"Implementing the Reduce Method in JavaScript: Line by Line Explanation"

Here's an implementation of the reduce method in JavaScript with a brief description for each line of code:

phpCopy codefunction reduce(array, callback, initialValue) {
  let accumulator = initialValue;

  // Check if accumulator is provided, if not use first item in array as accumulator
  if (accumulator === undefined) {
    accumulator = array[0];
    array = array.slice(1);
  }

  // Loop through the array and apply the callback
  for (let i = 0; i < array.length; i++) {
    accumulator = callback(accumulator, array[i], i, array);
  }

  return accumulator;
}

Line 1: Define the reduce function that takes in an array, a callback, and an optional initial value.

Line 2: Define a variable accumulator and initialize it with the initialValue argument.

Line 5: Check if the accumulator is undefined, and if so, set it to the first item in the array and slice the first item off the array.

Lines 8-11: Loop through the array and apply the callback to each item, updating the accumulator with the result of each iteration.

Line 14: Return the final value of the accumulator.