As we saw before, when a promise is fulfilled in a rejected state, the callback of the catch functions is triggered. In the preceding recipe, we see that this can happen when the reject method is called (as with performGuidanceDiagnostic). It also happens when a function in the chain throws an error (as will addBoosters).
This has similar benefit to how Promise.resolve can normalize asynchronous functions. This handling allows asynchronous functions to not know about the promise chain, and announce error states in a way that is familiar to developers who are new to promises.
This makes expanding the use of promises much easier.
