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.
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