doWhilst(fn, test, callback)

相当于do…while,较whilst而言，doWhilst交换了fn,test的参数位置，先执行一次循环，再做test判断。

var count = 0;
async.doWhilst(
    function (callback) {
        count++;
        setTimeout(callback, 1000);
    },
    function () { return count < 5; },
    function (err) {

}
);

var apple = 2,
    student = ['张三','李四','王五'];