Two types of undefined

undefined.js

new Array(10).length
// evaluates to `10`

new Array(10).map(function(current, i) { return i });
// evaluates to `[]`

var arr = [];
arr[2] = undefined;
arr[9] = undefined;

console.log(arr);
// logs `[undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined]

console.log(arr.map(function(current, i) { return i }));
// logs `[undefined, undefined, 2, undefined, undefined, undefined, undefined, undefined, undefined, 9]`

arr.map(function(current, i) { return i }).forEach(function(current, i) { console.log(i) });
// logs `2`
// logs `9`

for (var i in arr) console.log(i);
// logs `2`
// logs `9`

for (var i = 0; i < arr.length; ++i) console.log(arr[i]);
// logs 10*`undefined`

/*
 * Any guesses on how to tell the difference between `undefined` and, err, `undefined`?
 */