a more comprehensive bind() that covers prototype chain and constructor function

bindUltimate.js

/**
 * Here comes the hardest parts. Have you considered if the function you are
 * going to bind is a constructor function.In this scenario, the this value you
 * bind should get ignored.
 *
 * @param {*} context
 */
Function.prototype.bindUltimate = function(context) {
  if (typeof this !== 'function') {
    throw new Error('bind is only invoked on functions');
  }

  let fn = this;
  let args = [...arguments].slice(1);

  let resultFn = function() {
    let bindArgs = [...args];
    /**
     *  Here is tricky part:
     * 1. if the fn is an constructor,this points to the instance, fn
     * points to the function to be bound.However, we explicitly set the
     * prototype of resultFn.prototype to the fn.prototype. Therefore, it would
     * be true and the this is set to the instance.
     * 2. if fn is not an constructor function. this points to the global
     * object, fn points to the function to be bound. So the this set to the
     * context passed in
     */
    fn.apply(this instanceof fn ? this : context, args.concat(bindArgs));
  };
  //dealing with the prototype issues, we uses a bus function to link to the chain
  function bus() {}
  bus.prototype = this.prototype;
  resultFn.prototype = new bus();
  return resultFn;
};