var code //在全局定义验证码
//产生验证码
window.onload = function createCode(){
code = ""
var codeLength = 4//验证码的长度
var checkCode = document.getElementById("code")
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')//随机数
for(var i = 0i <codeLengthi++) {//循环操作
var index = Math.floor(Math.random()*36)//取得随机数的索引（0~35）
code += random[index]//根据索引取得随机数加到code上
}
checkCode.value = code//把code值赋给验证码
}
//校验验证码
function validate(){
var inputCode = document.getElementById("input").value.toUpperCase()//取得输入的验证码并转化为大写
if(inputCode.length <= 0) { //若输入的验证码长度为0
alert("请输入验证码！")//则弹出请输入验证码
}
else if(inputCode != code ) { //若输入的验证码与产生的验证码不一致时
alert("验证码输入错误！@_@")//则弹出验证码输入错误
createCode()//刷新验证码
document.getElementById("input").value = ""//清空文本框
}
else { //输入正确时
alert("^-^")//弹出^-^
}
}
简单代码如下：<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>JS生成验证码</title>
<style type="text/css">
.code {
background-image: url(code.jpg)
font-family: Arial
font-style: italic
color: Red
border: 0
padding: 2px 3px
letter-spacing: 3px
font-weight: bolder
}
.unchanged {
border: 0
}
</style>
<script language="javascript" type="text/javascript">
var code//在全局 定义验证码
function createCode() {
code = ""
var codeLength = 6//验证码的长度
var checkCode = document.getElementById("checkCode")
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 = 0i <codeLengthi++) {
var charIndex = Math.floor(Math.random() * 36)
code += selectChar[charIndex]
}
//alert(code)
if (checkCode) {
checkCode.className = "code"
checkCode.value = code
}
}
function validate() {
var inputCode = document.getElementById("input1").value
if (inputCode.length <= 0) {
alert("请输入验证码！")
} else if (inputCode != code) {
alert("验证码输入错误！")
createCode()//刷新验证码
} else {
alert("^-^ OK")
}
}
</script>
</head>
<body onload="createCode()">
<form action="#">
<input type="text" id="input1" /><input type="text" onclick="createCode()" readonly="readonly" id="checkCode" class="unchanged" style="width: 80px" /><br />
<input id="Button1" onclick="validate()" type="button" value="确定" />
</form>
</body>
</html>
注意别用在实际生产项目中
可以用JS做个简单的验证码 <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 =5//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=0i<codeLengthi++)
{
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="3px" //字体间距
document.getElementById("discode").style.color="#ff0000" //字体颜色
document.getElementById("discode").innerHTML=code // 显示
}</script>把上面一段JS代码放到<head></head>标签中在页面装载的时候，调用验证码创建函数 <body onload="createCode()
">在验证码输入框的后面放一个标签 <span id="discode"></span>
