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Testem aí.
<!DOCTYPE html>
<html>
    <canvas width='900' height='600'></canvas>
    <meta charset='utf-8'>
</html>

<script type='text/javascript'>
    var canvas = document.querySelector('canvas');
    var brush = canvas.getContext('2d');
    var screenWidth = canvas.width;
    var screenHeight = canvas.height;
    var bullseyeRadius = 0.000020 * (screenWidth * screenHeight);
    var bullseyeCoord = [screenWidth / 2, screenHeight / 2];
    var score = 0;
    var misses = 0;

    updateScreen();
    drawBullseye();
    setInterval(updateScreen, 800);
    canvas.onclick = hitDetect;

    function drawCircle (X, Y, radius, color) {
        brush.fillStyle = color;
        brush.beginPath();
        brush.arc(X, Y, radius, 0, 2 * Math.PI);
        brush.fill();
    }

    function drawBullseye () {
        drawCircle(bullseyeCoord[0], bullseyeCoord[1], bullseyeRadius * 3, 'red');
        drawCircle(bullseyeCoord[0], bullseyeCoord[1], bullseyeRadius * 2, 'white');
        drawCircle(bullseyeCoord[0], bullseyeCoord[1], bullseyeRadius, 'red');
    }

    function updateCoord () {
        var newX = Math.round(Math.random() * (screenWidth - bullseyeRadius * 3));
        var newY = Math.round(Math.random() * (screenHeight - bullseyeRadius * 3));
        if (newX - (bullseyeRadius * 3) - canvas.offsetLeft > 0 && newX + (bullseyeRadius * 3) < screenWidth) {
            bullseyeCoord[0] = newX;
        } else {
            if (newX < 0) {
                newX += bullseyeRadius * 3
            } else { newX -= bullseyeRadius * 3 }
        }
        if (newY - (bullseyeRadius * 3) - canvas.offsetTop > 35 && newY + (bullseyeRadius * 3) < screenHeight) {
            bullseyeCoord[1] = newY;
        } else {
            if (newY < 35) {
                newY += bullseyeRadius * 3
            } else { newY -= bullseyeRadius * 3 }
        }
    }

    function hitDetect (event) {
        var X = event.pageX - canvas.offsetLeft;
        var Y = event.pageY - canvas.offsetTop;
        if (X < bullseyeCoord[0] + bullseyeRadius && X > bullseyeCoord[0] - bullseyeRadius && Y < bullseyeCoord[1] + bullseyeRadius && Y > bullseyeCoord[1] - bullseyeRadius) {
            score ++;
        } else { misses++ }
    }

    function clearScreen () {
        brush.clearRect(0, 0, screenWidth, screenHeight);
    }

    function showScore () {
        brush.fillStyle = 'black';
        brush.fillRect(0, 0, screenWidth, 35);

        brush.fillStyle = 'white';
        brush.font = '30px monospace, Arial';
        brush.fillText('Hits: ' + score, 70, 27);

        brush.fillStyle = 'white';
        brush.font = '30px monospace, Arial';
        brush.fillText('Misses: ' + misses, screenWidth - 220, 27);
    }

    function updateScreen () {
        clearScreen();
        updateCoord();
        drawBullseye();
        showScore();
    }
</script>