jquery check if is function before executing

jquery-is-function.js

if ( $.isFunction($.fn.datepicker) ) {
    $('input.datepicker').datepicker({
        // The format you want
        altFormat: "yy-mm-dd",
        // The format the user actually sees
        dateFormat: "M dd, yy",
        changeYear: true,
        changeMonth: true
    });
}