Chrome Dev Tools - Modify javascript and reload

The Resource Override extension allows you to do exactly that:

• create a file rule for the url you want to replace
• edit the js/css/etc in the extension
• reload as often as you want :)

This is a bit of a work around, but one way you can achieve this is by adding a breakpoint at the start of the javascript file or block you want to manipulate.

Then when you reload, the debugger will pause on that breakpoint, and you can make any changes you want to the source, save the file and then run the debugger through the modified code.

But as everyone has said, next reload the changes will be gone - at least it lets you run some slightly modified JS client side.

1. In the devtools preferences check the Enable local overrides.
2. Go to network tab, find the file you want to edit, rigth click on it and select Save for overrides (on the sources/overrides tab you need to add a local folder)
3. The file appears in a new tab on the Sources tab as local copy, so you can edit this file, and after site reload the new (and edited) override file will load on the site!

Great news, the fix is coming in March 2018, see this link: https://developers.google.com/web/updates/2018/01/devtools

"Local Overrides let you make changes in DevTools, and keep those changes across page loads. Previously, any changes that you made in DevTools would be lost when you reloaded the page. Local Overrides work for most file types

How it works:

• You specify a directory where DevTools should save changes. When you make changes in DevTools, DevTools saves a copy of the modified file to your directory.
• When you reload the page, DevTools serves the local, modified file, rather than the network resource.

To set up Local Overrides:

1. Open the Sources panel.
2. Open the Overrides tab.
3. Click Setup Overrides.
4. Select which directory you want to save your changes to.
5. At the top of your viewport, click Allow to give DevTools read and write access to the directory.
6. Make your changes."

UPDATE (March 19, 2018): It's live, detailed explanations here: https://developers.google.com/web/updates/2018/01/devtools#overrides