[方法]随机获取六位验证码

验证码由大写字母 或小写字母 或  数字组成

function yanNum(){
    str = [];
    for( var i = 1;i <= 4; i++ ){
        var ma =  numRandom(122,48);
        if( ma >= 48 && ma <= 57 || ma >= 65 && ma <= 90 || ma >= 97 && ma <= 122 ){
            var st = String.fromCharCode(ma);
            str.push(st);
        }else{
            i--;
        }
    }
     return str    
 }