Examine source code of Ctrl+Number to switch tabs in reverse order

{
  "author": "Tom Rodriguez",
  "background": {
    "scripts": [
      "ctrlnumber.js"
    ]
  },
  "commands": {
    "switch-tab-1": {
      "description": "Switch to the last tab",
      "suggested_key": {
        "default": "Ctrl+1"
      }
    },
    "switch-tab-2": {
      "description": "Switch to the second to last tab",
      "suggested_key": {
        "default": "Ctrl+2"
      }
    },
    "switch-tab-3": {
      "description": "Switch to the third to last tab",
      "suggested_key": {
        "default": "Ctrl+3"
      }
    },
    "switch-tab-4": {
      "description": "Switch to the fourth to last tab",
      "suggested_key": {
        "default": "Ctrl+4"
      }
    },
    "switch-tab-5": {
      "description": "Switch to the fifth to last tab",
      "suggested_key": {
        "default": "Ctrl+5"
      }
    },
    "switch-tab-6": {
      "description": "Switch to the sixth to last tab",
      "suggested_key": {
        "default": "Ctrl+6"
      }
    },
    "switch-tab-7": {
      "description": "Switch to the seventh to last tab",
      "suggested_key": {
        "default": "Ctrl+7"
      }
    },
    "switch-tab-8": {
      "description": "Switch to the eighth to last tab",
      "suggested_key": {
        "default": "Ctrl+8"
      }
    },
    "switch-tab-9": {
      "description": "Switch to the ninth to last tab",
      "suggested_key": {
        "default": "Ctrl+9"
      }
    }
  },
  "description": "This extension adds keyboard shortcuts to switch tabs in reverse order. Ctrl+1 switches to the last tab, Ctrl+2 switches to the second to last tab, and so on.",
  "manifest_version": 2,
  "name": "Ctrl+Number to switch tabs in reverse order",
  "short_name": "Ctrl+Number reverse",
  "version": "1.0.0",
  "browser_specific_settings": {
    "gecko": {
      "id": "{cc98fa7d-8615-4264-a83a-21bc63f437ae}"
    }
  }
}