What are callbacks?
A callback is just a way to store some things to do at a later time.
A callback function is called at the completion of a given task.
The callback takes two arguments, resolve and reject , which are both functions. All your asynchronous code goes inside that callback. If everything is successful, the promise is fulfilled by calling resolve() . In case of an error, reject() is called with an Error object.
A promise is an object which can be returned synchronously from an asynchronous function. It will be in one of 3 possible states:
Fulfilled: onFulfilled() will be called (e.g., resolve() was called)
Rejected: onRejected() will be called (e.g., reject() was called)
Pending: not yet fulfilled or rejected
