是否有可能找到触发了.change（）的值？

Question

从<select>代码中，是否可以从中找到.change()触发的值？

Answer 1

使用变量缓存前一个值。

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// bind change event handler to the eleemnt 
// and cache the current value in `prev`
var prev = $('#test').change(function() {
  // get previous value from `prev`
  console.log('prev : ' + prev + ' , current : ' + this.value);

  //... do the rest here

  // update the `prev` variable with current value
  prev = this.value;
}).val(); // get the default value

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<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="test">
  <option value="1">1</option>
  <option value="11">11</option>
  <option value="111">111</option>
</select>

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Answer 2

以下是一个示例，您只需要在页面加载时获得value的{​​{1}}，然后每次更改时都只需使用新值更新此变量。

在任何更改事件中，变量select在更改前保留先前的值。

currValue

var currValue = $('select').val();

$('select').on('change',function(){
  var newValue = $(this).val();
  alert('value Before change : '+ currValue);
  alert('value After change : '+ newValue);
  
  currValue = newValue;
});

Answer 3

如果我没错，这就是你要求的：

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(function () {
    var previous;

    $("select[name=test]").focus(function () {
        // Store the current value on focus, before it changes
        previous = this.value;
    }).change(function() {
        // Do soomething with the previous value after the change
        document.getElementById("log").innerHTML = "<b>Previous: </b>"+previous;
        
        previous = this.value;
    });
})();

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<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select name="test">
    <option value="stack">Stack</option>
    <option value="overflow">Overflow</option>
    <option value="my">My</option>
    <option value="question">Question</option>
</select>
<div id="log"></div>

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