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From comments below your question:

我没有使用任何插件

这就是问题所在.我猜你只是从某处粘贴了一些代码，并没有查看依赖项.

So that is the problem. I guess you just pasted some code from somewhere and did not look at the dependencies.

如果加载了 jQuery-Confirm 插件，则您的代码有效.因此，只需添加以下代码段中使用的 CDN.

Your code works if the jQuery-Confirm plugin is loaded. So just add the CDNs used in the below snippet.

function checknag() {
  var x1 = document.getElementById('tsumnag').value;
  var x2 = document.getElementById('salenugsum').value;
  if (+x1 == +x2) {
    $.alert({
      title: 'Alert!',
      content: 'Please enter item name',
      onDestroy: function() {
        // here the button key 'hey' will be used as the text.
        $('#munshi_perc').focus();
      }
    });
    return false;
  }
}

<link href="https://cdnjs.cloudflare.com/ajax/libs/jquery-confirm/3.3.4/jquery-confirm.min.css" rel="stylesheet"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-confirm/3.3.4/jquery-confirm.min.js"></script>

<input required class="urdu" onfocusout='checknag();' value="" type="text" />
<input required class="urdu" id="tsumnag" value="" type="text" />
<input required class="urdu" id="salenugsum" value="" type="text" />
<input required class="urdu" id="munshi_perc" value="" type="text" />