JS验证码实现代码

<script language="javascript">var code; //在全局 定义验证码function createCode(){ //创建验证码函数 code = ""; var codeLength =4;//验证码的长度 var selectChar = 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');//所有候选组成验证码的字符，当然也可以用中文的 for(var i=0;i<codeLength;i++) {  var charIndex =Math.floor(Math.random()*36); code +=selectChar[charIndex];  }// 设置验证码的显示样式，并显示 document.getElementById("discode").style.fontFamily="Fixedsys"; //设置字体 document.getElementById("discode").style.letterSpacing="5px"; //字体间距 document.getElementById("discode").style.color="#0ab000"; //字体颜色 document.getElementById("discode").innerHTML=code; // 显示}function but(){//验证验证码输入是否正确 var val1=document.getElementById("t1").value; var val2=code; if(val1!=val2){  alert("验证码错误!"); document.getElementById('t1').value="";  } }</script>

 <div class="input">    <input id="t1" type="text" name="u" placeholder="验证码" onblur="but()" />    <span id="discode"></span>    <input type="button" value="换一换" class="c" style="height:20px;"onClick="createCode()">    </div>