AI solution for JavaScript Coding Question on BFE.dev
146. implement Array.prototype.reduce()

Array.prototype.myReduce = function(callback, initialValue) {
  let accumulator = initialValue === undefined ? undefined : initialValue;
  for(let i = 0; i < this.length; i++) {
    if(accumulator !== undefined) {
      accumulator = callback.call(undefined, accumulator, this[i], i, this);
    } else {
      accumulator = this[i];
    }
  }
  if(accumulator === undefined) {
      throw new TypeError('Reduce of empty array with no initial value');
  }
  return accumulator;
}

Explanation:

• First, we add a new method named myReduce to the existing Array.prototype method.
• myReduce accepts a callback function as its first parameter and an optional initialValue as its second parameter (it is undefined by default).
• Initialize the variable accumulator with initialValue or undefined, depending on whether an initialValue is passed or not.
• Loop through the array using a for loop.
• If the accumulator is not undefined, we call the callback function and pass in the current value this[i] as the second parameter, the current index i as the third parameter, and the original array this as the fourth parameter. We set the accumulator to the returned value.
• If the accumulator is undefined, we set it to this[i].
• If the accumulator remains undefined after the loop, we throw an error.
• Finally, we return the accumulator.