Commas in arrays

The specification of Javascript allows for elision - the omission of element values - when initializing an array. While this behavior has a history of browser-specific bugs, it is a useful way to increase array length when used carefully.

The empty space creates a missing element. Notice that an empty space is not created for a trailing comma.

The missing element is not store a null or undefined value - it simply isn't there.

The missing space is referred to as a hole. A hole is equivalent to undefined in comparison, not null.

The in operator will report false when checking for the index of a hole. In this way, a hole is functionally similar to an element that is out of bounds.

Even assigning the value undefined to the array will fill the hole.