enemy.js

class Enemy {
  constructor(x, y) {
    this.x = x;
    this.y = y;
    this.size = 30;
    this.speed = 1;
  }

  update() {
    this.move();
    this.draw();
  }

  move() {
    this.y += this.speed;
  }

  draw() {
    ctx.beginPath();
    ctx.arc(this.x, this.y, this.size, 0, Math.PI * 2);
    ctx.fillStyle = 'red';
    ctx.fill();
  }
}