Physics.Spring = function(config) {
  Physics.Constraint.call(this, config);
};
Physics.Spring.prototype = Object.create(Physics.Constraint.prototype);
Physics.Spring.prototype.update = function() {
  this.springEffect();
};

if(!Physics.Spring) {
  Physics.Spring = function(config) {
    Physics.Constraint.call(this, config);
  };
  Physics.Spring.prototype = Object.create(Physics.Constraint.prototype);
  Physics.Spring.prototype.update = function() {
    this.springEffect();
  };
}

Update spring.js