Returns the value of undefined.
The undefined property is a member of the Global object, and becomes available when the scripting engine is initialized. When a variable has been declared but not initialized, its value is undefined.
If a variable has not been declared, you cannot compare it to undefined, but you can compare the type of the variable to the string "undefined". Undeclared variables cannot be used in fast mode.
The undefined property is useful when explicitly testing or setting a variable to undefined.