Name
undefined — the undefined value
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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