Pure JavaScript

undefined

JavaScript 1.3+, ECMAScript 1.0+, JScript 5.0+ Nav4.06+, IE5+ Syntax

undefined

Description

The undefined property is a primitive value of the global object. It is returned by variables that have not had values assigned to them. It is also returned by methods if the variable being evaluated is not assigned a value. Browsers not supporting this property return null on the undefined variables.

Note

ECMAScript 1.0 only defines undefined as a primitive value, and does not define it as a property of the global object.

Example

Listing 6.268 creates the variable myVariable, and then checks to see if it is undefined in an if clause. Note that Opera 3 and later browsers, even though they do not officially support this property, return ...