With a content script you can inject a script tag into the DOM in order to access variables in the original page (as explained in this question).

I want to avoid injecting my code into every page and instead only do that when the user clicks on the extension icon.

When I tried using the same code as for the content script the values were undefined, although the script was inserted correctly.

Is this possible? Otherwise is using a content script and communicating with it the preferred solution?

Here is the code I'm using:

var scr = document.createElement(script);

scr.type=text/javascript;

scr.innerHTML = setInterval('console.log(window.testVar)', 1000)

document.body.appendChild(scr)

Manifest excerpt:

 permissions: [

    tabs,

    http://*/*, https://*/*

  ],

  background: {

    scripts: [inject.js]

  },