Edit in JSFiddle

(function($){
    jQuery.f2c = function(option)
    {
        var opts = jQuery.extend(
            {},
            jQuery.f2c.default, 
            option);
        return (opts.fahrenheit-32) * 5/9;
    };
        
    jQuery.f2c.default = 
    {
        fahrenheit: 32
    };
})(jQuery);

$(document).ready(function() 
{
    document.write($.f2c() +"<br/>" + $.f2c({fahrenheit: 212}));
});

<p>&nbsp;</p>