Node.Js makes extensive use of callbacks which enables it to trigger it at the execution of a given task. In this context, it is important to note that a majority of the APIs of the Node.Js is written in such a manner that it supports the feature of callbacks. Once the file read procedure gets completed, it would ask the call back function and can pass the contents of the file. Hence, it is evident that there is no wait or blocking due to the existence of File I/O. This is the reason that Node.js can be highly scalable as it is involved in processing a higher number of requests without waiting for the function to give back the result that is expected.
