August 2006
Intermediate to advanced
1018 pages
34h 13m
English
undefined: the undefined value — ECMAScript v3
undefined
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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