index2.html

<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <title>Document</title>
        <script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
    </head>
    <body>
        <div id="app">
            <p>{{ foo }}</p>
            <!-- 这里的 `foo` 不会更新！ -->
            <button v-on:click="foo = 'baz'">Change it</button>
        </div>
        <script>
            var obj = {
                foo: "bar",
            };

            Object.freeze(obj);

            new Vue({
                el: "#app",
                data: obj,
            });

            var data = { a: 1 };

            var vm = new Vue({
                el: "#example",
                data: data,
            });

            vm.$data == data;
            vm.$el == document.getElementById("example");

            vm.$watch("a", function (newValue, oldValue) {});

            new Vue({
                data: {
                    a: 1,
                },
                created: function () {
                    // `this` 指向 vm 实例
                    console.log("a is: " + this.a);
                },
            });
        </script>
    </body>
</html>