Professional Ajax, 2nd Edition

5.1. Priority Queues

Whenever pieces of data need to be arranged in order of priority, the typical solution is to use a priority queue. A standard queue is a first-in, first-out data structure: items are added at the back of the queue, wait in line, and eventually are removed from the front of the queue. A priority queue augments that methodology by inserting new values into the queue based on a priority, so a new value with a higher priority doesn't go to the back of the queue, but rather, gets inserted into an appropriate location. In a priority queue where 0 is the highest priority and 4 is the lowest, items with a priority of 3 will always be inserted into the queue before any items with a priority of 4. Likewise, items with a priority of 2 are inserted ahead of those with a priority of 3, and so on. This is the perfect paradigm for managing multiple XHR requests. Unfortunately, JavaScript doesn't have a built-in priority queue, so it's necessary to create one.

A generic priority queue can be made with a single Array object, making use of the built-in sort() method. When values are added to this custom priority queue, they are added to the array, which is then sorted. By providing a custom comparison function to the sort() method, it's possible to determine the order in which the values should appear within the array, making it perfect for assigning priorities. A comparison function has the following generic form:

function compare(oValue1, oValue2) { if (oValue1 < oValue2) ...