# Extensions
In jsPsych, extensions allow one to extend the functionality of various plugins, giving individual plugins the ability to collect more data, display additional stimuli, and more. A canonical example of an extension is [eye tracking](../extensions/webgazer.md), which allow plugins to gather gaze data and add it to the their respective data objects. For a full list of extensions directly included in the jsPsych release, see [here](../extensions/list-of-extensions.md).
## 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: {...} }
]
}
```
## Writing an Extension
See our [developer's guide for extensions](../developers/extension-development.md) for information about how to create a new extension.