debugger
The debugger statement invokes any available debugging
functionality, such as setting a breakpoint. If no debugging functionality is available,
this statement has no effect.
Syntax
debugger;
Examples
Using the debugger statement
The following example shows code where a debugger statement has been
inserted, to invoke a debugger (if one exists) when the function is called.
function potentiallyBuggyCode() {
  debugger;
  // do potentially buggy stuff to examine, step through, etc.
}
When the debugger is invoked, execution is paused at the debugger
statement. It is like a breakpoint in the script source.

Specifications
{{Specifications}} 
Browser compatibility
{{Compat}} 
See also
- The Firefox JavaScript Debugger¶ in the Firefox source docs