Skip to Main Content
JavaScript: The Definitive Guide, Fourth Edition
book

JavaScript: The Definitive Guide, Fourth Edition

by David Flanagan
November 2001
Intermediate to advanced content levelIntermediate to advanced
936 pages
68h 43m
English
O'Reilly Media, Inc.
Content preview from JavaScript: The Definitive Guide, Fourth Edition

Name

undefined — the undefined value

Availability

JavaScript 1.5; JScript 5.5; ECMAScript v3

Synopsis

undefined

Description

undefined is a global property that holds the JavaScript undefined value. This is the same value that is returned when you attempt to read the value of a nonexistent object property. The undefined property is not enumerated by for/in loops and cannot be deleted with the delete operator. Note that undefined is not a constant and can be set to any other value, something that you should take care not to do.

When testing a value to see whether it is undefined, use the === operator, because the == operator treats the undefined value as equal to null.

Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Start your free trial

You might also like

JavaScript: A Beginner's Guide, Fourth Edition, 4th Edition

JavaScript: A Beginner's Guide, Fourth Edition, 4th Edition

John Pollock
Coding with JavaScript For Dummies

Coding with JavaScript For Dummies

Chris Minnick, Eva Holland

Publisher Resources

ISBN: 0596000480Supplemental ContentCatalog PageErrata