Continue Javascript animation even if in another tab

Question

I have a rotating Wheel, which should turn no matter what. But, if I leave the Tab, the wheel stops rotating until I focus the tab in which the website is open in.

Here is my code:

var colors =["#DD7039", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE", "#171A21", "#FEFEFE"];
var numbers =["0","21","10","13","3","14","1","17","11","23","4","18","9","24","7","20","5","15","16","22","6","12","19","8","2"];

var startAngle = 0;
var arc = Math.PI / (colors.length / 2);
var spinTimeout = null;

var timeToSpin = 2000;

var spinArcStart = 10;  
var spinTime = 0;
var spinTimeTotal = 0;

var ctx;

function drawRouletteWheel() {
    var canvas = document.getElementById("canvas");
    if (canvas.getContext) {
        var outsideRadius = 200;
        var textRadius = 160;
        var insideRadius = 125;

        ctx = canvas.getContext("2d");
        ctx.clearRect(0,0,500,500);


        ctx.strokeStyle = "black";
        ctx.lineWidth = 2;

        ctx.font = 'bold 12px Helvetica, Arial';

        for(var i = 0; i < colors.length; i++) {
            var angle = startAngle + i * arc;
            ctx.fillStyle = colors[i];

            ctx.beginPath();
            ctx.arc(250, 250, outsideRadius, angle, angle + arc, false);
            ctx.arc(250, 250, insideRadius, angle + arc, angle, true);
            ctx.stroke();
            ctx.fill();

            ctx.save();
            var color;
            if (colors[i] == "#FEFEFE") {
                color = "black";
            }else{
                color ="white";
            }
            ctx.fillStyle = color;
            ctx.translate(250 + Math.cos(angle + arc / 2) * textRadius,
                250 + Math.sin(angle + arc / 2) * textRadius);
            ctx.rotate(angle + arc / 2 + Math.PI / 2);
            var text = numbers[i];
            ctx.fillText(text, -ctx.measureText(text).width / 2, 0);
            ctx.restore();
        }

        //Arrow
        ctx.strokeStyle = "#F0C24E";
        ctx.beginPath();
        ctx.moveTo(250 + 0, 250 - (outsideRadius + 5));
        ctx.lineTo(250 + 0, 250 - (outsideRadius - 80));
        ctx.lineWidth = 4;
        ctx.stroke();
    }
}

function spin() {
    spinAngleStart = Math.random() * 10 + 10;
    spinTime = 0;
    spinTimeTotal = Math.random() * 3 + 4 * timeToSpin;
    rotateWheel();
}

function rotateWheel() {
    spinTime += 30;
    if(spinTime >= spinTimeTotal) {
        stopRotateWheel();
        return;
    }
    var spinAngle = spinAngleStart - easeOut(spinTime, 0, spinAngleStart, spinTimeTotal);
    startAngle += (spinAngle * Math.PI / 180);
    drawRouletteWheel();
    spinTimeout = setTimeout('rotateWheel()', 30);
}

function stopRotateWheel() {
    clearTimeout(spinTimeout);
    var degrees = startAngle * 180 / Math.PI + 90;
    var arcd = arc * 180 / Math.PI;
    var index = Math.floor((360 - degrees % 360) / arcd);
    ctx.save();
    ctx.font = 'bold 30px Helvetica, Arial';
    ctx.strokeStyle = "black";
    ctx.fillStyle = "black";
    var text = numbers[index]
    ctx.fillText(text, 250 - ctx.measureText(text).width / 2, 250 + 10);
    ctx.restore();
}

function easeOut(t, b, c, d) {
    var ts = (t/=d)*t;
    var tc = ts*t;
    return b+c*(tc + -3*ts + 3*t);
}

drawRouletteWheel();

Basiclly this is for a Roulette Wheel.

The animation is a calculation which redraws the wheel every time, just with different values (to simulate spinning).

Is there a way to continue the animation(calculation), even if the Website is "not in focus"?

Answer 1

Well thanks to @Thomas I found my solution:

At the beginning of the script I added:

if (!Date.now) {
    Date.now = function() { return new Date().getTime(); }
}
var start = null;

Inside of spin() I set the start var:

start = Date.now();

now inside of rotateWheel() I added and changed:

// At the beginning of the function
timePassed = Date.now() - start;
spinTime += timePassed;

and at the end of rotateWheel():

// right before the '.setTimeout'
start += timePassed;

now the animation completes once I get back to the window