formwatchfieldCode date_of_join data = if datadate_of_join && datadate_of_joinlength var currentDate = new Date; var oneMonthEarlier = new DatecurrentDategetFullYear currentDategetMonth - 1 current

<p>要让上述JavaScript代码可以执行，你需要将其放置在HTML文件中的<script>标签中。例如：</p>
<pre><code class="language-html">&lt;!DOCTYPE html&gt;
&lt;html&gt;
&lt;head&gt;
  &lt;title&gt;My Form&lt;/title&gt;
&lt;/head&gt;
&lt;body&gt;
  &lt;form&gt;
    &lt;input type=&quot;date&quot; name=&quot;date_of_join&quot;&gt;
    &lt;button type=&quot;submit&quot;&gt;Submit&lt;/button&gt;
  &lt;/form&gt;

  &lt;script&gt;
    function watchForm() {
      var form = document.querySelector('form');

      form.watch([{
        'fieldCode': 'date_of_join'
      }], function(data) {
        if (data['date_of_join'] &amp;&amp; data['date_of_join'].length) {
          var currentDate = new Date();
          var oneMonthEarlier = new Date(currentDate.getFullYear(), currentDate.getMonth() - 1, currentDate.getDate());

          var minDate = oneMonthEarlier.toISOString().split('T')[0];
          var selectedDate = new Date(data['date_of_join']);

          var selectedTimestamp = selectedDate.getTime();
          console.log(&quot;selectedTimestamp=&quot;, selectedTimestamp)

          var oneMonthEarlierTimestamp = oneMonthEarlier.getTime();
          console.log(&quot;oneMonthEarlierTimestamp&quot;, oneMonthEarlierTimestamp)

          if (selectedTimestamp &lt; oneMonthEarlierTimestamp) {
            form.actions({
              type: 'msgbox',
              message: &quot;最早日期不得早于&quot; + minDate,
              data: { 'date_of_join': '' }
            });

            // alert(&quot;最早日期不得早于&quot; + minDate);
          }
        }
      });
    }

    watchForm();
  &lt;/script&gt;
&lt;/body&gt;
&lt;/html&gt;
</code></pre>
<p>在上面的示例中，我们将代码包裹在<code>watchForm</code>函数中，并在页面加载完成后调用该函数。这样，当表单中的日期字段发生变化时，将触发<code>watch</code>方法，并执行相应的逻辑</p>