# Extensions
Extensions are jsPsych modules that can interface with any plugin to extend the functionality of the plugin. A canonical example of an extension is eye tracking. An eye tracking extension allows a plugin to gather gaze data and add it to the plugin's data object.
## Using an Extension
To use an extension in an experiment, you'll load the extension file via a `
```
```js
initJsPsych({
extensions: [
{type: jsPsychExtensionExample, params: {...} }
]
})
```
To enable an extension during a trial, add the extension to the `extensions` list for the trial. Some extensions may also support or require an object of parameters to configure the extension:
```js
var trial = {
extensions: [
{type: jsPsychExtensionExample, params: {...} }
]
}
```
## List of Extensions
Extension | Description
------ | -----------
[jspsych‑ext‑webgazer.js](../extensions/jspsych-ext-webgazer.md) | Enables eye tracking using the [WebGazer](https://webgazer.cs.brown.edu/) library.
## Writing an Extension
See our [developer's guide for extensions](../developers/extension-development.md) for information about how to create a new extension.