我需要在 jQuery 的 get 函数之外使用 var

Question

考虑这段代码sn-p：

$('#selectone').change(function(){
    var amount;
    $.get('search.php', {search:'units'}, function(result){
        //this will return only one or zero for me.
        amount = result;
    })
    if(amount>0)
    {
        alert('This option has been selected, please select another one');
    }
})

我的变量amount 总是会出现undefined。如何修复或重构？

Answer 1

那是因为下面的代码在你的$.get()请求中的回调函数之前运行：

if(amount>0)
{
    alert('This option has been selected, please select another one');
}

AJAX 调用是异步的，这意味着它们周围的代码在 AJAX 调用等待响应时运行。所以 if(amount>0) 代码在 AJAX 回调触发之前运行（这意味着对于您的 if/then 语句 amount 将始终等于 null）。

要执行您想要的操作，我建议将该代码放入您的 $.get() 请求的回调函数中：

$('#selectone').change(function(){
    $.get('search.php', {search:'units'}, function(result){
        //this will return only one or zero for me.
        if(result>0)
        {
            alert('This option has been selected, please select another one');
        }
    });
});

--更新--

你也可以使用jQuery的$.when()方法：

$('#selectone').change(function(){
    var amount;
    var jqXHR = $.get('search.php', {search:'units'}, function(result){
        //this will return only one or zero for me.
        amount = result;
    });
    $.when(jqXHR).then(function () {
        if(amount>0)
        {
            alert('This option has been selected, please select another one');
        }
    });
});

Answer 2

你不能只在成功函数中使用它吗？最好的办法：

$('#selectone').change(function(){
 var amount;
    $.post({
        async : false, // this makes it possible
        url: 'search.php', 
        data: {search:'units'}, 
        success : function(result){
            //this will return only one or zero for me.
            amount = result;
        }
    });

    if(amount>0)
    {
        alert('This option has been selected, please select another one');
    }
})