本文详细介绍如何使用JavaScript在前端实现验证码功能。首先讲解基础版，通过数组随机抽取四位字符生成验证码并实现验证逻辑。随后进阶介绍利用CSS3 transform属性实现字符旋转及随机干扰线，并通过透明DIV层覆盖防止机器自动识别，从而提升前端验证的安全性。
基础版验证码实现
基础版
基于日常上网经验，验证码通常由四位数字和字母组成。接下来将逐步演示如何使用JavaScript编写一个验证码脚本。首先展示成品效果，以便直观理解：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<style>
#securityCode{
background-color: #008000;
width:70px;
height:30px;
font-family: '楷体', serif;
font-size: 20px;
color:white;
}
</style>
<script language="JavaScript" type="text/javascript">
function createCode(){
var code=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');
var codeNumber;
securityCode="";//全局变量,方便后续验证
for(var i=0;i<4;i++){
codeNumber=Math.floor(Math.random()*36);
securityCode+=code[codeNumber];
}
document.getElementById("securityCode").value=securityCode;
}
function verify(){
var enterCode=document.getElementById("enterCode").value;
if(enterCode.toUpperCase()==securityCode){
alert("输入正确,通过验证!");
}
else{
enterCode.value="";
createCode();
}
}
</script>
<title>Jizhen Tan</title>
</head>
<body onLoad="checkCookie()" >
<input type="text" id="enterCode"><br/>
<input type="button" id="securityCode" onclick="createCode()">
<a href="###" onclick="createCode()">看不清楚</a><br/>
<input type="button" style="background-color: #0099FF; font-size: 20px;"value="验证" onclick="verify()">
</body>
</html>
1. 既然需要生成四位验证码，思路需适当拓展。首先，我们需要创建一个数组，用于存储可用的字母和数字字符。
var code=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');
2. 接下来，需要实现从数组中随机选取元素的功能。这里定义一个变量 codeNumber 用于存储随机生成的数字索引。由于我们需要四位验证码，而数组元素是单个字符，因此可以建立另一个变量 securityCode 来拼接这些字符。具体代码如下：
var codeNumber;
securityCode="";//全局变量,方便后续验证
for(var i=0;i<4;i++){
codeNumber=Math.floor(Math.random()*36);
securityCode+=code[codeNumber];
}
可以看出，此时 securityNumber 变量（注：原文此处指代 securityCode 逻辑）存储的即为一个由四位随机字符组成的验证码。3. 经过上述简单两步，我们便得到了四位验证码。最后，将其封装在 createCode 函数中。
function createCode(){
var code=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');
var codeNumber;
securityCode="";//全局变量,方便后续验证
for(var i=0;i<4;i++){
codeNumber=Math.floor(Math.random()*36);
securityCode+=code[codeNumber];
}
document.getElementById("securityCode").value=securityCode;
}
4. 接下来，我们创建一个验证机制：
function verify(){
var enterCode=document.getElementById("enterCode").value;
if(enterCode.toUpperCase()==securityCode){
alert("输入正确,通过验证!");
}
else{
enterCode.value="";
createCode();
}
}
5. 对验证码进行简单的美化修饰：
<style>
#securityCode{
background-color: #008000;
width:70px;
height:30px;
font-family: '楷体', serif;
font-size: 20px;
color:white;
}
</style>
高级防机器人技巧
进阶：进一步阻止机器人的高级技巧 在接触的大部分项目中，验证码通常由后台处理。近期遇到一个需要验证码的页面，起初考虑后台实现，但考虑到部分项目对安全性要求不高且需具备一定防机器人技巧，因此决定使用前端实现。该方案利用CSS3的transform属性中的rotate设置字符旋转，并随机添加干扰线。最后，为了在所有DOM节点上方叠加一层 opacity=0 的DIV，从而构建出一个前端验证码。
vCode代码：
(function(){
var randstr = function(length){
var key = {
str : [
'a','b','c','d','e','f','g','h','i','j','k','l','m',
'o','p','q','r','s','t','x','u','v','y','z','w','n',
'0','1','2','3','4','5','6','7','8','9'
],
randint : function(n,m){
var c = m-n+1;
var num = Math.random() * c + n;
return Math.floor(num);
},
randStr : function(){
var _this = this;
var leng = _this.str.length - 1;
var randkey = _this.randint(0, leng);
return _this.str[randkey];
},
create : function(len){
var _this = this;
var l = len || 10;
var str = '';
for(var i = 0 ; i<l ; i++){
str += _this.randStr();
}
return str;
}
};
length = length ? length : 10;
return key.create(length);
};
var randint = function(n,m){
var c = m-n+1;
var num = Math.random() * c + n;
return Math.floor(num);
};
var vCode = function(dom, options){
this.codeDoms = [];
this.lineDoms = [];
this.initOptions(options);
this.dom = dom;
this.init();
this.addEvent();
this.update();
this.mask();
};
vCode.prototype.init = function(){
this.dom.style.position = "relative";
this.dom.style.overflow = "hidden";
this.dom.style.cursor = "pointer";
this.dom.title = "点击更换验证码";
this.dom.style.background = this.options.bgColor;
this.w = this.dom.clientWidth;
this.h = this.dom.clientHeight;
this.uW = this.w / this.options.len;
};
vCode.prototype.mask = function(){
var dom = document.createElement("div");
dom.style.cssText = [
"width: 100%",
"height: 100%",
"left: 0",
"top: 0",
"position: absolute",
"cursor: pointer",
"z-index: 9999999"
].join(";");
dom.title = "点击更换验证码";
this.dom.appendChild(dom);
};
vCode.prototype.addEvent = function(){
var _this = this;
_this.dom.addEventListener("click", function(){
_this.update.call(_this);
});
};
vCode.prototype.initOptions = function(options){
var f = function(){
this.len = 4;
this.fontSizeMin = 20;
this.fontSizeMax = 48;
this.colors = [
"green",
"red",
"blue",
"#53da33",
"#AA0000",
"#FFBB00"
];
this.bgColor = "#FFF";
this.fonts = [
"Times New Roman",
"Georgia",
"Serif",
"sans-serif",
"arial",
"tahoma",
"Hiragino Sans GB"
];
this.lines = 8;
this.lineColors = [
"#888888",
"#FF7744",
"#888800",
"#008888"
];
this.lineHeightMin = 1;
this.lineHeightMax = 3;
this.lineWidthMin = 1;
this.lineWidthMax = 60;
};
this.options = new f();
if(typeof options === "object"){
for(i in options){
this.options[i] = options[i];
}
}
};
vCode.prototype.update = function(){
for(var i=0; i<this.codeDoms.length; i++){
this.dom.removeChild(this.codeDoms[i]);
}
for(var i=0; i<this.lineDoms.length; i++){
this.dom.removeChild(this.lineDoms[i]);
}
this.createCode();
this.draw();
};
vCode.prototype.createCode = function(){
this.code = randstr(this.options.len);
};
vCode.prototype.verify = function(code){
return this.code === code;
};
vCode.prototype.draw = function(){
this.codeDoms = [];
for(var i=0; i<this.code.length; i++){
this.codeDoms.push(this.drawCode(this.code[i], i));
}
this.drawLines();
};
vCode.prototype.drawCode = function(code, index){
var dom = document.createElement("span");
dom.style.cssText = [
"font-size:" + randint(this.options.fontSizeMin, this.options.fontSizeMax) + "px",
"color:" + this.options.colors[randint(0, this.options.colors.length - 1)],
"position: absolute",
"left:" + randint(this.uW * index, this.uW * index + this.uW - 10) + "px",
"top:" + randint(0, this.h - 30) + "px",
"transform:rotate(" + randint(-30, 30) + "deg)",
"-ms-transform:rotate(" + randint(-30, 30) + "deg)",
"-moz-transform:rotate(" + randint(-30, 30) + "deg)",
"-webkit-transform:rotate(" + randint(-30, 30) + "deg)",
"-o-transform:rotate(" + randint(-30, 30) + "deg)",
"font-family:" + this.options.fonts[randint(0, this.options.fonts.length - 1)],
"font-weight:" + randint(400, 900)
].join(";");
dom.innerHTML = code;
this.dom.appendChild(dom);
return dom;
};
vCode.prototype.drawLines = function(){
this.lineDoms = [];
for(var i=0; i<this.options.lines; i++){
var dom = document.createElement("div");
dom.style.cssText = [
"position: absolute",
"opacity: " + randint(3, 8) / 10,
"width:" + randint(this.options.lineWidthMin, this.options.lineWidthMax) + "px",
"height:" + randint(this.options.lineHeightMin, this.options.lineHeightMax) + "px",
"background: " + this.options.lineColors[randint(0, this.options.lineColors.length - 1)],
"left:" + randint(0, this.w - 20) + "px",
"top:" + randint(0, this.h) + "px",
"transform:rotate(" + randint(-30, 30) + "deg)",
"-ms-transform:rotate(" + randint(-30, 30) + "deg)",
"-moz-transform:rotate(" + randint(-30, 30) + "deg)",
"-webkit-transform:rotate(" + randint(-30, 30) + "deg)",
"-o-transform:rotate(" + randint(-30, 30) + "deg)",
"font-family:" + this.options.fonts[randint(0, this.options.fonts.length - 1)],
"font-weight:" + randint(400, 900)
].join(";");
this.dom.appendChild(dom);
this.lineDoms.push(dom);
}
};
this.vCode = vCode;
}).call(this);
用法：
//container 为 验证码的DOM节点
var code = new vCode(container);
// 验证是否正确
// inputCode为用户输入的验证码
code.verify(inputCode); // return true or false