Promise States

pending, fulfilled and rejected states of Promises

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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

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