What is the extent of your experience with Promises and/or their polyfills?

Possess working knowledge of it. A promise is an object that may produce a single value sometime in the future: either a resolved value or a reason that it's not resolved (e.g., a network error occurred). A promise may be in one of 3 possible states: fulfilled, rejected, or pending. Promise users can attach callbacks to handle the fulfilled value or the reason for rejection.

Some common polyfills are $.deferred, Q and Bluebird but not all of them comply with the specification. ES2015 supports Promises out of the box and polyfills are typically not needed these days.

References

• Front End Interview Handbook