Examine source code of Prevent Closing Window

{
  "manifest_version": 2,
  "name": "Prevent Closing Window",
  "description": "Displays the \"Leave Page\" prompt to avoid accidentally closing the current window.",
  "version": "1.0",
  "applications": {
    "gecko": {
      "id": "{0bd65bd5-3bba-4d89-a2a8-7bb5d057244d}",
      "strict_min_version": "57.0"
    }
  },
  "developer": {
    "name": "Jefferson \"jscher2000\" Scher",
    "url": "https://github.com/jscher2000/"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "browser_style": true,
    "default_icon": "icons/close-stop-32.png",
    "default_title": "Open tab for Prevent Closing Window"
  },
  "permissions": [
    "menus",
    "tabs",
    "storage"
  ],
  "icons": {
    "16": "icons/close-stop-16.png",
    "32": "icons/close-stop-32.png",
    "64": "icons/close-stop-64.png"
  },
  "options_ui": {
    "page": "options.html",
    "browser_style": true
  }
}