Extending

11ty is extremely flexible and can be configured to do a whole lot of things. The Configuration API is behind most of this flexibility. While you can (and should) read through the above document and get started right away, Elfin includes a framework for structuring your configurations in a way that makes the easier to see and analyze, and doesn’t crowd up your 11ty config file.

This framework can be used (or expanded) for any use of the Configuration API, but the following example demonstrates how to use it for adding a filter.

Create a .js file for your new filter in 11ty/filters/. i.e. if you’re making an uppercase filter you might create 11ty/filters/uppercase.js. The name doesn’t matter, but it helps make your code more readable.
In your new file, add the following:
```
module.exports = conf => {}
```
conf is an object we pass through from elsewhere that contains the 11ty Configuration API.

Using the Configuration API, add your filter inside of the method body we just defined:

module.exports = conf => {
    conf.addFilter('uppercase', function(string) {
        return string.toUpperCase();
    });
}

Now we need to let the rest of 11ty know about the filter. Open up 11ty/filters/index.js, and add the following:
```
module.exports = conf => {
    // ...
    require('./uppercase')(conf);
};
```
(The name used in require('./uppercase') is the filename you created in step #1.)
Use your filter in a template, i.e. {{ some_variable | uppercase }}!