The debugger statement

The debugger statement in JavaScript will cause a debugger (like FireBug or the Chrome developer tools) to pause execution just as if there was a breakpoint at that line.  This is most useful when you have dynamically generated code or whenever you don't have a chance to set a breakpoint on a line before the code executes.  

Simple example:

var a = 44;

debugger;

alert(a);

In this case the debugger (if open) will pause before the alert is shown.  

Here are the MDN docs for the debugger statement.