jQuery.deferredEach()

This is a non-blocking, asynchronous implementation of jQuery.each() using jQuery's deferred/promise features. It allows you to iterate over huge arrays or objects without hogging the UI thread in which client-side JS runs. That kind of thread-hogging can freeze up the browser's user interface and lead to the dreaded long-running script error message.

If you'd like an example of $.deferredEach() in action, check out Flexitable -- the jQuery plugin for which I originally created it.

How Does It Work?

Instead of this:

results = processAllTheThings(massive_obj);

doMagic(results);
addPizzazz(results);
doBigFinish();
takeBow();

You do something like this:

$.deferredEach(massive_obj, processThing)
  .progress(function(amount_done, count, length) {
    var pct_complete = Math.round(amount_done * 100) + '%';
    updateProgressMeter(pct_complete, count, length);
  })
  .then(doMagic)
  .then(addPizzazz)
  .done(doBigFinish)
  .fail(appeaseAudience)
  .always(takeBow);

Progress Callbacks

.progress() callbacks get passed the following numeric arguments:

amount_done: A decimal representation of how much of the work has been completed on each iteration.
count: The integer count of the last-processed item in the collection. Unlike an array index, count starts at 1 rather than 0.
length: The length of the collection being processed.

Processing an object with four properties would run your progress callback four times with the following arguments

0.25, 1, 4
0.5, 2, 4
0.75, 3, 4
1, 4, 4

You can, for example, update a progress bar using only the amount_done, or do something specific when the process begins or ends by checking count === 1 or count === length.

Success & Failure Callbacks

All completion callback funcitons added with methods like .then(), .done(), .fail(), and .always() recieve two arguments:

collection: The array or object you're iterating over.
message: A string giving the reason for the deferred's resolution or rejection. Currently comes in three flavors:
- "done": The deferred was resolved because we're done iterating over the passed collection.
- "error: empty collection": The deferred was rejected because the passed collection was empty (had nothing to iterate over).
- "error: invalid callback": The deferred was rejected because the passed callback was falsy or not a function.

The object or array returned as collection will include any alterations that your processing callback made on each iteration.

Disclaimer

Using jQuery.deferredEach will make your script run slower than $.each() or a simple loop. That's because it's using setTimeout to take a short break after each iteration and let the browser handle other work, like responding to user interactions. This plugin is meant for those times you just can't avoid iterating over a huge object or array in the browser.

Credit Where Due