Examine source code of YouTube Playlist Duration

{
  "manifest_version": 2,
  "name": "YouTube Playlist Duration",
  "version": "0.6",
  "description": "Displays the duration of a playlist on YouTube",
  "permissions": [
    "tabs"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "app.js"
      ]
    }
  ],
  "browser_action": {
    "default_popup": "popup.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "yt-playlist-duration@noahdunbar.com"
    }
  }
}