Examine source code of Numbers in Words

{
  "manifest_version": 2,
  "name": "Numbers in Words",
  "description": "Replaces digits with words, as an accessibility tool.",
  "version": "1.1",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "activeTab",
    "contextMenus"
  ]
}