Examine source code of Close Previous Tab

{
  "name": "Close Previous Tab",
  "description": "Defines a shortcut to close the previously active tab without having to switch back to it.",
  "homepage_url": "https://github.com/y1n0/close-previous-tab",
  "manifest_version": 2,
  "version": "1.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "close-previous-tab@y1n0.ma",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "commands": {
    "close-previous": {
      "suggested_key": {
        "default": "Alt+Ctrl+W"
      },
      "description": "Close the previously active tab"
    }
  }
}