The topics are:

The undefined value is a special value used in some places to specify an absence of information. For example, accessing a property of a value which is not defined, or a local variable which has been declared but not initialized, yields the undefined value.

There is no way of referencing the undefined value in programs. Checking if a value is the undefined value can be done using the typeof operator:

typeof(value) == "undefined" −> true if value is undefined, false otherwise.

The only method of undefined is: