A copromise represents the eventual value of a coroutine. A coroutine is a generator function where the yield keyword is used to suspend execution to wait on a future value, allowing linear control flow logic for asynchronous code.

yield

Any kind of value can be yielded, either a promise or non-promise value. In other words, yield is very much like the when method exposed by some popular promise libraries. Internally, every yielded value is wrapped with Promise.resolve, and when this promise is fulfilled its value will be returned as the result of the yield expression. If an exeption is thrown or this promise is rejected the yield expression will throw in the coroutine, which can be caught with the standard try/catch syntax.

return

Running a coroutine returns a promise which will resolve to the return value of the coroutine, if any. An unhandled exeption in the coroutine will result in a rejected promise.

yield*

The yield* operator can be used to invoke to another coroutine. The result of the yield* epxression (if successful) will be the value returned from the delegated coroutine. If this value is a promise it will be resolved before execution resumes in the originating coroutine.