JavaScript回调函数不起作用

Question

I'm working with nodeJS and expressJS. I Have 2 functions, main functions:

download(imgUrl, imgMD5, function(fileLength, fileSize) {
    console.log(fileLength);

    var qb = data.Img.query();
    qb.where('img_md5', '=', imgMD5).update({
        img_downloaded: 1
    }).then(function() {});
});

and external function

module.exports = function() {
    return function(uri, filename) {
        request(uri, function(err, res, callback) {
            fileLength = res.headers['content-length'];
            var mkdirs = function(fold, callback) {

                var pf = path.dirname(fold);

                fs.exists(pf, function(exists) {
                    var create = function() {

                        fs.mkdir(fold, callback);
                    };
                    if (exists) {
                        create();
                    } else
                        mkdirs(pf, create);
                })
            };
            var folder = ('./downloaded/' + yy + '/' + mm + '/' + dd + '/' + ho + '/');

            mkdirs(folder, function() {
                var r = request(uri).pipe(fs.createWriteStream(folder + filename));
                r.on('close');
            });
            callback(fileLength);
        });
    };
};

but it's fired an error when running:

TypeError: string is not a function

I don't know if I'm uses the callback right or not?

thank you

Answer 1

Your request() callback parameters aren't quite labelled appropriately. The third argument passed to your callback is the entire buffered (string) body from the response, not a function. So that's why it complains at callback(fileLength); .

Also, because you used a callback (which receives the entire buffered response), there is no need to request the URL again. So you could change this:

mkdirs(folder, function(){
  var r = request(uri).pipe(fs.createWriteStream(folder + filename));
  r.on('close');
});

to this:

mkdirs(folder, function() {
  // `data` here is the third argument to the `request()` callback ...
  fs.writeFile(folder + filename, data);
});

to save an extra HTTP request.

Or if you really do want to stream the response data, you could do:

request(uri).on('response', function(res) {
  // ...

  mkdirs(folder, function() {
    res.pipe(fs.createWriteStream(folder + filename));
  });
});