The Firefox JavaScript Debugger

The JavaScript Debugger enables you to step through JavaScript code and examine or modify its state to help track down bugs.

You can use it to debug code running locally in Firefox or running remotely, for example on an Android device running Firefox for Android. See remote debugging to learn how to connect the debugger to a remote target.

To find your way around the debugger, here’s a quick tour of the UI.

How to

To find out what you can do with the debugger, refer to the following how-to guides.

• Open the debugger

• Pretty-print a minified file

• Search

• Use a source map

• Debug worker threads

Pause execution

You probably want to pause the execution of your code, in order to see what is going on at various points. There are multiple ways to tell the Debugger how and when to pause:

• Set a breakpoint

• Set a conditional breakpoint

• Set an XHR breakpoint

• Set even listener

• Break on exceptions

• Use watchpoints for property reads and writes

• Break on DOM Mutation

• Disable breakpoints

Control execution

What can you do after execution pauses?

• Step through code

• Black box a source

• Debug worker threads

• Debug eval sources

Look at values

You probably want to see the value of variables or expressions, either during execution or when it is paused.

• Set a logpoint

• Set watch expressions

Reference

• Keyboard shortcuts

• Source map errors