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// HackTricks · Web Pentesting

Debugging Client-Side JavaScript

Debugging Client-Side JavaScript

Client-side JavaScript debugging can become repetitive when navigation or parameter changes reload the page and invalidate temporary debugging state.

debugger;

When developer tools are open, a debugger; statement pauses execution at that point unless breakpoints are disabled. Adding the statement to a persistent local copy is one way to keep the pause point across reloads.[1]

Overrides

Chrome DevTools Local Overrides stores a local replacement for a network resource and serves that replacement on subsequent page loads.[2]

  1. Open DevTools > Sources > Overrides.
  2. Select an empty local folder and allow DevTools to access it.
  3. In the Page tree, right-click the target script and select Override content or Save for overrides, depending on the Chrome version.
  4. Add debugger;, save the file, and reload the page.

Selecting a JavaScript file in the Sources panel and saving it as a local override

The saved local copy now replaces the matching network resource while overrides are enabled. Changes therefore persist across reloads, but they affect only your local browser profile.[2]

A locally overridden JavaScript file containing a debugger statement

The XSS challenge walkthrough in reference 3 demonstrates this debugger; and Local Overrides workflow during a practical client-side analysis.[3]

References