node.js带文件下载器的回调地狱

Question

我有一些代码用360个产品解析XML文件，并从URL抓取图像，但在解析大型XML时遇到了一些麻烦。我的主要问题是我如何重新编写代码以便同步工作，所以我使用async.eachSeries，但它没有帮助。

var fs = require('fs');
var request = require('request');
var parseString = require('xml2js').parseString;
var baseUrl = 'http://test.com/uploads/catalog_item_image_main/';
var async = require('async');

var processImg = require('./downloader');

async.waterfall([
    function readFileSync(readFileSyncCallback) {
        var xml = fs.readFileSync("./test.xml", "utf8");
        readFileSyncCallback(null, xml);
    },
    function parse(xml, parseStringCallback) {
        parseString(xml, function(err, result) {
            if (err) parseStringCallback(err);
            var products = result.product_list.product;
            parseStringCallback(null, products);
        })
    },
    function iterate(products, iterateCallback) {
        console.log("PRODUCTS QNT - " + products.length);
        async.eachSeries(products, function(prdt, callbackDone) {
            // Perform operation on file here.
            console.log('Processing file ' + prdt.sku);
            var filename = prdt.sku + "";
            filename = filename.replace(/\//g, '_');
            processImg(baseUrl + filename + '_big.', filename, callbackDone());
        }, function(err) {
            // if any of the file processing produced an error, err would equal that error
            if (err) {
                // One of the iterations produced an error.
                // All processing will now stop.
                console.log('A prdt failed to process');
            } else {
                console.log('All prdt have been processed successfully');
            }
        });
    }
], function(error) {
    if (error) {
        console.log(error);
    }
});

然后我运行我的脚本，我看到所有360

console.log('Processing file ' + prdt.sku)

消息，然后我才看到消息下载了什么图像

console.log(uri + [extension] + " - downloaded");

以下是我的＆＃39; ./下载程序＆＃39; 模块中的代码：

/**
 * Created by Gabriel on 6/27/2015.
 */
var fs = require('fs'),
    request = require('request');

var processImg = function(uri, filename, callback) {
    checkHead(uri + 'png', function(length) {
        if (length > 2000) {
            download(uri + 'png', filename + '.png', function() {
                console.log(uri + 'png' + " - downloaded");
                callback;
            });
        } else {
            checkHead(uri + 'jpg', function(length) {
                if (length > 2000) {
                    download(uri + 'jpg', filename + '.jpg', function() {
                        console.log(uri + 'jpg' + " - downloaded");
                        callback;
                    });
                } else {
                    checkHead(uri + 'jpeg', function(length) {
                        if (length > 2000) {
                            download(uri + 'jpeg', filename + '.jpeg', function() {
                                console.log(uri + 'jpeg' + " - downloaded");
                                callback;
                            });
                        } else {
                            checkHead(uri, function(length) {
                                if (length > 2000) {
                                    download(uri, filename, function() {
                                        callback;
                                    });
                                    callback;
                                } else {
                                    console.log(uri + " - DOWNLOAD ERRROR!!!!!!!!!!!")
                                    callback;
                                }
                            });
                        }
                    });
                }
            });
        }

    });
};


var checkHead = function(uri, callback) {
    request.head(uri, function(err, res, body) {
        if (err) return console.log(err);
        var length = res.headers['content-length'];
        callback(length);
    });
}

var download = function(uri, filename, callback) {
    request(uri).pipe(fs.createWriteStream('./static/' + filename).on('close', callback));
};

module.exports = processImg;