Step through code

This page describes the JavaScript Debugger as it appears in Firefox 52 and above, in Firefox Nightly and Firefox Developer Edition.

To see what it's like in earlier versions of Firefox or in Firefox Beta and Release, see Debugger (before Firefox 52).

If you are getting this version of the Debugger and need to switch back to the old version, you can do so by visiting about:config and setting the "devtools.debugger.new-debugger-frontend" preference to false.

When the debugger is stopped at a breakpoint, you can step through it using four buttons in the toolbar:

In order, the buttons are:

Play: run to the next breakpoint
Step over: advance to the next line in the same function.
Step in: advance to the next line in the function, unless on a function call, in which case enter the function being called
Step out: run to the end of the current function

When paused, the "Toggle split console" button can be used to open and close the web console to gain more insight into errors and variables:

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