javascript - Hijacking a variable with a userscript for Chrome - Stack Overflow

admin管理员组

文章数量:1201370

I'm trying to change the variable in a page using a userscript. I know that in the source code there is a variable

var smilies = false;

In theory I should be able to change it like that:

unsafeWindow.smilies = true;

But it doesn't work. When I'm trying to alert or log the variable to the console without hijacking I get that it's undefined.

alert(unsafeWindow.smilies); // undefined !!!

EDIT: I'm using Chrome if it changes anything...

.html says:

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.

It's about Chrome Extensions but I guess it's the same story with Userscripts too?

Thank you, Rob W. So the working code for people who need it:

var scriptText = "smilies = true;";
var rwscript = document.createElement("script");
rwscript.type = "text/javascript";
rwscript.textContent = scriptText;
document.documentElement.appendChild(rwscript);
rwscript.parentNode.removeChild(rwscript);

I'm trying to change the variable in a page using a userscript. I know that in the source code there is a variable

var smilies = false;

In theory I should be able to change it like that:

unsafeWindow.smilies = true;

But it doesn't work. When I'm trying to alert or log the variable to the console without hijacking I get that it's undefined.

alert(unsafeWindow.smilies); // undefined !!!

EDIT: I'm using Chrome if it changes anything...

http://code.google.com/chrome/extensions/content_scripts.html says:

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.

It's about Chrome Extensions but I guess it's the same story with Userscripts too?

Thank you, Rob W. So the working code for people who need it:

var scriptText = "smilies = true;";
var rwscript = document.createElement("script");
rwscript.type = "text/javascript";
rwscript.textContent = scriptText;
document.documentElement.appendChild(rwscript);
rwscript.parentNode.removeChild(rwscript);

• javascript
• google-chrome
• userscripts
• content-script

Share Improve this question Follow edited May 7, 2012 at 23:20 Brock Adams 93.4k2323 gold badges240240 silver badges304304 bronze badges asked May 7, 2012 at 16:58 Badr HariBadr Hari 8,3742121 gold badges7070 silver badges101101 bronze badges 3

• smilies it's not a variable declared inside a function? – The Mask Commented May 7, 2012 at 17:01
• No, it's not inside the function. – Badr Hari Commented May 7, 2012 at 17:03
• For longer function, use a self-executing format scriptText = '('+ fn +')();'; where fn is any (possibly long) function within the extension. – Moos Commented Sep 22, 2014 at 23:16

Add a comment  | 

1 Answer 1

Sorted by: Reset to default 27

In Content scripts (Chrome extensions), there's a strict separation between the page's global window object, and the content script's global object.

• To inject the code, a script tag has to be injected.
• Overwriting a variable is straightforward.
  Overwriting a variable, with the intention of preventing the variable from being overwritten requires the use of Object.defineProperty ^{Example + notes}.

The final Content script's code:

// This function is going to be stringified, and injected in the page
var code = function() {
    // window is identical to the page's window, since this script is injected
    Object.defineProperty(window, 'smilies', {
        value: true
    });
    // Or simply: window.smilies = true;
};
var script = document.createElement('script');
script.textContent = '(' + code + ')()';
(document.head||document.documentElement).appendChild(script);
script.parentNode.removeChild(script);

本文标签： javascriptHijacking a variable with a userscript for ChromeStack Overflow