Stick.js

update() {
  // calculate the distance between two dots
  let dx = this.endPoint.pos.x - this.startPoint.pos.x;
  let dy = this.endPoint.pos.y - this.startPoint.pos.y;
  // pythagoras theorem
  let dist = Math.sqrt(dx * dx + dy * dy);
  // calculate the resting distance betwen the dots
  let diff = (this.length - dist) / dist * this.stiffness;

  // getting the offset of the points
  let offsetx = dx * diff * 0.5;
  let offsety = dy * diff * 0.5;

  // calculate mass
  let m1 = this.startPoint.mass + this.endPoint.mass;
  let m2 = this.startPoint.mass / m1;
  m1 = this.endPoint.mass / m1;

  // and finally apply the offset with calculated mass
  if (!this.startPoint.pinned) {
    this.startPoint.pos.x -= offsetx * m1;
    this.startPoint.pos.y -= offsety * m1;
  }
  if (!this.endPoint.pinned) {
    this.endPoint.pos.x += offsetx * m2;
    this.endPoint.pos.y += offsety * m2;
  }

}