JS ajax 执行先后的 问题。。。来帮个！！！！

var currencySelhtm = "";
$.ajax({
    url: "/servlet/CurrencyAction.do",
    type: "post",
    dataType: "xml",
    data: "action=showall",
    success: function(xmlResult) {
        var $xml = $(xmlResult).find("Currency");
        if ($xml.length < 1) {
        } else {
            for (var m = 0; m < $xml.length; m++) {
                var Currency_Name = $xml.eq(m).attr("Currency_Name");
                var Currency_Signcode = $xml.eq(m).attr("Currency_Signcode");
                var Currency_ID = $xml.eq(m).attr("Currency_ID");
                currencySelhtm += '<option value="' + Currency_ID + '">' + Currency_Name+'</option>';

            }
        }
    },
    error: function() {
        currencySelhtm = "";
    }
});
alert(currencySelhtm);

result += "币种:<select name='tcurrencys' id='tcurrencys'><option value='-1'>---</option >" + currencySelhtm + "</select><br/><br/>";

有以上代码，我想动态生成币种 这个下拉框的 OPT选项。

遇到的问题：
程序先执行了 result 的拼串 动作。使得currencySelhtm 后做 AJAX 获取的数据 无法拼凑进 目标SELECT 元素。大家有没有其他的 办法？谢谢了
[解决办法]

var currencySelhtm = "";
$.ajax({
    url: "/servlet/CurrencyAction.do",
    type: "post",
    dataType: "xml",
    data: "action=showall",
    success: function(xmlResult) {
        var $xml = $(xmlResult).find("Currency");
        if ($xml.length < 1) {
        } else {
            for (var m = 0; m < $xml.length; m++) {
                var Currency_Name = $xml.eq(m).attr("Currency_Name");
                var Currency_Signcode = $xml.eq(m).attr("Currency_Signcode");
                var Currency_ID = $xml.eq(m).attr("Currency_ID");
                currencySelhtm += '<option value="' + Currency_ID + '">' + Currency_Name+'</option>'; 
 
            }
                alert(currencySelhtm);
                //你要怎么使用这个currencySelhtm的方法放在这个回调函数中即可
        }
    },
    error: function() {
        currencySelhtm = "";
    }
});

 
result += "币种:<select name='tcurrencys' id='tcurrencys'><option value='-1'>---</option >" + currencySelhtm + "</select><br/><br/>";

[解决办法]

var currencySelhtm = "";
$.ajax({
    url: "/servlet/CurrencyAction.do",
    type: "post",
    dataType: "xml",
    data: "action=showall",
    success: function(xmlResult) {
        var $xml = $(xmlResult).find("Currency");
        if ($xml.length < 1) {
        } else {
            for (var m = 0; m < $xml.length; m++) {
                var Currency_Name = $xml.eq(m).attr("Currency_Name");
                var Currency_Signcode = $xml.eq(m).attr("Currency_Signcode");
                var Currency_ID = $xml.eq(m).attr("Currency_ID");
                currencySelhtm += '<option value="' + Currency_ID + '">' + Currency_Name+'</option>';
 
            }
            result += "币种:<select name='tcurrencys' id='tcurrencys'><option value='-1'>---</option >" + currencySelhtm + "</select><br/><br/>";
            alert(currencySelhtm);
        }
    },
    error: function() {
        currencySelhtm = "";
    }
});

AJAX 的本意，　就是异步JavaScript和XML。　所以前面的事情刚开始做，不管有没有做完，　马上就会执行下一条语句。

如果你想在提交之后才做某个事情，　两种解决方案：
1. 写在 $.ajax 的回调函数之中。
2. 将 $.ajax 设置为同步 async:false ，　不过这样并不好，　会造成页面短时间无法操作。
[解决办法]

var currencySelhtm = "";
$.ajax({
    url: "/servlet/CurrencyAction.do",
    type: "post",
async:false,///////加这句同步就行了
    dataType: "xml",
    data: "action=showall", 
 
    success: function(xmlResult) {
        var $xml = $(xmlResult).find("Currency");
        if ($xml.length < 1) {
        } else {
            for (var m = 0; m < $xml.length; m++) {
                var Currency_Name = $xml.eq(m).attr("Currency_Name");
                var Currency_Signcode = $xml.eq(m).attr("Currency_Signcode");
                var Currency_ID = $xml.eq(m).attr("Currency_ID");
                currencySelhtm += '<option value="' + Currency_ID + '">' + Currency_Name+'</option>';
 
            }
        }
    },
    error: function() {
        currencySelhtm = "";
    }
});
alert(currencySelhtm);
 
result += "币种:<select name='tcurrencys' id='tcurrencys'><option value='-1'>---</option >" + currencySelhtm + "</select><br/><br/>";