Here is my answer

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

$(function () {
    var message = "New Title!";
    var original;

    $(window).focus(function() {
      if(original) {
        document.title = original;
      }
    }).blur(function() {
      var title = $('title').text();
      if(title != message) {
        original = title;
      }
      document.title = message;
    });
  });

I have a problem about changing website title during two process.

When I enter webpage, its title has changed via the code shown below.

<script type="text/javascript">

    $(document).ready(function() {
        document.title = 'New Title';
    });

</script>

The sceranio,

• There are two blank page in my browser. a.html and b.html
• My website is a.html and its title “A page”
• When I enter to the a.html, its name changed as New title
• When I click to the b.html page, I want to change title of a.html as “A Page”

When I am at the any blank page in browser, I want to change its title again. How can I do it?

I found a code snippet about it . Is it right?

<script>

window.onblur = function () { document.title = 'A Page?'; }

window.onfocus = function () { document.title = 'New Title'; }
</script>