Examine source code of Rapidand

{
  "manifest_version": 2,
  "name": "Rapidand",
  "version": "1.1",
  "description": "Adds button to amazon webstores to purchase the same product on rapidand.com",
  "icons": {
    "16": "icons/16x16.png",
    "48": "icons/48x48.png",
    "128": "icons/128x128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://*/*/dp/*",
        "https://*/dp/*",
        "https://*/gp/product/*"
      ],
      "js": [
        "rapidand.js"
      ]
    }
  ]
}