JavaScript Reduce Concept.js

const _ = {};

_.reduce = (list, callback, initialValue) => {
  let storage = initialValue; // 0 => 1 => 3 => 6 => 10

  for (let i = 0; i < list.length; i++) { // 0 => 1 => 2 => 3

    if (i === 0 && storage === undefined) {
        storage = list[0];
    } else {
        storage = callback(list[i], storage); // (1 + 0) => (2 + 1) => (3 + 3) => (4 + 6)
    }
      
  }

  return storage; // 10
}

_.reduce([1,2,3,4], (value, sum) => value + sum, 0);