Сделать так, что-бы конфетти были поверх всех элементов страницы, а не внизу

var maxParticleCount = 150; 
var particleSpeed = 2; 
var startConfetti; 
var stopConfetti; 
var toggleConfetti; 
var removeConfetti; 

(function() {
    startConfetti = startConfettiInner;
    stopConfetti = stopConfettiInner;
    toggleConfetti = toggleConfettiInner;
    removeConfetti = removeConfettiInner;


    var colors = ["DodgerBlue", "OliveDrab", "Gold", "Pink", "SlateBlue", "LightBlue", "Violet", "PaleGreen", "SteelBlue", "SandyBrown", "Chocolate", "Crimson"]
    var streamingConfetti = false;
    var animationTimer = null;
    var particles = [];
    var waveAngle = 0;
    
    function resetParticle(particle, width, height) {
        particle.color = colors[(Math.random() * colors.length) | 0];
        particle.x = Math.random() * width;
        particle.y = Math.random() * height - height;
        particle.diameter = Math.random() * 10 + 5;
        particle.tilt = Math.random() * 10 - 10;
        particle.tiltAngleIncrement = Math.random() * 0.07 + 0.05;
        particle.tiltAngle = 0;
        return particle;
    }
    

    function startConfettiInner() {
        var width = window.innerWidth;
        var height = window.innerHeight;
        window.requestAnimFrame = (function() {
            return window.requestAnimationFrame ||
                window.webkitRequestAnimationFrame ||
                window.mozRequestAnimationFrame ||
                window.oRequestAnimationFrame ||
                window.msRequestAnimationFrame ||
                function (callback) {
                    return window.setTimeout(callback, 16.6666667);
                };
        })();
        var canvas = document.getElementById("confetti-canvas");
        if (canvas === null) {
            canvas = document.createElement("canvas");
            canvas.setAttribute("id", "confetti-canvas");
            canvas.setAttribute("style", "display:block;z-index:999999;pointer-events:none");
            document.body.appendChild(canvas);
            canvas.width = width;
            canvas.height = height;
            window.addEventListener("resize", function() {
                canvas.width = window.innerWidth;
                canvas.height = window.innerHeight;
            }, true);
        }
        var context = canvas.getContext("2d");
        while (particles.length < maxParticleCount)
            particles.push(resetParticle({}, width, height));
        streamingConfetti = true;
        if (animationTimer === null) {
            (function runAnimation() {
                context.clearRect(0, 0, window.innerWidth, window.innerHeight);
                if (particles.length === 0)
                    animationTimer = null;
                else {
                    updateParticles();
                    drawParticles(context);
                    animationTimer = requestAnimFrame(runAnimation);
                }
            })();
        }
    }

    function stopConfettiInner() {
        streamingConfetti = false;
    }
    setTimeout(stopConfettiInner, 5000)

    function removeConfettiInner() {
        stopConfetti();
        particles = [];
    }
    


    

    function toggleConfettiInner() {
        if (streamingConfetti)
            stopConfettiInner();
        else
            startConfettiInner();
    }

    function drawParticles(context) {
        var particle;
        var x;
        for (var i = 0; i < particles.length; i++) {
            particle = particles[i];
            context.beginPath();
            context.lineWidth = particle.diameter;
            context.strokeStyle = particle.color;
            x = particle.x + particle.tilt;
            context.moveTo(x + particle.diameter / 2, particle.y);
            context.lineTo(x, particle.y + particle.tilt + particle.diameter / 2);
            context.stroke();
        }
    }

    function updateParticles() {
        var width = window.innerWidth;
        var height = window.innerHeight;
        var particle;
        waveAngle += 0.01;
        for (var i = 0; i < particles.length; i++) {
            particle = particles[i];
            if (!streamingConfetti && particle.y < -15)
                particle.y = height + 100;
            else {
                particle.tiltAngle += particle.tiltAngleIncrement;
                particle.x += Math.sin(waveAngle);
                particle.y += (Math.cos(waveAngle) + particle.diameter + particleSpeed) * 0.5;
                particle.tilt = Math.sin(particle.tiltAngle) * 15;
            }
            if (particle.x > width + 20 || particle.x < -20 || particle.y > height) {
                if (streamingConfetti && particles.length <= maxParticleCount)
                    resetParticle(particle, width, height);
                else {
                    particles.splice(i, 1);
                    i--;
                }
            }
        }
    }
})();

#confetti-canvas {
  position: fixed;
  top: 0;
  left: 0;
}

<!doctype html>
  <html>
  <head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  </head>
  <body>
    <script src="confetti.js"></script>
    <button class="btn btn-danger">Start</button>

  </div>

</body>
</html>