What is the best way to retrieve a specific field from the observable data stream?

When working with observables, I often find myself using them like this:

...
const id = 1337;
this.service.getThing(id).subscribe(
  suc => doSomething(suc.name),
  err = doSomethingElse()
);

Lately, I've been utilizing the async pipe more frequently. This approach allows me to handle my observables in a cleaner way, as shown below.

thing$: Observable<Thing>;
...
ngOnInit(){
  this.thing$ = this.service.getThing(1337);
}

However, I am curious if there's a way to declare an operation that can extract a specific field from the observable value when it is realized.

<div *ngIf="thing$ | async as thing>
  {{thing.name}}
</div>

Instead of extracting the name manually, I wish to access only the name without additional steps.

I have attempted to explore the pipe(...) function in Angular thinking it might hold the solution. Unfortunately, my attempts were unsuccessful, and now I am unsure if that is the right approach.

Answer №1

If you need to transform data in an observable stream, you can leverage the power of the RxJS map operator:

import { map } from 'rxjs/operators';

itemName$: Observable<string>;

ngOnInit(){
  this.itemName$ = this.service.getItem(1337).pipe(map(item => item.name));
}
<div *ngIf="itemName$ | async as itemName>
  {{itemName}}
</div>

Answer №2

One alternative is to use the pluck function from the rxjs library.

import { Component, OnInit } from '@angular/core';
import { Observable } from 'rxjs';
import { ProductService } from '../shared/product.service';
import { pluck } from 'rxjs/operators';

@Component({
    selector: 'app-product',
    template: `<div *ngIf="productName$ | async as productName">
          {{productName}}
        </div>`,
    styleUrls: ['./product.component.css']
})
export class ProductComponent implements OnInit {
    productName$: Observable<string>;

    constructor(private productService: ProductService) { }

    ngOnInit() {
        this.productName$ = this.productService.getProduct(1).pipe(pluck('productName'));
    }

}

This specific example demonstrates how to extract the product name from a product fetched by the product service.

export interface Product {
    id: number;
    productCode: string;
    productName: string;
}

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