Examine source code of Select Tab Using Alt-Number

{
  "applications": {
    "gecko": {
      "id": "koteyur@gmail.com",
      "strict_min_version": "54.0"
    }
  },
  "manifest_version": 2,
  "name": "SelectTab Windows Shortcut Override",
  "version": "1.0.1",
  "description": "An extension that overrides the default select-tab modifier key. This is a modified version of original extension by selecttab@braniecki.net in order to use alt+[0-9] to select tab",
  "permissions": [
    "tabs"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "commands": {
    "selectTab1": {
      "suggested_key": {
        "default": "Alt+1"
      },
      "description": "Activate Tab 1"
    },
    "selectTab2": {
      "suggested_key": {
        "default": "Alt+2"
      },
      "description": "Activate Tab 2"
    },
    "selectTab3": {
      "suggested_key": {
        "default": "Alt+3"
      },
      "description": "Activate Tab 3"
    },
    "selectTab4": {
      "suggested_key": {
        "default": "Alt+4"
      },
      "description": "Activate Tab 4"
    },
    "selectTab5": {
      "suggested_key": {
        "default": "Alt+5"
      },
      "description": "Activate Tab 5"
    },
    "selectTab6": {
      "suggested_key": {
        "default": "Alt+6"
      },
      "description": "Activate Tab 6"
    },
    "selectTab7": {
      "suggested_key": {
        "default": "Alt+7"
      },
      "description": "Activate Tab 7"
    },
    "selectTab8": {
      "suggested_key": {
        "default": "Alt+8"
      },
      "description": "Activate Tab 8"
    },
    "selectTab9": {
      "suggested_key": {
        "default": "Alt+9"
      },
      "description": "Activate Tab 9"
    },
    "toggleHotkeys": {
      "suggested_key": {
        "default": "Ctrl+Alt+0"
      },
      "description": "Toggle Hotkeys"
    }
  }
}