Consider the following code snippet

function f(o) 
{
     if (o === undefined) debugger;
}

What could be the task of the statement debugger?

The debugger statement normally does nothing. If, however, a debugger program is available and is running, then an implementation may (but is not required to) perform some kind of debugging action. In practice, this statement acts like a breakpoint: execution of JavaScript code stops and you can use the debugger to print variable’s values.