Understanding JS Promises

Question

I would like to get a deeper understanding of how Promises work internally. Therefore I have some sample code:

var p1          


        

Answer 1

You can think of Promise as a wrapper on some background task. It takes in a function which needs to be executed in the background.

The most appropriate place to use a promise is where some code is dependent on some background processing and it needs to know the status of the background task which was executed. For that, the background task itself accepts two callback resolve and reject in order to convey its status to the code which is dependent on it. In layman terms, this code is the one behind it in the promise chain.

When a background task invokes resolve callback with some parameter. it's marking the background operation successful and passing the result of the background operation to the next then block which will be executed next. and if it calls reject, marking it as unsuccessful then the first catch block will be executed. In your custom promise, you can pass an error obj to the reject callback so that next catch block is aware of the error happened in the background task.