Examine source code of Alt-Left Closes Empty Tab

{
  "manifest_version": 2,
  "name": "Back Arrow Close Tab",
  "version": "1.0",
  "description": "Closes the tab if there is no previous page when using the back button or Alt+Left.",
  "permissions": [
    "tabs",
    "activeTab",
    "webNavigation"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "commands": {
    "check-back-and-close": {
      "suggested_key": {
        "default": "Alt+Left"
      },
      "description": "Check back history and close tab if none"
    }
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "back-arrow-close-tab@domain.com"
    }
  }
}