Examine source code of Currency EX

{
  "manifest_version": 2,
  "name": "Currency EX",
  "version": "1.1.1",
  "description": "Get exchange rates.",
  "homepage_url": "https://github.com/saintograph/currency-ex",
  "icons": {
    "32": "icons/32.png",
    "48": "icons/48.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/32.png",
    "default_title": "Currency EX",
    "default_popup": "popup/index.html"
  },
  "content_scripts": [
    {
      "matches": [
        "*://api.exchangeratesapi.io/*"
      ],
      "js": [
        "popup/main.js"
      ]
    }
  ]
}