The callback is an asynchronous equivalent for a function. It is called following every task. In Node.js, callbacks are frequently used. All APIs of Node are drafted in a way that supports callbacks. To illustrate, when a function commences reading a file, it instantly returns the control to the execution environment to execute the succeeding instruction.

In Node.js, once file I/O is concluded, it would call the callback function. There is no hindrance or waiting for File I/O. This presents Node.js as highly scalable, as it can process an extraordinary number of requests without pausing for any function to return results.

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