Examine source code of focusTab

{
  "manifest_version": 2,
  "name": "Focus a tab",
  "version": "1.2",
  "description": "Focus a tab with window.focusTab() function",
  "developer": {
    "name": "Tomofumi Inoue",
    "url": "https://github.com/GitHub30/focusTab"
  },
  "icons": {
    "48": "focus-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "run_at": "document_start",
      "js": [
        "content_scripts.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "permissions": [
    "tabs"
  ]
}