Distinguish empty element and `undefined` valued one in JavaScript array

gistfile1.md

Real undefined valued element

Way 1: in operator, and for...in loop

Due to indexes are not even defined for empty element, in operator return false.

var arr = new Array(5);
console.log(1 in arr); // false

arr[1] = undefined;
console.log(1 in arr); // true

for (var i in arr) {
  console.log(i);
}
// 1

Tested in Chrome 25.

Way 2: forEach, filter, every, some etc. iterator method.

These methods do not invoke the function for indexes at which no element actually exists.

var arr = new Array(5);
arr.forEach(function () { console.log('Never log this') });
arr[1] = undefined;
arr.forEach(function () { console.log('Only log 1') });