> For the complete documentation index, see [llms.txt](https://demirels-organization.gitbook.io/javascript-tutorial/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://demirels-organization.gitbook.io/javascript-tutorial/the-callback-pattern.md).

# The callback pattern

The callback pattern is a way of performing an action after a function has completed. A callback function is a function that is passed as an argument to another function, and is executed after the first function has completed.

Here is an example of the callback pattern:

```javascript
function doSomething(callback) {
  // Perform some action
  console.log('Doing something...');

  // Call the callback function
  callback();
}

// This function will be passed as a callback
function doSomethingElse() {
  console.log('Doing something else');
}

doSomething(doSomethingElse);
```

In this example, the `doSomething` function takes a callback function as an argument. When the `doSomething` function is called, it performs some action and then calls the callback function.

The callback function, `doSomethingElse`, is defined separately and is passed as an argument to the `doSomething` function when it is called.

The output to the console will be:

```javascript
Doing something...
Doing something else
```

The callback pattern is often used to perform an action after an asynchronous operation has completed. For example:

```javascript
function getData(callback) {
  // This is an asynchronous operation
  fetch('https://example.com/data').then(response => {
    const data = response.json();

    // Call the callback function with the data
    callback(data);
  });
}

// This function will be passed as a callback
function displayData(data) {
  console.log(data);
}

getData(displayData);
```

In this example, the `getData` function performs an asynchronous operation to retrieve data from a server, and calls the callback function (`displayData`) with the data when it has been retrieved.

The callback pattern is a way of organizing code and performing actions after a function has completed. It allows you to write flexible and reusable code, and is a common pattern.