data does not update in vue js

Question

i have a simple form like so

<form @submit.prevent="getSummary">
    <input type="text" v-model="userAcccount" placeholder="Enter Account 
    Number">
    <button type="submit" class="btn btn-xs btn-default">Submit</button>
</form>

and the getSummary method in my methods object in vue is like this

methods: {
        getSummary () {               
            axios.get('core/handle.php', {
                params: {
                  accountNumber: this.userAcccount
                }
              })
              .then(function (response) {
                this.userData = response.data;
                console.log(this.userData);
              })
              .catch(function (error) {
                alert(error);
            });
        }
    }

my data object contains

data: {
        userAcccount: '',
        userData: []
    },

i am trying to update the userData with the response from axios and then use it to populate a table for testing purposes i just tried this

<span v-if="userData.length > 0">
      some data 
</span>
<span v-else>
      data not found
</span>

console.log shows me the userData has be updated but this bit of code above doesn't change.

Answer 1

Using ES6 arrow functions you can access the response in the callback by rewriting slightly, like so:

.then(response => {
    this.userData = response.data;
    console.log(this.userData);
})

This is because you are currently reaching the axios request context with this, which obviously is null as userData isn't in your scope. By using the arrow function syntax you get access to the parent component where userData resides.

Answer 2

You have no access to the VM via this inside your then function. You have three ways to fix this:

1. Explicitly bind the outer this as execution context to the function (ES5 functions have their own this):

   .then(function (response) {
       this.userData = response.data; 
       console.log(this.userData);
   }.bind(this))
   

2. Store the this context in a local variable e.g. self or that:

   getSummary () {
     const self = this;
     axios.get('core/handle.php', {
       params: {
         accountNumber: this_.userAcccount
       }
     })
     .then(function (response) {
       self.userData = response.data;
       console.log(self.userData);
     })
     .catch(function (error) {
       alert(error);
     });
   }
   

3. Use ES6 arrow functions which don't have their own this:

   .then(response => {
     this.userData = response.data;
     console.log(this.userData);
   })
   

The concept of this in Javascript differs alot from many other programming languages as it describes the execution context of a function. It is not an easy-to-grab concept, but this helped me get into it:

http://javascriptissexy.com/understand-javascripts-this-with-clarity-and-master-it/

For reference, MDN is always a good place to go:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this