<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <input type="button" value="选择" id="btn">
    <br>
    <select id="food">
        <option>烤肉</option>
        <option>拉面</option>
        <option>麻辣烫</option>
        <option>小龙虾</option>
        <option>火锅</option>
        <option>外卖</option>
    </select>
    <script>
        // 1.获取元素
        var btn = document.getElementById("btn");
        var food = document.getElementById("food");
        var opts = food.getElementsByTagName("option");

        // 2.给按钮添加点击事件
        btn.onclick = function () {
            // 3.随机选择一个 option
            // 每次点击需要获取一个 opts 数组的随机下标
            // Math.random()   [0,1)
            // Math.random()*6   [0,6)
            // Math.floor()  向下取整数
            var n = Math.floor(Math.random() * opts.length);
            // console.log(n);
            // 设置对应的随机项的属性
            opts[n].selected = true;
        };
    </script>
</body>
</html>