Question

Currently, I have a function defined like:

function a(b,c) {
 if (typeof(b) === "undefined") { b = 1; }
 if (typeof(c) === "undefined") { c = 2; }
 ...
}

Before, it set to default if the value was falsy. Changing this breaks the following call:

a(null, 3);

There is no undefined object. How can I pass only the second argument?

There is, actually, an undefined object:

There may be an undefined object normally, but clearly in my environment something has gone awry

Answer 1

A good test for checking the existence of a variable is this:

(typeof b === "undefined" || b === null)

Thus, your function can be rewritten like this :

function a(b,c) {
   if (typeof(b) === "undefined" || b === null) { b = 1; }
   if (typeof(c) === "undefined" || b === null) { c = 2; }
   ...
}

This would account for any of undefined, null or void 0 passed as arguments. If you find that repetitive, you can always define a utility function :

function exists(obj) {
    return typeof(obj) === "undefined" || obj === null;
}

And this precise test is what Coffee-script does with its existential operator ?. Coffee-script offers default arguments as well, which would make your life even easier.