1const window {
2  width: window.innerWidth,
3  height: window.innerHeight
4}

1// better than using window.innerWidth / window.innerHeight 
2// because of scrollbars
3const client = {
4      width: document.documentElement.clientWidth,
5      height: document.documentElement.clientHeight
6}

1window.screen.width
2//or just
3screen.width

1var win = window,
2    doc = document,
3    docElem = doc.documentElement,
4    body = doc.getElementsByTagName('body')[0],
5    x = win.innerWidth || docElem.clientWidth || body.clientWidth,
6    y = win.innerHeight|| docElem.clientHeight|| body.clientHeight;
7alert(x + ' × ' + y);

1// Size of browser viewport.
2$(window).height();
3$(window).width();
4
5// Size of HTML document (same as pageHeight/pageWidth in screenshot).
6$(document).height();
7$(document).width();

1<p id="demo"></p>
2
3<script>
4document.getElementById("demo").innerHTML = 
5"Screen width is " + screen.width;
6</script>