trianta.dev/js/pong.js

const canvas = document.getElementById('pong');
const context = canvas.getContext('2d');
const grid = 15;
const paddleHeight = grid * 5; // 80
const maxPaddleY = canvas.height - grid - paddleHeight;

var paddleSpeed = 6;
var ballSpeed = 5;

var leftScore = 0;
var rightScore = 0;

const leftPaddle = {
  // start in the middle of the game on the left side
  x: grid * 2,
  y: canvas.height / 2 - paddleHeight / 2,
  width: grid,
  height: paddleHeight,

  // paddle velocity
  dy: 0
};
const rightPaddle = {
  // start in the middle of the game on the right side
  x: canvas.width - grid * 3,
  y: canvas.height / 2 - paddleHeight / 2,
  width: grid,
  height: paddleHeight,

  // paddle velocity
  dy: 0
};
const ball = {
  // start in the middle of the game
  x: canvas.width / 2,
  y: canvas.height / 2,
  width: grid,
  height: grid,

  // keep track of when need to reset the ball position
  resetting: false,

  // ball velocity (start going to the top-right corner)
  dx: ballSpeed,
  dy: -ballSpeed
};

// check for collision between two objects using axis-aligned bounding box (AABB)
// @see https://developer.mozilla.org/en-US/docs/Games/Techniques/2D_collision_detection
function collides(obj1, obj2) {
  return obj1.x < obj2.x + obj2.width &&
         obj1.x + obj1.width > obj2.x &&
         obj1.y < obj2.y + obj2.height &&
         obj1.y + obj1.height > obj2.y;
}

function gameOver() {
    context.clearRect(0, 0, canvas.width, canvas.height);
    let restartGame = document.getElementById("restart");
    let gameOverText = document.getElementById("gameover");
    restartGame.hidden = false;
    gameOverText.hidden = false;
}

function restartGame() {
    leftScore = 0;
    rightScore = 0;
    let restartGame = document.getElementById("restart");
    restartGame.hidden = true;
    requestAnimationFrame(loop);
}

function BotMovesPaddle(paddle) {
  ballRegion = Math.floor(ball.y / 15);
  paddleRegion = Math.floor(paddle.y / 15);
  if (ballRegion < paddleRegion) paddle.dy = -paddleSpeed; 
  if (ballRegion > paddleRegion) paddle.dy = paddleSpeed; 
  if (ballRegion === paddleRegion) paddle.dy = 0; 
}

// game loop
function loop() {
        //if (leftScore < 7 && rightScore < 7) {
            requestAnimationFrame(loop);
        //} else if (leftScore >= 7 || rightScore >= 7) {
        //    gameOver();
        //}
        context.clearRect(0,0,canvas.width,canvas.height);

        if (ball.dx < 0) {
            BotMovesPaddle(leftPaddle);
            document.getElementById("leftPaddle").innerHTML = leftPaddle.y;
        }
        if (ball.dx > 0) {
            BotMovesPaddle(rightPaddle);
            document.getElementById("rightPaddle").innerHTML = rightPaddle.y;
        }

        // move paddles by their velocity
        leftPaddle.y += leftPaddle.dy;
        rightPaddle.y += rightPaddle.dy;

        // prevent paddles from going through walls
        if (leftPaddle.y < grid) {
            leftPaddle.y = grid;
        }
        else if (leftPaddle.y > maxPaddleY) {
            leftPaddle.y = maxPaddleY;
        }

        if (rightPaddle.y < grid) {
            rightPaddle.y = grid;
        }
        else if (rightPaddle.y > maxPaddleY) {
            rightPaddle.y = maxPaddleY;
        }

        // draw paddles
        context.fillStyle = 'white';
        context.fillRect(leftPaddle.x, leftPaddle.y, leftPaddle.width, leftPaddle.height);
        context.fillRect(rightPaddle.x, rightPaddle.y, rightPaddle.width, rightPaddle.height);

        // move ball by its velocity
        ball.x += ball.dx;
        ball.y += ball.dy;

        // prevent ball from going through walls by changing its velocity
        if (ball.y < grid) {
            ball.y = grid;
            ball.dy *= -1;
        }
        else if (ball.y + grid > canvas.height - grid) {
            ball.y = canvas.height - grid * 2;
            ball.dy *= -1;
        }

        // reset ball if it goes past paddle (but only if we haven't already done so)
        if ( (ball.x < 0 || ball.x > canvas.width) && !ball.resetting) {
            ball.resetting = true;

            if (ball.x < 0) {
                rightScore++;
            }
            if (ball.x > canvas.width) {
                leftScore++;
            }

            // give some time for the player to recover before launching the ball again
            setTimeout(() => {
                ball.resetting = false;
                ball.x = canvas.width / 2;
                ball.y = canvas.height / 2;
            }, 400);
        }

        // check to see if ball collides with paddle. if they do change x velocity
        if (collides(ball, leftPaddle)) {
            ball.dx *= -1;
            leftPaddle.dy = 0;

            // move ball next to the paddle otherwise the collision will happen again
            // in the next frame
            ball.x = leftPaddle.x + leftPaddle.width;
        }
        else if (collides(ball, rightPaddle)) {
            ball.dx *= -1;
            rightPaddle.dy = 0;

            // move ball next to the paddle otherwise the collision will happen again
            // in the next frame
            ball.x = rightPaddle.x - ball.width;
        }

        // draw score
        context.font = "30px Arial";
        context.fillText(leftScore, 200, 50);
        context.fillText(rightScore, 560, 50);

        // draw ball
        context.fillRect(ball.x, ball.y, ball.width, ball.height);

        // draw walls
        context.fillStyle = "#f4dbd6";
        context.fillRect(0, 0, canvas.width, grid);
        context.fillRect(0, canvas.height - grid, canvas.width, canvas.height);

        // draw dotted line down the middle
        for (let i = grid; i < canvas.height - grid; i += grid * 2) {
            context.fillRect(canvas.width / 2 - grid / 2, i, grid, grid);
        }
}

// listen to keyboard events to move the paddles
document.addEventListener('keydown', function(e) {
    // up arrow key
    if (e.which === 38) {
        rightPaddle.dy = -paddleSpeed;
    }
    // down arrow key
    else if (e.which === 40) {
        rightPaddle.dy = paddleSpeed;
    }

    // w key
    if (e.which === 87) {
        leftPaddle.dy = -paddleSpeed;
    }
    // a key
    else if (e.which === 83) {
        leftPaddle.dy = paddleSpeed;
    }
});

// listen to keyboard events to stop the paddle if key is released
document.addEventListener('keyup', function(e) {
    if (e.which === 38 || e.which === 40) {
        rightPaddle.dy = 0;
    }
    if (e.which === 83 || e.which === 87) {
        leftPaddle.dy = 0;
    }
});

// start the game
requestAnimationFrame(loop);