HTML DOM Select 对象:https://www.w3school.com.cn/jsref/dom_obj_select.asp
js随机数的生成:https://www.runoob.com/w3cnote/js-random.html
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Title</title> <script> window.onload = function () { // 1,给按钮注册事件 var btnSet = document.getElementById('btnSet'); btnSet.onclick = function () { // 2,获取下拉框的所有option:伪数组 var selCity = document.getElementById('selCity'); var options = document.getElementsByTagName('option'); // 3,随机生成一个索引 范围在0-4 // Math.random() 范围[0,1) var randomIndex = parseInt(Math.random() * options.length); // 4,根据索引获取option 并让option选中 selected = "selected" options[randomIndex].selected = true; } } </script> </head> <body> <input type="button" value="设置" id="btnSet"> <select name="" id="selCity"> <option value="1">北京</option> <option value="2">上海</option> <option value="3">加油</option> <option value="4">你好</option> <option value="5">努力啊</option> </select> </body> </html>