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The promise object represents the completion or failure of an asynchronous operation and the value that results from this. We use promises when we will have a value that we do not yet know. This way you can associate handlers with an action that will occur.
A promise can have 1 of 3 states:
- pending: This is the initial state and it has been neither fulfilled or rejected.
- fulfilled: The operation was successfully completed.
- rejected: The operation failed.
The eventual state of a pending promise can be either fulfilled with a value or rejected with an error. When either of these occur, the associated handler that is queued by a promise’s then method are called.