[zlodiak]

а вот так получше стало? что ещё из явных косяков можно доработать?

Tank = function(id){ 
  this.x_coord = helper.randomIntFromZero(481);
  this.y_coord = helper.randomIntFromZero(481);
  this.id = id; 
  this.arrow; 
  this.direction;
  this.DIRECTION = ['up', 'right', 'bottom', 'left'];

  this.Create();
}

Tank.prototype.Create = function(){
  this.direction = this.DIRECTION[helper.randomIntFromZero(4)];

  var tank = $('<div class="tank" id="' + this.id + '"></div>').css({
    left: this.x_coord + 'px',
    top: this.y_coord + 'px'
  });

  $('#board').append(tank);
}

Tank.prototype.checkArrowDirection = function(){
  switch (this.direction) {
   case 'up':
      this.arrow = '▲';
      break
   case 'right':
      this.arrow = '►';
      break
   case 'bottom':
      this.arrow = '▼';
      break
   case 'left':
      this.arrow = '◄';
      break         
   default:
      console.log('error arrow direction');
      break
  }
}

Tank.prototype.checkBorderCollision = function(){
  switch (this.direction) {
   case 'up':
      this.y_coord -= 10;
      if(this.y_coord <= 0) this.y_coord = 0; 
      break
   case 'right':
      this.x_coord += 10;
      if(this.x_coord >= 480) this.x_coord = 480;
      break
   case 'bottom':
      this.y_coord += 10;
      if(this.y_coord >= 480) this.y_coord = 480;
      break
   case 'left':
      this.x_coord -= 10;
      if(this.x_coord <= 0) this.x_coord = 0;
      break         
   default:
      console.log('error direction definition');
      break
  }
}

Tank.prototype.checkChangeDirection = function(){
  if(helper.randomIntFromZero(100) > 75){
    this.direction = this.DIRECTION[helper.randomIntFromZero(4)];
  }
}

Tank.prototype.Offset = function(){
  $('#' + this.id).css({
    left: this.x_coord + 'px',
    top: this.y_coord + 'px'
  }).html(this.arrow);
}

Tank.prototype.Move = function(){
  this.checkChangeDirection();
  this.checkBorderCollision();
  this.checkArrowDirection();
  this.Offset();
}