How can I edit javascript in my browser like I can use Firebug to edit CSS/HTML?

javascript browser firebug

Ryan · Jul 12, 2011 · Viewed 192.2k times · Source

Within JSP files, I have some pretty complicated Javascript. On a production machine, we're seeing a very weird bug that we have not been able to understand. We have never been able to replicate it in a local or development environment. It might be related to the javascript, but I haven't found a good way to do this: use my browser to visit the page (on the production website) and then use browser tools to edit the javascript that runs on that page, including on reloads of the page.

I'm always able to do this to tweak CSS etc, but as these questions point out, it's not obvious how to tweak JS client-side:

However, those answers don't help me because:

"Execute JS" (Firefox addon) doesn't seem to work (doesn't do more than the console in Chrome already can do),
"Charles" might work if I'd used separated js files, but my javascript is embedded in JSP

It seems like How to modify javascript code on the fly in browser in debugging mode? is the closest thing to what I'm talking about, but that guy isn't able to talk about what he did because it was for his employer.

Thanks for your help! Ryan

Answer

The problem with editing JavaScript like you can CSS and HTML is that there is no clean way to propagate the changes. JavaScript can modify the DOM, send Ajax requests, and dynamically modify existing objects and functions at runtime. So, once you have loaded a page with JavaScript, it might be completely different after the JavaScript has run. The browser would have to keep track of every modification your JavaScript code performs so that when you edit the JS, it rolls back the changes to a clean page.

But, you can modify JavaScript dynamically a few other ways:

JavaScript injections in the URL bar: javascript: alert (1);
Via a JavaScript console (there's one built into Firefox, Chrome, and newer versions of IE
If you want to modify the JavaScript files as they are served to your browser (i.e. grabbing them in transit and modifying them), then I can't offer much help. I would suggest using a debugging proxy: http://www.fiddler2.com/fiddler2/

The first two options are great because you can modify any JavaScript variables and functions currently in scope. However, you won't be able to modify the code and run it with a "just-served" page like you can with the third option.

Other than that, as far as I know, there is no edit-and-run JavaScript editor in the browser. Hope this helps,

How can I edit javascript in my browser like I can use Firebug to edit CSS/HTML?

Answer

Related questions