Array.prototype.myReduce = function (callback, initialValue) {

  const argsLength = arguments.length;

  //If array is empty and there is no initial value, return an error

  if (argsLength === 1 && this.length === 0) {

    throw new Error();

  }

​

  let index = argsLength === 1 ? 1 : 0;

  let resultValue = argsLength === 1 ? this[0] : initialValue;

​

  //Call the callback function for every element and replace the resultValue

  for (let i = index; i < this.length; i += 1) {

    resultValue = callback(resultValue, this[i], i, this);

  }

​

  return resultValue;

};

​