RasulCrose

var clamp = function(val, min, max) {
  return Math.min(Math.max(val, min), max);
}

// ...

// new position tank
this.pos = function () {
  this.x += this.stepX;
  this.y += this.stepY;

  this.x = clamp(this.x, 0, canvas.width);
  this.y = clamp(this.y, 0, canvas.height);
}