Why are the data fields of the parent component available in the child - when using the debugger?

Question

Vue.js

Hello, my parent component (view) has child components and I am using this. in the child component I can access the data of the parent, why is this happening if I do not use props
For example, in the parent

data() {
    return {
      test: "context",
    };
  },

on the child component

mounted() {
    alert(this.page);
  },

In the debugger, I see the value test
However, when executing undefined

Answer 1

The problem was fixed by using bind(this);

global.context.authorization.$on(
      "authorizationMessage",
      function (msg) {
        this.totalAmount++;
        this.messages.set(this.totalAmount, msg);
      }.bind(this)
    );