老师请检查，有关于等于与冒号的问题！

<!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="./vue.js"></script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="num1">
        <select v-model="opt">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">x</option>
        <option value="/">÷</option>
        </select>
        <input type="text" v-model="num2">
        <button @click="cal">=</button>
        <span>{{result}}</span>

    </div>

    <script>
        const app = {
            data(){
                return{
                   num1:"",
                   num2:"",
                   opt:"",
                   result:""
                }
            },

            methods:{
                cal () {
                switch (this.opt) {
                    case '+':
                        this.result = parseFloat(this.num1) + parseFloat(this.num2);
                        break;
                    case '-':
                        this.result = parseFloat(this.num1) - parseFloat(this.num2);
                        break;
                    case '*':
                        this.result = parseFloat(this.num1) * parseFloat(this.num2);
                        break;
                    case '/':
                        this.result = parseFloat(this.num1) / parseFloat(this.num2);
                        break;

   }
}


                }
            }

        Vue.createApp(app).mount("#app")
    </script>

</body>
</html>