Using rxjs withLatestFrom function

Question

Using rxjs withLatestFrom function

Attempting to execute the following code, which should:

Get a list of all user chats
get the latest message for all these chats
Get recipient information for all these chats

Concatenate everything together as an array where each item is a combination of chat, last message, and recipient information.

 this.msgService.getUserChatList(this.uid).do((chats) => {
    this.chats = [];
    if (!(chats.length > 0))
      this.loading = false;
    this.getLastMessages$ = this.getLastMessagesForChats(chats);
    this.getRecipients$ = this.getRecipientsForChats(chats);
}).switchMap( chats => Observable.from(chats) ).withLatestFrom(
   this.getLastMessages$,
   this.getRecipients$,
   (chat, lastMessages, recipients) => ({
     chat: chat,
     last: lastMessages[chat['id']],
     recipient: recipients[chat['id']]
    })
).subscribe( chats => {
  console.log('chats ', chats);
  this.chats.push(chats);
  this.loading = false;
});

Additional functions

getLastMessagesForChats(chats: any): Observable<any[]> {
    let lastMessages$ = [];
    for (let chat of chats) {
      let obs = this.msgService.getLastMessage(chat.id)
        .map( last => ({chat: chat.id, last: last}) );
      lastMessages$[chat.id] = obs;
    }
    return Observable.from(lastMessages$).merge().toArray();
  }

  getRecipientsForChats(chats: any): Observable<any[]> {
    let recipients$ = [];
    for (let chat of chats) {
      let obs = this.userService.getUserPublicInfo(chat.recipient)
        .map( recipient => ({chat: chat.id, recipient: recipient}) );
      recipients$[chat.id] = obs;
    }
    return Observable.from(recipients$).merge().toArray();
  }

I am getting the following error

Uncaught (in promise): TypeError: Cannot read property 'subscribe' of undefined
    TypeError: Cannot read property 'subscribe' of undefined

I can't find what is going wrong ... I tried to reproduce the following JSBIN

Any idea?

+3

observable rxjs rxjs5

Manuel rodriguez Jul 29. 17 at 11:41

source to share

1 answer

martin · Answer 1 · 2017-07-29T12:31:16+0000

The chain is built from bottom to top. This means the order is subscribe()

→ withLatestFrom

→ switchMap

→ do

.

So, at the point where it withLatestFrom

tries to subscribe to getLastMessages$

and getRecipients$

, they are undefined

, because they are only assigned after the first value propagates from the Observable that originates in do()

.

Edit:

// Execution
getLastMessages$ = Rx.Observable.of(1);
getRecipients$ = Rx.Observable.of(2);
chats = [];

getUserChatList('uC')
  .do( (chats) => {
    getLastMessages$ = getLastMessagesForChats(chats);
    getRecipients$ = getRecipientsForChats(chats);
  } )
  .switchMap( chats => Rx.Observable.from(chats) )
  .withLatestFrom(
    getLastMessages$,
    getRecipients$,
    (chat, lastMessages, recipients) => ({
        chat: chat,
        last: lastMessages[chat['id']],
        recipient: recipients[chat['id']]
      }))
  .subscribe( c => {
    console.log('chats ', c);
    chats.push(c);
  });

http://jsbin.com/sulatar/3/edit?js,console

Using rxjs withLatestFrom function

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