[Vuejs]-Vm.attribute vs. this.attribute

You can see the following after code

    export default{
        data(){
            return{
                title:"Form Register",
                formdata:{},
                message:"",
                success:0,
            }
        },
        methods:{
           register(){
                this.axios.post("http://localhost:8888/form-register",this.formdata).then((response) => {
                       console.log(response);
                       if(response.data.success>0){
                           this.message="You register success";
                           this.success=response.data.success;
                       }
                       else{
                           this.message="Register to failed";
                           this.success=response.data.success;
                       }
                  });
                    
            },
 
        }
    }

If you use arrow functions like () => {...} it binds current context. And this will point to right context. So if you use it instead of function() {...} where context is not binded it will work. Like

.then(response => {this.attribute = 'value'}

This is a really common stumbling block for developers. The reason is that inside an axios function your code out of scope of the object containing the axios method call. This can more easily be see if you rewrite this block of code to be like the one below it:

 var vm = {
      function doWork(){
        axios.post('my/route', {
              attribute: this.attribute //this works
        }).then(function (response) {
              vm.attribute = 'value'; //this works
              this.attribute = 'value'; //this does not work
         });
      }
 }

is functionally equivalent to:

 var vm = {
      function doWork(){
          axios.post('my/route', {
                attribute: this.attribute //this works
           }).then(followOnWork(response));
      }
 }

 function followOnWork(response){
      vm.attribute = 'value'; //this works
      this.attribute = 'value'; //this does not work
 }

As you can see in this refactored code, followOnWork is running totally independently of the vm object. So any use of a this variable in followOnWork isn’t going to relate to the vm object. Where as vm is the actual name of the object and once the object is created it can be accessed from anywhere via the vm variable.

You can get around this “out of scope” problem by using an arrow function (as mentioned by @MakarovSergey) if ES6 is an option for you.