js随机生成一个验证码

<p>验证码：</p>
 <div id="login" onclick="change()">
  <a href="#" rel="external nofollow" ></a>
 </div>

 function getCode(n) {
  var all = "azxcvbnmsdfghjklqwertyuiopZXCVBNMASDFGHJKLQWERTYUIOP0123456789";
  var b = "";
  for (var i = 0; i < n; i++) {
   var index = Math.floor(Math.random() * 62);
   b += all.charAt(index);

  }
  return b;
 };

 function change() {
  document.getElementById("login").innerHTML = getCode(4);

 }
 window.onload = change;