Callbacks Concept

A Callback in Node.js is an asynchronous equivalent for a function. It is a special type of function passed as an argument to another function. Node.js makes heavy use of callbacks. Callbacks help us make asynchronous calls. All the APIs of Node are written in such a way that they support callbacks.

Programming instructions are executed synchronously by default. If one of the instructions in a program is expected to perform a lengthy process, the main thread of execution gets blocked. The subsequent instructions can be executed only after the current I/O is complete. This is where callbacks come in to the picture.

The callback is called when the function that contains the callback as an argument completes its execution, and allows the code in the callback to run in the meantime. This makes Node.js highly scalable, as it can process a high number of requests without waiting for any function to return results.