How to show fullscreen popup window in javascript?

Use screen.availWidth and screen.availHeight to calculate a suitable size for the height and width parameters in window.open()

Although this is likely to be close, it will not be maximised, nor accurate for everyone, especially if all the toolbars are shown.

What about this:

var popup = window.open(URL);
if (popup == null)
   alert('Please change your popup settings');
else  {
  popup.moveTo(0, 0);
  popup.resizeTo(screen.width, screen.height);
}

What about this, I gave width and height value to a big number and it works

window.open("https://www.w3schools.com", "_blank","toolbar=yes,scrollbars=yes,resizable=yes,top=500,left=500,width=4000,height=4000");

More than bad design - this "feature" is a recipe for UI disaster. There were a number of malicious web sites which exploited the full screen view features in JavaScript to hijack browser windows and display a screen indistinguishable from the user's desktop. While there may still be a way to do this, please for the love of all things decent, do not implement this.