使用以下代码，我已经测试了它和它的工作

var prev_val;
$('.dropdown').focus(function() {
    prev_val = $(this).val();
}).change(function(){
            $(this).unbind('focus');
            var conf = confirm('Are you sure want to change status ?');

            if(conf == true){
                //your code
            }
            else{
                $(this).val(prev_val);
                $(this).bind('focus');
                return false;
            }
});

如何使用一个自定义的jQuery事件与角手表类型的接口;

// adds a custom jQuery event which gives the previous and current values of an input on change
(function ($) {
    // new event type tl_change
    jQuery.event.special.tl_change = {
        add: function (handleObj) {
            // use mousedown and touchstart so that if you stay focused on the
            // element and keep changing it, it continues to update the prev val
            $(this)
                .on('mousedown.tl_change touchstart.tl_change', handleObj.selector, focusHandler)
                .on('change.tl_change', handleObj.selector, function (e) {
                // use an anonymous funciton here so we have access to the
                // original handle object to call the handler with our args
                var $el = $(this);
                // call our handle function, passing in the event, the previous and current vals
                // override the change event name to our name
                e.type = "tl_change";
                handleObj.handler.apply($el, [e, $el.data('tl-previous-val'), $el.val()]);
            });
        },
        remove: function (handleObj) {
            $(this)
                .off('mousedown.tl_change touchstart.tl_change', handleObj.selector, focusHandler)
                .off('change.tl_change', handleObj.selector)
                .removeData('tl-previous-val');
        }
    };

    // on focus lets set the previous value of the element to a data attr
    function focusHandler(e) {
        var $el = $(this);
        $el.data('tl-previous-val', $el.val());
    }
})(jQuery);

// usage
$('.some-element').on('tl_change', '.delegate-maybe', function (e, prev, current) {
    console.log(e);         // regular event object
    console.log(prev);      // previous value of input (before change)
    console.log(current);   // current value of input (after change)
    console.log(this);      // element
});

 $("#dropdownId").on('focus', function () {
    var ddl = $(this);
    ddl.data('previous', ddl.val());
}).on('change', function () {
    var ddl = $(this);
    var previous = ddl.data('previous');
    ddl.data('previous', ddl.val());
});

我选择Avi Pinto的解决方案，它使用jquery.data()

使用焦点并不是一个有效的解决方案。它在你第一次改变选项时起作用，但如果你停留在选择元素上，并按“上”或“下”键。它不会再次通过焦点事件。

所以解应该是这样的，

//set the pre data, usually needed after you initialize the select element
$('mySelect').data('pre', $(this).val());

$('mySelect').change(function(e){
    var before_change = $(this).data('pre');//get the pre data
    //Do your work here
    $(this).data('pre', $(this).val());//update the pre data
})

我需要根据选择显示一个不同的div

这就是如何使用jquery和es6语法来做到这一点

HTML

<select class="reveal">
    <option disabled selected value>Select option</option>
    <option value="value1" data-target="#target-1" >Option 1</option>
    <option value="value2" data-target="#target-2" >Option 2</option>
</select>
<div id="target-1" style="display: none">
    option 1
</div>
<div id="target-2" style="display: none">
    option 2
</div>

JS

$('select.reveal').each((i, element)=>{
    //create reference variable 
    let $option = $('option:selected', element)
    $(element).on('change', event => {
        //get the current select element
        let selector = event.currentTarget
        //hide previously selected target
        if(typeof $option.data('target') !== 'undefined'){
            $($option.data('target')).hide()
        }
        //set new target id
        $option = $('option:selected', selector)
        //show new target
        if(typeof $option.data('target') !== 'undefined'){
            $($option.data('target')).show()
        }
    })
})

将焦点事件与更改事件结合起来以实现您想要的结果:

(function () {
    var previous;

    $("select").on('focus', function () {
        // Store the current value on focus and on change
        previous = this.value;
    }).change(function() {
        // Do something with the previous value after the change
        alert(previous);

        // Make sure the previous value is updated
        previous = this.value;
    });
})();

工作示例:http://jsfiddle.net/x5PKf/766