Get pixel ratio

(function() {
var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d")
var devicePixelRatio = window.devicePixelRatio || 1;
var backingStoreRatio = ctx.webkitBackingStorePixelRatio ||
   ctx.mozBackingStorePixelRatio ||
   ctx.msBackingStorePixelRatio ||
   ctx.backingStorePixelRatio || 1
console.log("devicePixelRatio: " + devicePixelRatio + "; backingStoreRatio: " + backingStoreRatio);
}());