<div> 
  <input type = "text" id = "input" value="" /> 
  <input type = "button" id="code" onclick="createCode()"/> 
  <input type = "button" value = "验证" onclick = "validate()"/> 
 </div> 

 #code{ 
    font-family:Arial; 
    font-style:italic; 
    font-weight:bold; 
    border:0; 
    letter-spacing:2px; 
    color:blue; 
   }

//设置一个全局的变量，便于保存验证码
 var code;
 function createCode(){
  //首先默认code为空字符串
  code = '';
  //设置长度，这里看需求，我这里设置了4
  var codeLength = 4;
  var codeV = document.getElementById('code');
  //设置随机字符
  var random = new Array(0,1,2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R', 'S','T','U','V','W','X','Y','Z');
  //循环codeLength 我设置的4就是循环4次
  for(var i = 0; i < codeLength; i++){
   //设置随机数范围,这设置为0 ~ 36
    var index = Math.floor(Math.random()*36);
    //字符串拼接 将每次随机的字符 进行拼接
    code += random[index]; 
  }
  //将拼接好的字符串赋值给展示的Value
  codeV.value = code;
 }
 //下面就是判断是否== 的代码，无需解释
 function validate(){
  var oValue = document.getElementById('input').value.toUpperCase();
  if(oValue ==0){
   alert('请输入验证码');
  }else if(oValue != code){
   alert('验证码不正确，请重新输入');
   oValue = ' ';
   createCode();
  }else{
   window.open('http://www.baidu.com','_self');
  }
 }
 //设置此处的原因是每次进入界面展示一个随机的验证码，不设置则为空
 window.onload = function (){
  createCode();
 }