karolmajta/background.js

## background.js
var current = null;
chrome.tabs.query({active: true}, function (tabs) {
    current = tabs[0].id;
})

chrome.tabs.onCreated.addListener(function(t) {
    current = t.id;
    console.log(current);
});

chrome.tabs.onActivated.addListener(function(t) {
    current = t.tabId;
    console.log(current);
});

## manifest.json
{
  "manifest_version": 2,

  "name": "One-click Kittens",
  "description": "This extension demonstrates a browser action with kittens.",
  "version": "1.0",

  "permissions": [
    "tabs"
  ],
  "browser_action": {
    "default_icon": "favicon.png",
    "default_popup": "popup.html"
  },
  "background": {
    "scripts": ["background.js"]
  },
  "content_security_policy": "script-src 'self' 'unsafe-eval'; object-src 'self'"
}

## popup.html

<!doctype html>
<html>
  <head>
  </head>
  <body>
    <script src="popup.js"></script>
  </body>
</html>

## popup.js
document.write(chrome.extension.getBackgroundPage().current);
	var current = null;
	chrome.tabs.query({active: true}, function (tabs) {
	current = tabs[0].id;
	})

	chrome.tabs.onCreated.addListener(function(t) {
	current = t.id;
	console.log(current);
	});

	chrome.tabs.onActivated.addListener(function(t) {
	current = t.tabId;
	console.log(current);
	});
	{
	"manifest_version": 2,

	"name": "One-click Kittens",
	"description": "This extension demonstrates a browser action with kittens.",
	"version": "1.0",

	"permissions": [
	"tabs"
	],
	"browser_action": {
	"default_icon": "favicon.png",
	"default_popup": "popup.html"
	},
	"background": {
	"scripts": ["background.js"]
	},
	"content_security_policy": "script-src 'self' 'unsafe-eval'; object-src 'self'"
	}