One Pager Cheat Sheet

• Using the built-in JavaScript methodreduce()you can create a single value from the elements in an array by running a user-supplied "reducer" callback function on each element, optionally passing an initial value first.
• We created a custom function, myReduce, that takes a callback function and an optional initialValue as arguments, and loops through the array elements by calling the callback with the adequate parameters, returning the final result value.

This is our final solution.

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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;

};

You're doing a wonderful job. Keep going!

If you had any problems with this tutorial, check out the main forum thread here.