How to use a .catch in firebase onSnapshot

Question

I've created an Ionic Firebase chat app. The problem I think I'm having is I've set up a query snapshot on initialization of the message page with the following:

  ngOnInit() {
      this.messageService.getAllMessages()
      .doc(`${this.userId[0] + '-' + this.userId[1]}`)
      .collection('message')
      .orderBy('createdAt', 'asc')
      .onSnapshot((doc) => {
        this.messages = [];
        doc.forEach((snap) => {
          this.messages.push({
            content: snap.data().content,
            createdAt: snap.data().createdAt,
            userId: snap.data().userId
          });
        });
        console.log('messages', this.messages);
      });
  }

The problem is if there are no messages on the initial attempt to send a message the page doesn't load the first time I attempt to go to it.

The message is a little obscure but I'm pretty sure I'm having this issue because Firebase isn't returning any data but there's no way to add a .catch in the query to catch the error and handle it so that the user can still navigate to the message page.

ERROR Error: Uncaught (in promise): Error: No value accessor for form control with unspecified name attribute
Error: No value accessor for form control with unspecified name attribute
    at _throwError (vendor.js:70776)
    at setUpControl (vendor.js:70646)
    at NgModel._setUpStandalone (vendor.js:73693)
    at NgModel._setUpControl (vendor.js:73668)
    at NgModel.ngOnChanges (vendor.js:73604)
    at checkAndUpdateDirectiveInline (vendor.js:63813)
    at checkAndUpdateNodeInline (vendor.js:65493)
    at checkAndUpdateNode (vendor.js:65432)
    at debugCheckAndUpdateNode (vendor.js:66400)
    at debugCheckDirectivesFn (vendor.js:66343)
    at resolvePromise (polyfills.js:3193)
    at resolvePromise (polyfills.js:3150)
    at polyfills.js:3254
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (polyfills.js:2785)
    at Object.onInvokeTask (vendor.js:57358)
    at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (polyfills.js:2784)
    at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (polyfills.js:2557)
    at drainMicroTaskQueue (polyfills.js:2963)
defaultErrorLogger @ vendor.js:55398
handleError @ vendor.js:55450
next @ vendor.js:57952
schedulerFn @ vendor.js:52576
__tryOrUnsub @ vendor.js:133918
next @ vendor.js:133857
_next @ vendor.js:133804
next @ vendor.js:133781
next @ vendor.js:133566
emit @ vendor.js:52556
(anonymous) @ vendor.js:57389
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invoke @ polyfills.js:2753
push../node_modules/zone.js/dist/zone.js.Zone.run @ polyfills.js:2512
runOutsideAngular @ vendor.js:57315
onHandleError @ vendor.js:57389
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.handleError @ polyfills.js:2757
push../node_modules/zone.js/dist/zone.js.Zone.runGuarded @ polyfills.js:2526
_loop_1 @ polyfills.js:3056
api.microtaskDrainDone @ polyfills.js:3065
drainMicroTaskQueue @ polyfills.js:2970
Promise.then (async)
scheduleMicroTask @ polyfills.js:2946
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.scheduleTask @ polyfills.js:2775
onScheduleTask @ polyfills.js:2663
push../node_modules/zone.js/dist/zone.js.ZoneDelegate.scheduleTask @ polyfills.js:2766
push../node_modules/zone.js/dist/zone.js.Zone.scheduleTask @ polyfills.js:2600
push../node_modules/zone.js/dist/zone.js.Zone.scheduleMicroTask @ polyfills.js:2620
scheduleResolveOrReject @ polyfills.js:3241
resolvePromise @ polyfills.js:3187
(anonymous) @ polyfills.js:3103
webpackJsonpCallback @ runtime.js:26
(anonymous) @ pages-messages-messages-module.js:1
vendor.js:116244 POST https://firestore.googleapis.com/google.firestore.v1.Firestore/Write/channe

So my question is, is there a way to handle null return values in the .Snapshot query in firebase?

I've tried this:

  ngOnInit() {
    try {
      this.messageService.getAllMessages()
      .doc(`${this.users[0] + '-' + this.users[1]}`)
      .collection('message')
      .orderBy('createdAt', 'asc')
      .onSnapshot((doc) => {
        this.messages = [];
        doc.forEach((snap) => {
          this.messages.push({
            content: snap.data().content,
            createdAt: snap.data().createdAt,
            userId: snap.data().userId
          });
        });
        console.log('messages', this.messages);
      });
    } catch (error) {
      console.log('Message page error', error);
    }
  }

Answer 1

While Doug answer is correct, you still need to catch error for onSnapshot when permission is denied

For that, use the following syntax:

const unsubscribeMe = firestoreInstance
    .collection('example')
    .onSnapshot(
        querySnapshot => {
            if (querySnapshot.empty) {
                return
            }
            const organizations = querySnapshot.docs.map(ref => ({
                id: ref.id,
                ...ref.data(),
            }))
        },
        error => {
            console.log(error)
        }
    )

See the javascript reference here which contain additional method signature that may fit your need.

Answer 2

A Firestore query doesn't fail if it finds no matching documents. It simply provides a QuerySnapshot that contains no documents.

The doc variable in your code is a QuerySnapshot type object (so, it's not a "document"). It has a method called empty() that you can use to see if it contains no documents. Alternately, you can check its docs property for zero length.

.onSnapshot(querySnapshot => {
  if (!querySnapshot.empty()) {
    this.messages = [];
    querySnapshot.forEach((snap) => {
      this.messages.push({
        content: snap.data().content,
        createdAt: snap.data().createdAt,
        userId: snap.data().userId
      });
    });
  }
  else {
    // what do you want to do if there are no documents?
  }
});

Based on the suggestions I've updated my code with the following and having the same issue:

ngOnInit() {
  this.messageService.getAllMessages()
    .doc(`${this.users[0] + '-' + this.users[1]}`)
    .collection('message')
    .orderBy('createdAt', 'asc')
    .onSnapshot((doc) => {
      if (!doc.empty) {
        this.messages = [];
        doc.forEach((snap) => {
          this.messages.push({
            content: snap.data().content,
            createdAt: snap.data().createdAt,
            userId: snap.data().userId
          });
        });
      } else {
        this.messages = [];
      }

      console.log('messages', this.messages);
    });
}

<ion-list lines="none" *ngIf="messages.length > 0">
  <ion-item *ngFor="let msg of messages; index as i; trackBy: trackByCreated">
    <div size="9" *ngIf="theirItinerary.userId === msg.userId" class="message other-user">
      <span>{{msg.content}}</span>
      <div class="time" text-right><br> {{msg.createdAt | date: 'short'}}</div>
    </div>
    <div offset="3" size="9" *ngIf="theirItinerary.userId !== msg.userId" class="message me" slot="end">
      <span>{{msg.content}}</span>
      <div class="time" text-right><br> {{msg.createdAt | date: 'short'}}</div>
    </div>
  </ion-item>
</ion-list>
</ion-content>

<ion-footer>
  <ion-toolbar light="light">
    <ion-row align-items-center no-padding>
      <ion-col size="8">
        <textarea autosize maxRows="3" [(ngModel)]="newMsg" class="message-input"></textarea>
      </ion-col>
      <ion-col size="3">
        <ion-button expand="block" fill="clear" color="primary" [disabled]="newMsg === ''" class="msg-btn" (click)="sendMessage()">
          <ion-icon name="ios-send" slot="icon-only"></ion-icon>
        </ion-button>
      </ion-col>
    </ion-row>
  </ion-toolbar>
</ion-footer>