lesson04.html

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>Gamedev Canvas Workshop - lesson 4: paddle and keyboard controls</title>
    <style>* { padding: 0; margin: 0; } canvas { background: #eee; display: block; margin: 0 auto; }</style>
</head>
<body>

<canvas id="myCanvas" width="480" height="320"></canvas>

<script>
    var canvas = document.getElementById("myCanvas");
    var ctx = canvas.getContext("2d");
    var ballRadius = 10;
    var x = canvas.width/2;
    var y = canvas.height-30;
    var dx = 2;
    var dy = -2;
    var paddleHeight = 10;
    var paddleWidth = 75;
    var paddleX = (canvas.width-paddleWidth)/2;
    var rightPressed = false;
    var leftPressed = false;

    document.addEventListener("keydown", keyDownHandler, false);
    document.addEventListener("keyup", keyUpHandler, false);

    function keyDownHandler(e) {
        if(e.code  == "ArrowRight") {
            rightPressed = true;
        }
        else if(e.code == 'ArrowLeft') {
            leftPressed = true;
        }
    }
    function keyUpHandler(e) {
        if(e.code  == "ArrowRight") {
            rightPressed = false;
        }
        else if(e.code == 'ArrowLeft') {
            leftPressed = false;
        }
    }

    function drawBall() {
        ctx.beginPath();
        ctx.arc(x, y, ballRadius, 0, Math.PI*2);
        ctx.fillStyle = "#0095DD";
        ctx.fill();
        ctx.closePath();
    }
    function drawPaddle() {
        ctx.beginPath();
        ctx.rect(paddleX, canvas.height-paddleHeight, paddleWidth, paddleHeight);
        ctx.fillStyle = "#0095DD";
        ctx.fill();
        ctx.closePath();
    }

    function draw() {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        drawBall();
        drawPaddle();

        if(x + dx > canvas.width-ballRadius || x + dx < ballRadius) {
            dx = -dx;
        }
        if(y + dy > canvas.height-ballRadius || y + dy < ballRadius) {
            dy = -dy;
        }

        if(rightPressed && paddleX < canvas.width-paddleWidth) {
            paddleX += 7;
        }
        else if(leftPressed && paddleX > 0) {
            paddleX -= 7;
        }

        x += dx;
        y += dy;
    }

    setInterval(draw, 10);
</script>

</body>
</html>