Before promises, callbacks were used to handle asynchronous operations. But due to limited functionality of callback, using multiple callbacks to handle asynchronous code can lead to unmanageable code.
Promise object has four states –
Pending – Initial state of promise. This state represents that the promise has neither been fulfilled nor been rejected, it is in the pending state.
Fulfilled – This state represents that the promise has been fulfilled, meaning the async operation is completed.
Rejected – This state represents that the promise has been rejected for some reason, meaning the async operation has failed.
Settled – This state represents that the promise has been either rejected or fulfilled.
A promise is created using the Promise constructor which takes in a callback function with two parameters, resolve and reject respectively.
resolve is a function that will be called, when the async operation has been successfully completed.
reject is a function that will be called, when the async operation fails or if some error occurs.
We can consume any promise by attaching then() and catch() methods to the consumer.
then() method is used to access the result when the promise is fulfilled.
catch() method is used to access the result/error when the promise is rejected.