A promise is always in one of these states:
- Fulfilled: If the resolve callback is invoked with a non-promise object as the argument or no argument, then we say that the promise is fulfilled
- Rejected: If the rejecting callback is invoked or an exception occurs in the executor scope, then we say that the promise is rejected
- Pending: If the resolve or reject callback is yet to be invoked, then we say that the promise is pending
- Settled: A promise is said to be settled if it's either fulfilled or rejected, but not pending
Once a promise is fulfilled or rejected, it cannot be transitioned back. An attempt to transition it will have no effect.
