+ el), err => console.error(err), () => console.log("Complete") ), filter(n => n % 2 === 0) ).subscribe(el => console.log("Even number: "+ el));
Process 1 Process 2 Even number: 2 Process 3 Process 4 Even number: 4 Complete
2. map
map is a RxJS pipeable operator. map applies a given function to each element emitted by the source Observableand emits the resulting values as an Observable. map is imported as following.import { map } from 'rxjs/operators';
map is used with pipe which is an instance method of Observable. Suppose we have an
Observable of a string that contains comma separated names.getStdNames(): Observable<string> { return of("Mahesh, Krishna, Ram"); }
Observable of array of names.stdNames$: Observable<string[]>; getStdNames() { this.stdNames$ = this.bookService.getStdNames().pipe( map(res => res.split(",")) ); }
stdNames$ as following.<ul> <li *ngFor="let name of stdNames$ | async" > {{name}} </li> </ul>
It will print
Mahesh
Krishna
Ram
