[Vuejs]-Vue + axios returns undefined

You can’t use arrow function for methods declaration.

See https://v2.vuejs.org/v2/api/#methods

Note that you should not use an arrow function to define a method
(e.g. plus: () => this.a++). The reason is arrow functions bind the
parent context, so this will not be the Vue instance as you expect and
this.a will be undefined.

These are the 2 ways to properly define a method

1.

        getPosts: function() {
    
        }

2. (if you can use ES6)

        getPosts() {
   
        }