I've seen this style of library come up quite a few times whilst hacking away in JavaScript where developers try to introduce a sense of strict typing (I even wrote a similar one myself when I first moved over from ActionScript). In the end I learned to stop worrying and love the <strike>bomb</strike> duck typing[0].

For me; one of the "joys" of coding JavaScript is the expressiveness that comes with a dynamic languages; should you throw an ArgumentError when your function that expects a number is invoked with a String? Maybe - sure it can help catch problems early and effectively "stop-the-line" in your public API, but then again it will probably end up throwing the classic `TypeError: Object foo has no method 'bar'` for you anyway.

For "public" methods which form part of an API (especially when that API is going to be shared outside of my team) I try to make my functions handle failure early (before they delegate off to the "private" internal methods), even better if the public methods can repair any unexpected usage, ie:

  function convertToHex(value) {
    var result;
    
    if (typeof value !== "number") {
      result = convertToHex(parseInt(value, 10));
    }
    else {
      result = "0x" + value.toString(16);
    }
    
    return result;
  }

Also, with regards to default argument values, I've always felt the "native" JavaScript approach was fairly compact and descriptive when required:

  function doFoo(bar) {
    bar = (bar !== undefined) ? bar : "default_value";
  }

[0] http://en.wikipedia.org/wiki/Duck_typing