undefined

The undefined global property represents the primitive value undefined. It is one of JavaScript’s primitive types.

undefined is a property of the global object. That is, it is a variable in global scope.

In all non-legacy browsers, undefined is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it.

A variable that has not been assigned a value is of type undefined. A function returns undefined if a value was not returned. Accessing a property that does not exist also returns undefined. The void operator always returns undefined.

Note: While you can use undefined as an identifier (variable name) in any scope other than the global scope (because undefined is not a reserved word), doing so is a very bad idea that will make your code difficult to maintain and debug.