The pointing problem of this in data return function and mounted function in Vue

Test and reflection triggered by the "Communication between non-parent and Child components" section of the official website tutorial:
is about creating two child components, and the value of component an is passed to component b.

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Vue</title>
    <script src="https://cdn.bootcss.com/vue/2.5.16/vue.min.js"></script>
</head>
<body> 
    <div id="example-7">
        :
        Vue
        <a-component></a-component>
        <b-component></b-component>
    </div>
</body>
<script>
    //
    let bus = new Vue();
    Vue.component("a-component", {
        template: `<button @click="idSelected">A</button>`,
        methods: {
            //June[Question]this?
            idSelected: function () {
                bus.$emit("id-selected", 1);//A
            }
        }
    });
    Vue.component("b-component", {
        data() {
            //June[Question]this?
            console.log(this);
            return {
                value: ""
            }
        },
        template: `<p v-model="value">BA:{{value}}`,
        //June[Question]mountedmethods
        mounted() {
            //A
            bus.$on("id-selected", function (id) {
                console.log(this);
                this.value = id;
                console.log("BA:" + id);
                //thisthis o{_uid: 30, _isVue: true, $options: {}, _renderProxy: o, _self: o,}
            // }.bind(this));
            //thisvalue,this hn{_uid: 27, _isVue: true, $options: {}, _renderProxy: hn, _self: hn,}
        });
        }
    });
    let example7 = new Vue({
        el: "-sharpexample-7"
    });

</script>
</html>

demo means that when you click component A"s button "pass the value of component A", the emit function can pass the number 1 to component B, which then assigns it to its own property and displays it on the page. Passing is not a problem, but it is found that without binding this, to the callback function of bus.$on in B at the beginning, the value in component B cannot be assigned, so it is suspected that this points to the problem, so this.

is printed out.

 bus.$on("id-selected", function (id) {
                console.log(this);
                this.value = id;
                console.log("BA:" + id);
 });

BidBvalue$onthisdata(){}thisvalue

Bthis,

you can see that the value of component B can be assigned successfully this time, and the direction of this is the same.

< H1 > Question? < / H1 >

1. My understanding is that this is bound to a global window object before it is assigned, right?
2. Who can explain what o {_ uid:30.}, hn {_ uid:27.} mean?

Thank you!