The JavaScript documentation states:

Iterates a specified variable over all the properties of an object, in arbitrary order. For each distinct property, the specified statement is executed.

… and

Although it may be tempting to use this as a way to iterate over Array elements, because the for…in statement iterates over user-defined properties in addition to the array elements, if you modify the Array object, such as adding custom properties or methods, the for…in statement will return the name of your user-defined properties in addition to the numeric indexes. Thus it is better to use a traditional for loop with a numeric index when iterating over arrays.

I made a simple example to show the difference between iterating over Arrays and Objects. This is with Mootols included on the page.

JavaScript

console.log('===== Array Example ===== ');

// Array
var a = ['alpha', 'bravo', 'charlie'];
console.log('Creating a new Array:', a);
console.log('The array length is:', a.length);

// For
console.log('--- Start For --- ');
for (var i = 0, j = a.length; i < j; i++) {
    console.log(i, ':', a[i]);
}
console.log('--- End For --- ');

// For-in
console.log('--- Start For-in --- ');
for (var ii in a) {
    console.log(ii, ':', a[ii]);
}
console.log('--- End For-in --- ');

Output

===== Array Example =====
Creating a new Array: ["alpha", "bravo", "charlie"]
The array length is: 3
--- Start For ---
0 : alpha
1 : bravo
2 : charlie
--- End For ---
--- Start For-in ---
0 : alpha
1 : bravo
2 : charlie
$family : Object name=array
each : forEach()
clean : function()
associate : function()
link : function()
contains : function()
extend : function()
getLast : function()
getRandom : function()
include : function()
combine : function()
erase : function()
empty : function()
flatten : function()
hexToRgb : function()
rgbToHex : function()
toJSON : function()
--- End For-in ---

And here using an object.

JavaScript

console.log('===== Object Example =====');

// Object
var o = { 'one': 'alpha', 'Bravo': 'bravo', 'three': 'charlie' };
console.log('Creating a new Array:', o);

// For-in
console.log('--- Start For-in --- ');
for (var item in o) {
    console.log(item, ':', o[item]);
}
console.log('--- End For-in --- ');

Output

===== Object Example =====
Creating a new Array: Object one=alpha Bravo=bravo three=charlie
--- Start For-in ---
one : alpha
Bravo : bravo
three : charlie
--- End For-in ---

Read more about the for…in operation at MDC Core JavaScript 1.5 Guide: Object Manipulation.

Unfortunately this stupid mistake prevents us from using Mootools on this site. And we might have to reconsider the use of Mootols on more sites in the future — AND THAT SUCKS!