Adding extra parameters to a callback

Question

Adding extra parameters to a callback

So after reading up I see that I need a closure or to use bind but I am not fully clear on the proper syntax and which I should use.

I am using the imagemagick module from https://github.com/rsms/node-imagemagick and need to pass the filename to the resize callback.

Syntax is:

var filename = 'myfile.jpeg';
im.resize(options, callback(err, stdout, stderr))

Now I want my callback to accept extra parameters such as the filename.

I would like to be able to call another function and have the filename along with the 3 callback parameters (also note that this is in a loop so the filename is from an array - not sure if that matters)

So I have a callback function:

var callback = function(err, stdout, stderr) {
    //how do I access filename?
}

Can I get an explanation of which I should use (closure or bind) and pros/cons and, if a closure, how would I actually write the callback function - I am unclear on how I access err, stdout and stderr in that way.

javascript
node.js
closures

Answer 1

You can do it with an extra anonymous function if that's what you mean:

im.resize(options, function(err, stdout, stderr) {
  callback(err, stdout, stderr, filename) // do something with filename
});

Answer 2

I'd say you could use closure, in your callback you could do something like this:

var filename = 'myfile.jpg';
im.resize(options, function(err,stdout,stderr) {
    console.log('filename',filename,'err',err);
});