VueJS: why is "this" undefined?

Both of those examples use an arrow function () => { }, which binds this to a context different from the Vue instance.

As per the documentation:

Don’t use arrow functions on an instance property or callback (e.g. vm.$watch('a', newVal => this.myMethod())). As arrow functions are bound to the parent context, this will not be the Vue instance as you’d expect and this.myMethod will be undefined.

In order to get the correct reference to this as the Vue instance, use a regular function:

mounted: function () {  console.log(this);}

Alternatively, you can also use the ECMAScript 5 shorthand for a function:

mounted() {  console.log(this);}

You are using arrow functions.

The Vue Documentation clearly states not to use arrow functions on a property or callback.

Unlike a regular function, an arrow function does not bind this. Instead, this is bound lexically (i.e. this keeps its meaning from its original context).

var instance = new  Vue({    el:'#instance',  data:{    valueOfThis:null  },  created: ()=>{    console.log(this)  }});

This logs the following object in the console:

Window {postMessage: ƒ, blur: ƒ, focus: ƒ, close: ƒ, frames: Window, …}

Whereas... If we use a regular function (which we should on a Vue instance)

var instance = new  Vue({    el:'#instance',  data:{    valueOfThis:null  },  created: function(){    console.log(this)  }});

Logs the following object in the console:

hn {_uid: 0, _isVue: true, $options: {…}, _renderProxy: hn, _self: hn, …}

If you want to keep using the arrow function, you could pass the component instance (this) as parameter like :

computed: {  foo: (vm) => { //vm refers to this     return vm.bar + 1;   } }