Question

If I have a function

foo()

that I call with no arguments most of the time, but one argument in special cases, is

var arg1 = arguments[0];
if (arg1) {
     <special case code>
}

inside the function a completely safe thing to do?

Answer 1

Yes, that’s fine. A reasonable alternative is to name the argument, and not use the arguments object:

function foo(specialArg)
{
    if (specialArg)
    {
        // special case code
    }
}

Note that if(bar) tests the truthiness of bar. If you call foo with any falsy value, such asfoo(0), foo(false), foo(null), etc., the special case code will not execute in the above function (or your original function, for that matter). You can change the test to

if (typeof specialArg !=== 'undefined')
{
    // ...
}

to make sure that the special case code is executed when the argument is supplied but falsy.