Here are the steps:
Suppose you want make a change to SomeLibrary.js to fix a bug.
1. Clear the browser cache
2. Open Fiddler
3. Navigate to a page the loads SomeLibrary.js
4. Right click on the list item in Fiddler that requests SomeLibrary.js from the server. Select Save, Response, Response Body… and let’s use c:\temp\SomeLibrary.js as the file name
5. In the AutoResponder tab check “Enable automatic responses” and “Permit passthrough for unmatched requests”
6. Drag the list item from step #5 to the AutoResponder window. At the bottom type c:\temp\SomeLibrary.js and press Save.
7. Make change to c:\temp\SomeLibraryjs to fix bug
8. Clear the browser cache
9. Repro steps for bug in browser. Since Fiddler is a proxy for the browser, Fiddler will respond to a request for SomeLibraryjs by returning your modified file, c:\temp\SomeLibrary.js.
If your fix did not work, simply repeat starting at step 7.