Javascript variable scope inside callbacks

Question

I'm using formidable (github) and I'm not sure about the scope of some variables inside the callbacks. Part of my code is:

UploadHandler.prototype.upload = function(req, res){
    var query = url.parse(req.url, true).query;
    var form = new formidable.IncomingForm();
    var id = query['X-Progress-ID'];

    self.uploads.add(id);

    form.parse(req, function(err, fields, files){
        self.uploads.remove(id);
        res.writeHead(200, { 'Content-type': 'text/plain' });
        return res.end('upload received');
    });

    ...

}

My question is, what will be the value of id inside the callback of parse? Also, will that code work as expected if more than 1 person is uploading a file? (As in, will id change it's value for both the first and the second person if they're both using the uploader at the same time.

Denys Séguret · Accepted Answer · 2013-06-24 10:48:37Z

2

id is what you defined and yes, it will work if there's more than one call to upload : the id variable is local to the invocation of the upload function. The scope here is the function call which forms what's called a closure.

Here's a simplified version of your code :

function upload(i){
   var id=i; // id is local to the invocation of upload
   setTimeout(function(){ console.log(id) }, 100*i);
}
for (var i=0; i<3; i++) {
    upload(i);
}

It logs 0, 1, 2.

answered Jun 24, 2013 at 10:48

Denys Séguret

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