Definition and Usage. The undefined property indicates that a variable has not been assigned a value.
What is the best way to check if a property or variable is
While it is possible to use it as an identifier (variable name) in any scope other than the global scope (because undefined is not a reserved word), doing so is a
All variables by default have a null value which is equal to undefined, false, 0 and of course null, to ensure a variable has a true value meaning something
Check if a variable is undefined or null - CodeCodex