Examine source code of OpenInSteam

{
  "manifest_version": 2,
  "name": "OpenInSteam",
  "version": "1.2",
  "description": "Adds a button to open the current Steam page inside Steam.",
  "icons": {
    "48": "icons/openinsteam-48.png"
  },
  "content_scripts": [
    {
      "exclude_matches": [
        "*://store.steampowered.com/cart/*",
        "*://store.steampowered.com/checkout/*"
      ],
      "matches": [
        "*://store.steampowered.com/*",
        "*://steamcommunity.com/*"
      ],
      "js": [
        "openinsteam.js"
      ]
    }
  ]
}