Responsive Screen Size Checker

screen-size.js

// Get visible window sizes
function displayWindowSize(){
	var	e=window.innerWidth,
		t=window.innerHeight;

	document.getElementById("dev").innerHTML=t+"x"+e;
}

// Create DIV
var div=document.createElement("div");
	div.id="dev";

// Append and style DIV to top right
document.body.appendChild(div);
[].forEach.call(document.querySelectorAll("#dev"),
	function(e){
		e.style.color="red";
		e.style.position="fixed";
		e.style.top="0";
		e.style.right="0";
		e.style.fontWeight="bold";
});

// Insert window sizes into the DIV
window.onload=displayWindowSize;

// Update the window size on resize of window
window.addEventListener("resize",displayWindowSize,!1);