well.js

Well.js is lightweight Promises/A+ implemention. It evolved from when-1.8.1 for getting higher performance. The article Promises/A+ Performance Hits You Should Be Aware Of described the benchmark defference of some common promise implementations. In addition, well.js also provide:

• Native cancelable deferred
• Extendable promise through well.extend
• Throw exception for uncaught error

It is very fast and compact, and has no external dependencies.