Race conditions

[JeanHuguesRobert]

Due to the fact that it is preferred to have callbacks called asynchronously, there is a delay between when a promise is resolved/rejected and when the callbacks are called. If during that delay .then is called, depending on the implementation, the newly added callbacks could very well be called twice: once by the delayed handler and once by the code that, in .then(), detects that the promise is already done.

A test to check against that would basically use .then(), .resolve() and .then() again, making sure that two callbacks are called only, in the proper order.

This is slightly pedandic, but I ran into that bug when implementing promises in my multitasker and other may have the same issue with this corner case. See https://github.com/JeanHuguesRobert/l8

[domenic]

Yep, noted a similar issue over in promises-aplus/promises-tests#6.