Bug with clearTimeout()?

I vote for handling undefined and missing the same way. Default function parameters are implemented in javascript using a check for undefined - an example:

function multiply(a, b) {
  b = (typeof b !== 'undefined') ?  b : 1;
  return a * b;
}

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Default_parameters

I believe adding a distinction between undefined and missing breaks the spec too.