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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