Examine source code of Focus left tab on close

{
  "manifest_version": 2,
  "name": "Focus left tab on close",
  "description": "When the current tab is closed, focus the tab on the left.",
  "version": "0.4",
  "browser_specific_settings": {
    "gecko": {
      "strict_min_version": "65.0"
    }
  },
  "icons": {
    "48": "icons/close-16.svg"
  },
  "background": {
    "scripts": [
      "focus-on-close-background.js"
    ]
  },
  "permissions": [
    "tabs"
  ]
}