array methods polyfill

array.js

Array.prototype.forEach1 = function(fn) {
  const arr = this
  for (let i = 0; i < arr.length; i++) {
    fn.call(this, arr[i], i, arr)
  }
}
Array.prototype.map1 = function(fn) {
  const arr = [...this]
  for (let i = 0; i < arr.length; i++) {
    arr[i] = fn.call(this, arr[i], i, arr)
  }
  return arr
}
Array.prototype.reduce1 = function(fn) {
  const initialValue = arguments.length === 2 && arguments[1]
  const hasInitialValue = typeof initialValue !== 'undefined'
  const arr = this
  let acc, cur

  if (hasInitialValue) {
    for (let i = 0; i < arr.length; i++) {
      if (i === 0) {
        acc = initialValue
      } 
      cur = arr[i]
      acc = fn.call(this, acc, cur, i, arr)
    }
  } else {
    for (let i = 0; i < arr.length - 1; i++) {
      if (i === 0) {
        acc = arr[i]
      }
      cur = arr[i + 1]
      acc = fn.call(this, acc, cur, i, arr)
    }
  }
  
  return acc
}

var arr = [1,2,3]
// arr.forEach1((e, i ,a) => console.log(e, i, a))
// arr.map1(i => i*2)
console.log(arr.reduce1((acc, cur) => acc+cur))
console.log(arr.reduce1((acc, cur) => acc+cur, 4))