constructor(x, y, angle, length, child) {

    this.x = x;

    this.y = y;

    this.length = length;

    this.angle = angle;

    this.child = child;

  }

​

  draw() {

    push();

    translate(this.x, this.y);

    rotate(this.angle);

    strokeWeight(this.length / 5);

    colorMode(HSB);

    let clr = color(map(abs(this.angle % (2*PI)), 0, 2*PI, 0, 255), 255, 255);

    colorMode(RGB);

    stroke(clr);

    line(0, 0, this.length, 0);

    this.child && this.child.draw();

    pop();

  }

​

  // takes in: a target point in the parent coordinate space

  // returns:  the endpoint of the chain, in that same parent

  //           coordinate space

  updateIK(target) {

    // convert from parent to local coordinates

    const localTarget = rotatePoint(translatePoint(target, -this.x, -this.y), -this.angle);

​

    let endPoint;

    if (this.child) {

      endPoint = this.child.updateIK(localTarget);

    } else {

      // base case:  the end point is the end of the current bone

      endPoint = [this.length, 0];

    }

​

    // point towards the endpoint

    const shiftAngle = angle(localTarget) - angle(endPoint);

    this.angle += shiftAngle;

​

    // convert back to parent coordinate space

    return translatePoint(rotatePoint(endPoint, this.angle), this.x, this.y);

  }

}