fillCircle(x, y, radius, fillStyle) {
    this._ctx.beginPath();
    this._ctx.fillStyle = fillStyle;
    this._ctx.arc(x, y, radius, 0, 2 * Math.PI);
    this._ctx.fill();
    this._ctx.closePath();
  }
Create a filled circle
@param {number} x
@param {number} y
@param {number} radius
@param {string} fillStyle