Body animation isn't smooth

// Im' assuming most of visitors here have recent browsers, so I removed the rAF polyfill for readibility

// If you wrap it after the canvas element decalaration, you can already populate this variable, it will avoid that you make a lot of calls to document.getElementById()
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
// ship data
var shipPositionX = canvas.width / 2;
// Just for the snippet height
var shipPositionY = 0;
var deltaShipPositionX = 10;
var deltaShipPositionY = 10;

//Removed the init() function, since our elements are loaded.


function draw() {
  clear();
  createRectangleToCoverCanvas();
  createSpaceShip(shipPositionX, shipPositionY, 10);
}

function clear() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
}

function createRectangleToCoverCanvas() {
  ctx.fillStyle = 'black';
  ctx.strokeStyle = 'black';
  ctx.beginPath();
  ctx.rect(0, 0, canvas.width, canvas.height);
  ctx.fill();
  ctx.stroke();
}

function createSpaceShip(x, y, radius) {
  ctx.fillStyle = 'white'
  ctx.strokeStyle = 'white'
  ctx.beginPath();
  ctx.rect(x, y, 20, 20);
  ctx.fill();
  ctx.stroke();
}
// instantiate a variable that will store our animationFrame id, so we can cancel it further
var raf, 
// the direction object, with an x and y values
    direction = {
      x: 0,
      y: 0
    };
// we can set a speed variable
var speed = 2.5;
function triggerMoveSpaceShip(event) {
  switch (event.keyCode) {
    // left
    case 37:
      // update the direction object
      direction.x = -speed;
      // avoid the scroll in the snippet
      event.preventDefault();
      break;

      // up
    case 38:
      direction.y = -speed;
      event.preventDefault();
      break;

      // right
    case 39:
      direction.x = speed;
      event.preventDefault();
      break;

      //down
    case 40:
      direction.y = speed;
      event.preventDefault();
      break;
  }
  // if we haven't initiated the animation yet, and that our direction is not 0, then do it now
  if (!raf && (direction.x || direction.y)) moveSpaceShip();
}

function releaseMoveSpaceShip(event) {;
  switch (event.keyCode) {
    // left
    case 37:
      //reset this direction
      direction.x = 0;
      break;

      // up
    case 38:
      direction.y = 0;
      break;

      // right
    case 39:
      direction.x = 0;
      break;

      //down
    case 40:
      direction.y = 0;
      break;
  }
  if (!direction.x && !direction.y) {
    // if none of the directions is set, stop the animation
    cancelAnimationFrame(raf);
    raf = undefined;
  }
}

function moveSpaceShip() {
  // declare our animation function
  var move = function() {
    // update the positions without going out of the screen
    // Sorry, this is dirty...
    if(direction.x){
    	if(
	    	(shipPositionX > 0 && shipPositionX < canvas.width-20) ||
			(shipPositionX <= 0 && direction.x > 0) ||
			(shipPositionX >= canvas.width-20 && direction.x < 0))
				shipPositionX += direction.x;
		}
	if(direction.y){
    	if(
	    	(shipPositionY > 0 && shipPositionY < canvas.height-20) ||
			(shipPositionY <= 0 && direction.y > 0) ||
			(shipPositionY >= canvas.width-20 && direction.y < 0))
				shipPositionY += direction.y;
		}

    // finally draw ou ship
    draw();
    // update our raf id
    raf = requestAnimationFrame(move);
  };
  // let's go !
  raf = requestAnimationFrame(move);
}


draw();

window.addEventListener('keydown', triggerMoveSpaceShip, true);
window.addEventListener('keyup', releaseMoveSpaceShip, true);

canvas {
  margin: auto;
  position: absolute;
  left: 0;
  top: 0;
  right: 0;
  bottom: 0;
  border: 1px solid black;
}
body{
   overflow: none;
  }

<canvas id="canvas" width="400" height="200"></canvas>