I'm casually reading application.js from the express source code.

Where is this event .on coming from? Is this plain old javascript or is there some library that provides this event pattern?

```
 this.on('mount', function onmount(parent) {
    // inherit trust proxy
    if (this.settings[trustProxyDefaultSymbol] === true
      && typeof parent.settings['trust proxy fn'] === 'function') {
      delete this.settings['trust proxy'];
      delete this.settings['trust proxy fn'];
    }

```

<https://github.com/expressjs/express/blob/master/lib/application.js#L89>