Promise States

Promises represent the eventual result of an asynchronous operation. They give us a way to handle asynchronous processing in a more synchronous fashion. A promise represents a value we can handle in the future, with the following guarantees:

• Promises are immutable.
• Promises are either kept or broken.
• When a promise is kept, we are guaranteed to receive a value.
• When a promise is broken, we are guaranteed to receive the reason why the promise cannot be fulfilled.

Promise States

• pending: may transition to fulfilled or rejected
• fulfilled (kept promise): must have a value
• rejected (broken promise): must have a reason for rejecting the promise