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.keyCode == 39) {
    rightPressed = true;
  }
  else if(e.keyCode == 37) {
    leftPressed = true;
  }
}
function keyUpHandler(e) {
  if(e.keyCode == 39) {
    rightPressed = false;
  }
  else if(e.keyCode == 37) {
    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);

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.keyCode == 39) {
    rightPressed = true;
  }
  else if(e.keyCode == 37) {
    leftPressed = true;
  }
}
function keyUpHandler(e) {
  if(e.keyCode == 39) {
    rightPressed = false;
  }
  else if(e.keyCode == 37) {
    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 < ballRadius) {
    dy = -dy;
  } else if (y + dy > canvas.height-ballRadius) {
    if(x > paddleX && x < paddleX + paddleWidth) {
      dy = -dy;
    } else {
      alert("GAME OVER");
      document.location.reload();
    }
  }

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

  x += dx;
  y += dy;
}

setInterval(draw, 10);

Add ability to lose