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A deferred version of jQuery.each() that can iterate over large data sets without locking up the browser. You can add callbacks using the standard jQuery methods .then(), .done(), .fail(), .always(), and .progress().
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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);


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);

Progress Callbacks

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

  1. amount_done: A decimal representation of how much of the work has been completed on each iteration.
  2. count: The integer count of the last-processed item in the collection. Unlike an array index, count starts at 1 rather than 0.
  3. 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

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

  1. collection: The array or object you're iterating over.
  2. 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.


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

jQuery.deferredEach is based on $.yieldingEach -- posted by colinmarc to the now-defunct -- and the source for jQuery.each().

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