Center Twitter login popup window

twitter-login-popup.js

// Source: https://www.codeofaninja.com/2011/04/how-to-center-screen-pop-up-window-with.html

function showcontactusform() {
  //set the width and height of the 
  //pop up window in pixels
  var width = 500;
  var height = 500;
  
  //Get the TOP coordinate by
  //getting the 50% of the screen height minus
  //the 50% of the pop up window height
  var top = parseInt((screen.availHeight/2) - (height/2));
  
  //Get the LEFT coordinate by
  //getting the 50% of the screen width minus
  //the 50% of the pop up window width
  var left = parseInt((screen.availWidth/2) - (width/2));
  
  //Open the window with the 
  //file to show on the pop up window
  //title of the pop up
  //and other parameter where we will use the
  //values of the variables above
  window.open('contact_form.php', 
              "Contact The Code Ninja", 
              "menubar=no,resizable=no,width=500,height=500,scrollbars=yes,left="  
              + left + ",top=" + top + ",screenX=" + left + ",screenY=" + top);    
}