Angela Angela - 28 days ago 7
Node.js Question

Node.JS asynch call back

I am just learning node.js so for my first app like so many people I am creating a music library so I can listen to my music via a web browser from my server at home.

I doing okay... But I am having a hard time understanding how not to create a spagetti nightmare. I am employing promises and using an async library to help with this. but I have this current scenario I am trying to understand how to know when it is finished so it can then preform the necessary call back function.

Basically the gist is I have employed an async library to do two calls one to the db and one to the folder so I can do a comparison of files that I have in the db to the ones indicated on the hard drive. After each async method is done it comes back to the "complete" callback provided by the async library I am using.

At this point I want to run through compare the two list and insert an music files that are missing from the db but where found on the hard drive. then I want to send back to the client the entire list for displaying in a grid.

I am using metamusicdata library to do this. and here is my code so far (untested/not fully complete)

function(err, results) {
console.log("ASYNC COMPLETED");
if (err) $this.callback(err, null);
var db = linq.asEnumerable(results.dbList);
var files = linq.asEnumerable(results.foldersList);
var filesToInsert = db.Where(d => !files.Contains(d.LocFile));
var newMusicList = [];
for (var file in filesToInsert) {
if (file.indexOf('mp3') || file.indexOf('mp4')) {
var fileData = musicMetaData(fs.createReadStream(file), function (err, metadata) {
if (err) throw err;
newMusicList.push(new Music ({
locFile : file,
artist : fileData.artist,
album : fileData.album,
albumArtist : fileData.albumartist,
title : fileData.title,
track : fileData.track.no,
totalTracks : fileData.track.of,
genre : fileData.genre,
image : fileData.picture.data,
imageType : fileData.picture.format,
duration : fileData.duration,
createdDate : Date.now(),
classification : "unknown"
}));
console.log(metadata);
});
}
}
//insert into database here the array I created.
var insertMyFiles.
//query database for the entire list
var musicListDb;
//return to the callback so it can send the results back to the client browser.
return $this.callback(err, musicListDb);
}


The question is how can I set this up to know when the looping through the files is entirely done? So I can take my array created and insert the new entries into the db? So I can then invoke another query and get the update list of the music available?

I can think of what I consider a really messed up way of doing it. Which would be replace the for in with a for i=to length. then in the callback for the musicmetadata at the end check to see if i+1=length if so then invoke the callback inside there.

That does not seem right to me and is rather spagettish in my eyes.

What solution am I failing to see or understand here?

Answer

Consider to use async, with it it's really easy to get rid of so-called "callback hell" and much more.

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