生成四位不重复的验证码。

 var block = document.getElementById();
    var link = document.getElementById();
    function queryCode () {
        var codaArea = 'qwertyuiopasdfghjklzxcvbnm' +
                'QWERTYUIOPASDFGHJKLZXCVBNM' +
                '1234567890';
        var result='';
        for (var i = 1;i<4;i++) {
            var n = Math.round(Math.random()*61);
            var m=codaArea.charAt(n);
            if (result.indexOf(m)>-1) {
                i--;
                continue;
            }
            result+=m;
        }
        return result;
    }
    block.innerHTML = queryCode();
    link.onclick=function () {
        block.innerHTML = queryCode();
    }