https://www.chiba-biyo.com/common/js/overlay.js

1/*LINEモーダルウィンドウ*/
2$(function() {
3  $('.line_open').on('click', function() {
4    $('#overlay, #modalWindow').fadeIn();
5  });
6  
7  $('#overlay,#modalWindow .close').on('click', function() {
8    $('#overlay, #modalWindow').fadeOut();
9  });
10  
11  locateCenter();
12  $(window).resize(locateCenter);
13
14  function locateCenter() {
15    let w = $(window).width();
16    let h = $(window).height();
17    
18    let cw = $('#modalWindow').outerWidth();
19    let ch = $('#modalWindow').outerHeight();
20   
21    $('#modalWindow').css({
22      'left': ((w - cw) / 2) + 'px',
23      'top': ((h - ch) / 2) + 'px'
24      
25    });
26  }
27});