您可以重写代码以使其更优雅：

var dateformat = document.getElementById("dateformat"),
    options = [{
        text: month + "/" + day + "/" + fullyear,
        value: "M/d/yyyy"
    }, {
        text: monthwz + "/" + daywz + "/" + fullyear,
        value: "MM/dd/yyyy"
    }, {
        text: day + "/" + month + "/" + fullyear,
        value: "d/M/yyyy"
    }];

for (var i = 0, len = options.length; i < len; i++) {
    var option = options[i],
        element = new Option(option.text, option.value);

    dateformat.options[dateformat.options.length] = element;
}

// one way to set the selected value:
dateformat.value = <?php print json_encode($value); ?>;

演示：http: //jsfiddle.net/vPerj/