Elements of array literal expressions that are omitted will be initialized to undefined. However, such omitted elements are easy to miss and make code hard to read. They should be avoided.

Explicitly initialize the array elements to undefined. If you want to create an array with a specific number of uninitialized elements, use the Array constructor instead.

The following code snippet initializes a to an array of size ten by using omitted elements.

This code is clearly hard to read, and should be replaced by the following snippet:

D. Crockford, JavaScript: The Good Parts, Appendix C.6. O'Reilly, 2008.