How do I define the callback in the Async library's "Each" function?

Question

In Caolan's Async library, the documentation says that the each function:

Applies the function iteratee to each item in arr, in parallel. The iteratee is called with an item from the list, and a callback for when it has finished.

This example is given:

async.each(arr, iteraree, function(err){
    //Do stuff
});

My question is how is the "callback", in this case iteraree, defined? How do you declare which callback function you would like called when it has finished?

If you were able to define functions as a parameter I could see it working but you can't (i.e):

async.each(openFiles, function (item, function () {console.log("done")}), function(err){
  //Do stuff
});

But how could I define that callback not inline?

If I do something like

var iteraree = function (item, callback) {};

And passed in iteraree I still do not see where I would be able to define the behavior of callback.

Felix Kling · Accepted Answer · 2016-02-22 01:55:39Z

2

I still do not see where I would be able to define the behavior of callback.

You don't. callback is just a parameter. async.each calls iteratee and passes a value for callback. It's your responsibility to call callback when you are done. This lets async.each know that it can continue to the next iteration.

function (item, function () {console.log("done")}) is invalid JavaScript btw. You cannot have a function definition in place of a parameter.

answered Feb 22, 2016 at 1:55

Felix Kling

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Comments

Robert Moskal · Accepted Answer · 2016-02-22 01:50:04Z

2

That iteratee function doesn't have the signature you think it does. It looks like this:

async.each(openFiles, function (item, cb){
       console.log(item);
       cb();   
    }, function(err){
      //Do stuff
 });

It receives each item and a callback function from asynchronous.each() that has to be called when you finish whatever you are doing. You don't provide the callback.

So if you are using a named function it's just:

 function foo (item, cb){
       console.log(item);
       cb();   
    }

async.each(openFiles, foo, function(err){
      //Do stuff
 });

edited Feb 22, 2016 at 1:50

answered Feb 22, 2016 at 1:39

Robert Moskal

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Comments

Louie Almeda · Accepted Answer · 2016-02-22 01:40:05Z

Here's a modified version of example from their docs with added comments

//I personally prefer to define it as anonymous function, because it is easier to see that this function will be the one iterated
async.each(openFiles, function(file, callback) {

    callback(); //this is just like calling the next item to iterate

}, then);

//this gets called after all the items are iterated or an error is raised
function then(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 file failed to process');
    } else {
      console.log('All files have been processed successfully');
    }
}

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