Examine source code of Tab Numbers

{
  "name": "Tab Numbers",
  "description": "This extension writes the tab number to the tabs, the ones accessible with ctrl/cmd + number",
  "version": "1.0.0",
  "permissions": [
    "tabs",
    "http://*/*",
    "https://*/*",
    "*://*/*"
  ],
  "icons": {
    "512": "tab_numbers.png"
  },
  "manifest_version": 2,
  "background": {
    "scripts": [
      "tab_numbers.js"
    ]
  }
}