Examine source code of Medium Word Count

{
  "manifest_version": 2,
  "name": "Medium Word Count",
  "version": "1.0.1",
  "description": "Counts words in a medium.com article.",
  "permissions": [
    "activeTab"
  ],
  "content_scripts": [
    {
      "matches": [
        "https://medium.com/*",
        "https://*.medium.com/*",
        "https://towardsdatascience.com/*",
        "https://hackernoon.com/*",
        "https://medium.freecodecamp.org/*",
        "https://psiloveyou.xyz/*",
        "https://betterhumans.coach.me/*",
        "https://codeburst.io/*",
        "https://theascent.pub/*",
        "https://medium.mybridge.co/*",
        "https://uxdesign.cc/*",
        "https://levelup.gitconnected.com/*",
        "https://itnext.io/*",
        "https://entrepreneurshandbook.co/*",
        "https://proandroiddev.com/*",
        "https://blog.prototypr.io/*",
        "https://thebolditalic.com/*",
        "https://blog.usejournal.com/*",
        "https://blog.angularindepth.com/*",
        "https://blog.bitsrc.io/*",
        "https://blog.devartis.com/*",
        "https://blog.maddevs.io/*",
        "https://blog.getambassador.io/*",
        "https://uxplanet.org/*",
        "https://instagram-engineering.com/*",
        "https://calia.me/*",
        "https://productcoalition.com/*",
        "https://engineering.opsgenie.com/*",
        "https://android.jlelse.eu/*",
        "https://robinhood.engineering/*",
        "https://blog.hipolabs.com/*",
        "https://ux.shopify.com/*"
      ],
      "js": [
        "medium_word_count.js"
      ]
    }
  ],
  "icons": {
    "16": "images/medium-word-count16.png",
    "32": "images/medium-word-count32.png",
    "48": "images/medium-word-count48.png",
    "128": "images/medium-word-count128.png"
  }
}