In traditional web development, a new process is created to handle each coming request. NodeJS makes heavy use of callbacks. In an example, a function to call the web service may start calling the service api and return the control to the environment immediately so it handles the next instruction. When web service call is finished, it calls the callback function. This avoids blocking or waiting of calls. It helps handle a high number of requests without waiting.
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NodeJS library are asynchronous and non-blocking. This means it will never wait for returning data. It handles the next task and when previous tasks are done, it calls the callback function and executes it.