Fix: Use `for of` instead of `for in`

Hi, I'd like to take the oportunity here to clarify the difference between for ... in and for ... of on arrays. Hope not bothering you about that.

for ... in loops over the array using the "keys" of the array

const a = ['a', 'b', 'c'] 
for (let i in a) {
  console.log(i)
}
// 0, 1, 2

as long as you are not doing monkey-patching on Array (with is not recommended) all will work as expected.

for ... of loops differently using the "content" of the array

const a = ['a', 'b', 'c'] 
for (let i of a) {
  console.log(i)
}
// 'a', 'b', 'c'

Your changes will always returns undefined as

const a = ['a', 'b', 'c'] 
for (let i of a) {
  console.log(a[i])
}
// undefined, undefined, undefined

My recommendation is don't monkey-patch global objects as this will cause side-effects, try extending from those objects instead:

// extending with own class is the better option ...
class MyArray extends Array {
  get doubleLength () {
    return this.length * 2
  }
}

const better = new MyArray('a','B','c')
print('extends', better)

// ... than Monkey-patching which causes side-effects
Object.defineProperty(Array.prototype, 'doubleLength', {
  configurable: true,
  enumerable: true,
  get: function () {
    console.log(this)
    return this.length * 2
  }
})

const monkey = new Array('a','B','c')
print('monkey', monkey)

function print (name, a) {
  console.log('------', name)
  console.log(a.doubleLength, a)

  for (let i in a) {
    console.log(i)
  }

  for (let i of a) {
    console.log(i)
  }
}

I did the changes for you in the esm branch, but I really would recommend to look where you monkey-patch Array with some "custom" methods.

commenthol / date-holidays-parser

Fix: Use `for of` instead of `for in` #27