Examine source code of Timefy

{
  "manifest_version": 2,
  "name": "Timefy",
  "version": "2.1",
  "description": "Display the sum of all video length on a YouTube playlist.",
  "icons": {
    "48": "icons/time-48.png",
    "96": "icons/time-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/playlist*"
      ],
      "js": [
        "thirdParty/jquery-3.5.1.slim.min.js",
        "timefy.js"
      ]
    }
  ]
}