JSFiddle - React, Tailwind, and code Playground

JavaScript

const arr = [0]
arr[5] = undefined
arr[6] = null

let out = 'length: ' + arr.length

let cnt = 0
for (var i in arr) cnt++
out += '\nObject iter.: '+cnt

cnt = 0
for (var i of arr) cnt++
out += '\nArray iter.: '+cnt

arr.pop()
out += '\nNew length: '+arr.length

out += '\nArray: '+arr.toString()
alert(out)