Promises.js is a JavaScript library that promises asynchronous I/O operations such as reading and writing on a file. Whenever there is a callback method that involves making all operations related to I/O, they are to be made asynchronous. This extra callback parameter confuses our idea of what is the input and what will be its return value. It never works with control flow primitives. It also doesn't handle errors thrown by a callback method.

So, we need to handle errors thrown by a callback method, but also need to be careful not to handle errors thrown by the callback method. By the time we are done with this, our code will be a mess of error handling.

Despite all this mess of error handling code, we are still left with the problem of the extra callback parameter hanging around. Promises help you naturally handle errors, and write cleaner code by not having callback parameters.