​
function verify(a = 6,b = "num"){
  //定义三个随机验证码验证码库
  var num ="0123456789"
  var str ="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNIPQRSTUVWXYZ"
  var mixin = num +str;
  
  //定义一个空字符串用来存放验证码
  var verify=""
  if(a == undefined || b == undefined){
    //验证输入是否合法  不通过就抛出一个异常
    throw new Error("参数异常");
  }else{
      if(a ==""||b==""){
        //判断用户是否没有输入
        throw new Error("参数非法.");
      }else{
        //检测输入的类型来判断是否进入
        var typea = typeof(a);
        var typeb = typeof(b);
        if(typea =="number" && typeb =="string"){
            if(b == "num"){
                  
                //定义一个循环来接收验证码    纯数字验证码
                for(var i=0;i<a;i++){
                     //定义一个变量来存储颜色的随机值
                     var r1 = Math.random()*255;
                     var g1 = Math.random()*255;
                     var b1 = Math.random()*255;
 
                     //确定随机索引
                     var index = Math.floor(Math.random()*(num.length-1))
                     //确定随机的验证码
                     var char = num[index];
                     //给随机的验证码加颜色
                     verify += `<span style ='color:rgb(${r1},${g1},${b1})'>${char}</span>`
                 }
                 //返回到数组本身
                return verify;
            }else if(b =="str"){
                 for(var i=0;i<a;i++){
                   //纯字母的验证码
                    var r1 = Math.random()*255;
                    var g1 = Math.random()*255;
                    var b1 = Math.random()*255;
 
                    var index = Math.floor(Math.random()*(str.length-1));
                    var char = str[index];
 
                    verify += `<span style ='color:rgb(${r1},${g1},${b1})'>${char}</span>`
                 }
                 return verify;   
            }else if(b == "mixin"){
                 // 混合型的验证码
                for(var i=0;i<a;i++){
                    var r1 = Math.random()*255;
                    var g1 = Math.random()*255;
                    var b1 = Math.random()*255;
 
                    var index = Math.floor(Math.random()*(mixin.length-1));
                    var char = mixin[index];
 
                    verify += `<span style ='color:rgb(${r1},${g1},${b1})'>${char}</span>`
                }
                return verify;
            }else{
                //验证没通过抛出一个异常
                throw new Error("输入类型非法.")
            }
        
        }else{
            //验证没通过抛出一个异常
            throw new Error("输入类型非法.")
        }
      }
  }
}
 
​

  //第一个值为用户输入的长度，第二个为类型！ 
 var arr = verify(8,"mixin");
     document.write(arr)