Though it has been already answered, i would like to give some deep level understanding of event loop in node.js.

In actual Event Loop is a loop internally more precisely a while loop which is running always. So whenever any event(any msg etc) happens this loop get executed.

More summarize overview of event loop

Hi , Node.js is a single-threaded application, but can support concurrency through the concept of event and callback. Each Node.js API is asynchronous and single-threaded, using async function calls to maintain concurrency. The node uses the observer pattern. The node thread holds an event loop and executes the corresponding event, which sends a signal to the function that listens for the event when a task completes.