spaceinvaders.js

  function Bullet() {
    this.x = null;
    this.y = spaceship.y - 5;
    this.speed = 4;
    this.length = 8;
    this.prototype.draw = function(){
      ctx.beginPath();
      ctx.moveTo(this.x, this.y);
      ctx.lineTo(this.x, this.y + this.length);
      ctx.stroke();
      this.y -= this.length;
    };
  };