Simple way to calculate the FPS when using requestAnimationFrame

fps.js

window.onload = function() {
	var lastCalledTime;
	var counter = 0;
	var fpsArray = [];

	function update(timestamp) {
		var fps;
	
		if (!lastCalledTime) {
			lastCalledTime = new Date().getTime();
			fps = 0;
		}
	
		var delta = (new Date().getTime() - lastCalledTime) / 1000;
		lastCalledTime = new Date().getTime();
		fps = Math.ceil((1/delta));
	
		if (counter >= 60) {
			var sum = fpsArray.reduce(function(a,b) { return a + b });
			var average = Math.ceil(sum / fpsArray.length);
			console.log(average);
			counter = 0;
		} else {
			if (fps !== Infinity) {
				fpsArray.push(fps);
			}
	
			counter++;
		}
	
		window.requestAnimationFrame(update);
	}

	window.requestAnimationFrame(update);
};