Question

I need to be able to read a variable set on a google chrome extension from any web page.

For example on a firefox extension I am doing this :

XPCNativeWrapper.unwrap(contentWindow).myExtensionName_varName = "value";

Then on any web page if I do alert(myExtensionName_varName); I get the correct value.

How could I achieve the same thing on a chrome extension?

Answer 1

Everything in Chrome is sandboxed for security and to avoid conflicts, so you don’t have direct access from extension to any variable created by website scripts.

Content scripts execute in a special environment called an isolated world. They have access to the DOM of the page they are injected into, but not to any JavaScript variables or functions created by the page. It looks to each content script as if there is no other JavaScript executing on the page it is running on. The same is true in reverse: JavaScript running on the page cannot call any functions or access any variables defined by content scripts.
[source]

Check out ‘Communication with the embedding page’ section here to learn how to communicate between scripts on webpages and extension content scripts.