flatten array

flatten.js

// loops through the array recursively and appends items until the 'current' list item is not an array
const flatsy = (arr, carry=[]) =>
   Array.isArray(arr) ? arr.reduce((acc, curr) =>
      Array.isArray(curr)
        ? flatsy(curr, acc)
        : [...acc, curr]
   ,
   carry
) : arr

const isEqual = (a,b) => {
    const length = a.length
    if(!length || a.length !== b.length) {
        return false
    }
    for(let i = 0; i <= length; i++) {
        if (a[i] !== b[i]){
            return false
        }
    }

    return true
}

// tests
const arr1 = [[1,2,[3]],4]
const des1 = [1, 2, 3, 4]
if (isEqual(des1, flatsy(arr1))){
  console.log('can flatten example array')
}

const arr2 = [[1,2,[3,[2,3,[3]]],4], [3,[2,5,23,8,1]]]
const des2 = [ 1, 2, 3, 2, 3, 3, 4, 3, 2, 5, 23, 8, 1 ]

if (isEqual(des2, flatsy(arr2))) {
  console.log('can handle deeply nested weirdy array')
}

const one = 1
if (flatsy(one) == one) {
  console.log('returns the input if not an array')
}