How to transform an array using observables in RXJS?

Question

I have this problem. I have an interface called Person which has a optional key called asked. I need to pass an array of that interface to a function which return an observable of type Person[] using RXJS.


interface Person {
    name string;
    asked?: boolean;
}

const persons: Person[] = [
    {name: "Wako"},
    {name: "Yako"},
    {name: "Dot"},
];

const persons$ = (persons: Person[]): Observable<Person[]> {
    return of(persons).pipe(
        map((p: Person) => {
            p.asked = true;
            return p;
        })
    );
}

But I have this problem:

Type 'Observable<Person>' is not assignable to type 'Observable<Person[]>'.
  Type 'Person' is missing the following properties from type 'Person[]': length, pop, push, concat, and 16 more.ts(2322)

What does my function persons$ has to do to return a Observable<Person[]>.

You seem to be confusing an observable map with an array map. You probably want both: map((people) => people.map((person) => ({ ...person, asked: true }))). — jonrsharpe
– jonrsharpe, Commented Nov 11, 2020 at 23:55
This would be inside the pipe? Because I need to do it using rxjs. — akko
– akko, Commented Nov 12, 2020 at 0:00
Yes, that would be inside the pipe. map is from RxJS, Array#map is not. — jonrsharpe
– jonrsharpe, Commented Nov 12, 2020 at 0:01
Thank you! I was using the map of rxjs as a regular map. This solved my problem. Thanks — akko
– akko, Commented Nov 12, 2020 at 6:39

Heartbit · Accepted Answer · 2020-11-12 08:42:39Z

3

You can do it with another solution:

import { from, Observable } from "rxjs";
import { map, toArray } from "rxjs/operators";

interface Person {
  name: string;
  asked?: boolean;
}

const persons: Person[] = [
  { name: "Wako" },
  { name: "Yako" },
  { name: "Dot" },
];

const persons$ = (persons: Person[]): Observable<Person[]> => {
  return from(persons).pipe(
    map((p: Person) => ({ ...p, asked: true })),
    toArray(),
  );
};

persons$(persons)
  .subscribe((x) => console.log(x));

answered Nov 12, 2020 at 8:42

Heartbit

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Comments

SrThompson · Accepted Answer · 2020-11-12 23:10:40Z

1

Observable.of(...args) will always unwrap the array and turn into Observable<Person>. If you want an Observable<Person[]> you need to do something like:

const persons$ = (persons: Person[]): Observable<Person[]> {
    return of(persons).pipe(
        map((people: Person[]) => people.map(p => ({...p, asked: true})))
    );
}

edited Nov 12, 2020 at 23:10

answered Nov 12, 2020 at 0:09

SrThompson

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3 Comments

akko Over a year ago

Thank you! I was using the rxjs map as a regular map, but now I can see the difference.

jonrsharpe Over a year ago

Only if they'd done of(...persons) would each person be emitted separately; it's from that emits array elements one-by-one. I think what you show now would emit an array containing an array.

SrThompson Over a year ago

You're right @jonrsharpe, I think the actual error was from OP's wrong typecasting in the map operator, declaring the callback as (person: Person) => instead of (person: Person[]) =>

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