How to: Q. What is callback function in Node.js?
A callback is a function which is called when a task is completed, thus helps in preventing any kind of blocking and a callback function allows other code to run in the meantime.

Callback is called when task get completed and is asynchronous equivalent for a function. Using Callback concept, Node.js can process a large number of requests without waiting for any function to return the result which makes Node.js highly scalable.

**Example:**

```js
/**
 * Callback Function
 */
function message(name, callback) {
  console.log("Hi" + " " + name);
  callback();
}

// Callback function
function callMe() {
  console.log("I am callback function");
}

// Passing function as an argument
message("Node.JS", callMe);
```

**Output:**

```js
Hi Node.JS
I am callback function
```
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