Examine source code of Youtube audio_only

{
  "name": "Youtube audio_only",
  "version": "24.1.29",
  "manifest_version": 2,
  "description": "listen to Youtube audio (no video is downloaded)",
  "homepage_url": "https://github.com/craftwar/youtube-audio",
  "icons": {
    "38": "img/icon38.png",
    "128": "img/icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{dccd64f0-f884-496c-af8b-1a66330e1a67}"
    },
    "gecko_android": {}
  },
  "background": {
    "scripts": [
      "js/global.js"
    ]
  },
  "permissions": [
    "tabs",
    "webRequest",
    "*://*/*",
    "webRequestBlocking",
    "storage"
  ],
  "browser_action": {
    "default_title": "Youtube audio_only"
  },
  "commands": {
    "_execute_browser_action": {
      "description": "Toggle Youtube audio only mode on/off"
    }
  },
  "content_scripts": [
    {
      "all_frames": true,
      "matches": [
        "*://*.youtube.com/*",
        "*://*.youtube-nocookie.com/*"
      ],
      "js": [
        "js/youtube_audio.js"
      ],
      "run_at": "document_end"
    }
  ]
}