Examine source code of Alt to title

{
  "manifest_version": 2,
  "name": "Alt to title",
  "version": "1.0",
  "description": "Copies images alt attributes to their title, so everyone can see the alt text while hovering the image",
  "icons": {
    "48": "alt-to-title.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "script.js"
      ]
    }
  ]
}