Get the size of the screen, current web page and browser window

You can get the size of the window or document with jQuery:

// Size of browser viewport.
$(window).height();
$(window).width();

// Size of HTML document (same as pageHeight/pageWidth in screenshot).
$(document).height();
$(document).width();

For screen size you can use the screen object:

window.screen.height;
window.screen.width;

This has everything you need to know: Get viewport/window size

but in short:

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

Fiddle

Please stop editing this answer. It's been edited 22 times now by different people to match their code format preference. It's also been pointed out that this isn't required if you only want to target modern browsers - if so you only need the following:

const width  = window.innerWidth || document.documentElement.clientWidth || 
document.body.clientWidth;
const height = window.innerHeight|| document.documentElement.clientHeight|| 
document.body.clientHeight;

console.log(width, height);