Function _concat

public/javascripts/async.js:766–776

(eachfn, arr, fn, callback)

    });

    function _concat(eachfn, arr, fn, callback) {
        var result = [];
        eachfn(arr, function (x, index, cb) {
            fn(x, function (err, y) {
                result = result.concat(y || []);
                cb(err);
            });
        }, function (err) {
            callback(err, result);
        });
    }
    async.concat = doParallel(_concat);
    async.concatSeries = doSeries(_concat);