screenshots.gif

<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

    </style>
</head>
<body>
    <input type="button" value="随机选择一个" 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>

//获取元素
var btn = document.getElementById("btn");
var food = document.getElementById("food");
var opts = food.getElementsByTagName("option");

//按钮添加点击事件
btn.onclick = function(){
    //随机选择一个option
    //每次点击需要获一个opts数组的随机下标
    //Math.random()    [0,1]
    //Math.random()*6    [0,6]
    var n = Math.floor(Math.random() * opts.length);
    //console.log(n);
    opts[n].selected = true;

};

    </script>
</body>
</html>