JavaScript Type checking

I know, I know. You should be doing duck typing and letting things blowup. Or only checking for the specific things you want. And yet I tend to find myself doing a bit of type checking around argument parsing.

Anyway I've had some success with type checking using the parent Object.prototype.toString with a few special cases. Here's the code I ended up using in my argument parsing library "funs":

function getType(o) {
    var rawTypeString = 
        Object.prototype.toString.call(o);

    var typeString = 
        rawTypeString.replace('[object ', '')
                     .replace(']', '')
                     .toLowerCase();

    if (typeString === 'number' && Number.isNaN(o)) {
        return 'nan';
    }

    if (o instanceof Error) {
        return 'error';
    }

    if (typeString.indexOf('html') === 0) {
        return 'dom';
    }

    return typeString;
}

Full code in use at: https://github.com/copenhas/funs