How can I update a Vue app's or component's property in a promise call back?

Question

I need to update a Vue property in a Firebase callback as follow, but it's not working. This code

methods: {
    sign_out : function(e) {
        this.item_count = 0
    }
}

works, but when the property is set in a promise callback:

methods: {
  sign_out : function(e) {
  firebase.auth().signOut().then(function() {
      this.item_count = 0
    })
  },

How can I set a property's value in this case?

Answer 1

Your this in your callback is pointing to the wrong object. There are a few ways you can fix this.

1. Capture this in a closure.

   methods: {
     sign_out : function(e) {
       var self = this;
       self.item_count = 0
       firebase.auth().signOut().then(function() {
         self.item_count = 0
     })
   }
   

2. Use a fat arrow.

   methods: {
     sign_out : function(e) {
       this.item_count = 0
       firebase.auth().signOut().then(() => this.item_count = 0)
     }
   }
   

3. Use bind().

   methods: {
     sign_out : function(e) {
       this.item_count = 0
       firebase.auth().signOut().then(function() {
         this.item_count = 0
       }.bind(this))
   }
   

Fat arrows will not work in all modern browsers yet, so only use them if you are compiling to es5.