jq Adding an instance of an overlay mask to a page

The jq code is introduced first, and then the code is as follows:

$(function(){
 
 var docHeight = $(document).height(); // Get window height 
  
 $('body').append('<div id="overlay"></div>');
  
 $('#overlay')
  .height(docHeight)
  .css({
   'opacity': .9, // Transparency 
   'position': 'absolute',
   'top': 0,
   'left': 0,
   'background-color': 'black',
   'width': '100%',
   'z-index': 5000 // Ensure that this suspension layer is above other contents 
  });
   
  setTimeout(function(){$('#overlay').fadeOut('slow')}, 3000); // Settings 3 The coating fades out automatically after seconds 
});