Examine source code of Addify

{
  "manifest_version": 2,
  "name": "Addify",
  "version": "1.6.7",
  "description": "Searches Spotify with Youtube video title and displays results in the popup where they can be added to your Spotify library.",
  "icons": {
    "48": "assets/icons/toolbar-icon-48.png",
    "96": "assets/icons/toolbar-icon-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://www.spotify.com/*?code=*"
      ],
      "js": [
        "src/authorization-content-script.js"
      ]
    }
  ],
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_action": {
    "default_icon": {
      "32": "assets/icons/toolbar-icon-32.png",
      "48": "assets/icons/toolbar-icon-48.png"
    },
    "default_title": "Addify",
    "default_popup": "src/index.html"
  }
}