如何使用JavaScript创建自定义弹出窗口

我有一个SendMail按钮，单击它可以调用JavaScript函数。验证后会发出警报消息。但这是我正在使用的默认警报。如何在JavaScript中创建自定义警报

我的HTML代码：

       <div id ="sendmail" style="display: none;">
        <div class="container">
        <div style="text-align:right; width:100%; padding:0;">
        <button id ="cancel" style='margin-right:16px' class="btn btnprimary btn-lg pull-right" a href="javascript:window.history.back()">Cancel</button>
        <button id ="sendMail" style='margin-right:16px'  class="btn btn-primary btn-lg pull-right" onclick="sendMail()">SendMail</button>

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请给出建议，谢谢。

您的问题与此非常相似，我知道您在这个问题中得到了答案。

        function sendMail(){
        console.log(employee_id);
        var employeeid = employee_id;
$.ajax({
  url:'http://localhost:8088/JirasTrackingApp/reporter/
  Reportees/JiraNames/'+employeeid,
    type:'GET',

    success: function(Jiranumbers){
             alert("Mail is sent successfully");  //here I need to be able to call the customized alert instead of that default alert
            },
      error: function(Jiranumbers){
            alert("Mail is not sent successfully");
        }        
    });