With bug 988122 landing soon, you'll now find a "Promise" object
available by default in the global scope of JavaScript modules.
However, this default implementation is still limited, and you're
strongly recommended to import Promise.jsm explicitly in new modules:
Cu.import("resource://gre/modules/Promise.jsm");
This will give you a number of advantages, among others:
- Errors don't risk disappearing silently (bug 966452)
- Tests will fail if errors are accidentally uncaught (bug 976205)
- You can inspect the current state in the debugger (bug 966471)
- You can see registered "then" handlers (bug 966472)
- You get better performance on long Promise chains
Promise.jsm and Promise are still interoperable from the functional
point of view, the difference is in the above non-functional
characteristics. Promise.jsm also has better performance due to the
fact that it avoids the JavaScript / C++ / JavaScript turnaround
time on chain resolution, with an optimized resolution loop handling,
but I don't think this performance part should prevent us from
migrating to C++ Promises when the other limitations are addressed.
Cheers,
Paolo
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