Is javascript safe?

The javascript extension lets users store custom JavaScript per domain and injects it into every page on that domain on each navigation.

Users write arbitrary JavaScript in the extension popup, which is saved to local storage keyed by hostname. On every page load matching any URL, the content script reads the stored script for that hostname and appends it as a <script> element to the page body. The options page also allows direct editing of the raw storage JSON. No data is sent to external servers.