<style>
#code{ ?
font-family:Arial; ?
font-style:italic; ?
font-weight:bold; ?
border:0; ?
letter-spacing:2px; ?
color:blue; ?
??}
</style>
<div> ?
<input type = "text" id = "input" value="" /> ?
<input type = "button" id="code" onclick="createCode()"/> ?
<input type = "button" value = "验证" onclick = "validate()"/> ?
</div>
<script>
//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{
???????????alert(‘验证码正确‘);
???????}
???}
???//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();
???}
</script>
js编写验证码以及验证
原文地址：http://www.cnblogs.com/timluo/p/7485554.html