May 28th 2014
It has some pretty cool and unique features:
Speed - It's currently one of the fastest promise implementations. In a current set of benchmarks from another promise vendor, when.js 3.2 is 10x to over 100x faster than the native Promise implementations in Firefox, Chrome, and Node (v0.11.13).
Scalability - Most other implementations don't scale well. As the amount of parallelism and number of promises increases, their performance increases linearly or worse. When.js's speed and memory usage scale better than linear, even for tens of thousands of parallel promises.
We're planning followup posts to dig a bit deeper into each of these 3 aspects. Specifically, we'll talk about the combination of architectural and code-level optimizations that make when.js fast and scalable, and the trade-offs and decisions we've made when dealing with debuggability.