Decide on using new ES6 keyed collections (e.g. Map, Set)

[Yves]

I'd like to have a team decision if we start using the new ​keyed collections which are available with SpiderMonkey 24 and later. These features are marked "experimental".

Overview

Mozilla writes this:

This is an experimental technology, part of the Harmony (ECMAScript 6) proposal. Because this technology's specification has not stabilized, check the compatibility table for >usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

Check the descriptions of these objects and also the browser compatibility list at the end of the articles:

• ​Map
• ​WeakMap
• ​Set
• ​WeakSet

Why should we use them?

It's mainly for performance reasons.

Performance example

Because I don't like generic benchmarks, here's an example of a test with our code I made today.

It's a non-visual replay of a 2vs2 AI game with aegis bots on Oasis 04 going over around 15000 turns. I've measured the total runtime in seconds of the get function in entity.js. I've used my v31 testing branch for the measurements but I don't know if it makes a difference compared to v24. The function gets called 51'206'230 times in total and the hash is the same with and without patch. The patch is attached. It's very simple and we could improve performance even further if we used it at some more key locations that use key/value pairs.

Before:
Time in seconds for get function:                      43.91533856391563
Total execution time  (some variantion is normal):    995 seconds

Get taking 4.4% of total execution time

After:
Time in seconds for get function:                     30.976013190725517
Total execution time (some variation is normal):     991 seconds

Get taking 3.1% of total execution time

[Yves]

performance testpatch (can be committed if the decision is positive)