null is not undefined.

nullisnotundefined.js

// http://traceur-compiler.googlecode.com/git/demo/repl.html
// open the console
// paste this into the text area.
//
// or
// 
// Run in Firefox 15 (Nightly)


function nullIsNotUndefined( arg = "foo" ) {
  return arg;
}


console.log( nullIsNotUndefined() === "foo" );
console.log( nullIsNotUndefined(null) === null );

It does make sense, but it also means that the difference between undefined and null is only semantic.

(That, and the fact that you cannot assign null.)

Technically, undefined isn't writable or configurable either, but unfortunately implementation adherence isn't consistent

The end result is, undefined and null have been spec'ed as having very similar behavior. Their main difference is in their meaning. The standard library uses null very sparingly, but quite frankly each of its use could be replaced by undefined without losing anything.