Examine source code of RickRoll Detector

{
  "manifest_version": 2,
  "name": "Rick Roll Detector",
  "version": "1",
  "description": "Detects Rick Rolls",
  "icons": {
    "48": "icon.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*",
        "*://youtu.be/*"
      ],
      "js": [
        "redir.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "rrdetect@script-js.repl.co"
    }
  }
}