while looking for a way to conditionally push something into an array, i discovered the following javascript black hole

jsarrayblackhole.js

var arr = [];

arr.push(1);
arr.push(arr);
arr.push(3);

arr.forEach(function (item, i) {
  console.log(i, item);
});

// 0 1
// 2 3
// will not iterate the self-referential member -- like it's not even there!

console.log(arr.join(';'));
// "1;;3"

console.log(arr.toString());
// "1,,3"


// but will return an empty string when joined or toString'd

Saw the same results except for the arr.forEach statement, which for me (in Chrome 7 on Mac) returned:
0 1
1 [1, >Array[3], 3]
2 3

and then I tried arr.forEach(function(item, i) { console.log(i,item.toString());}), which yielded:
0 "1"
1 "1,,3"
2 "3"

It's OK with JS (by the spec and in SM implementation) -- all elements are visited correctly; only non-existing elements shouldn't be visited. It's a bug of the console object. Replace console.log with alert for check.

Dmitry.