reduce.js

const reduce = (array, fn, initialValue) => {
  let accumlator
  if (initialValue != undefined) {
    accumlator = initialValue
  } else {
    accumlator = array[0]
  }
  if (initialValue === undefined) {
    for (let i = 1; i < array.length; i++) {
      accumlator = fn(accumlator, array[i])
    }
  } else {
    for (const value of array) accumlator = fn(accumlator, value)
  }
  return [accumlator]
}