The value undefined.

Core property

Syntax

Description

undefined is a top-level property and is not associated with any object.

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value.

You can use undefined to determine whether a variable has a value. In the following code, the variable x is not defined, and the if statement evaluates to true.

undefined is also a primitive value.