Cuál es la diferencia en JavaScript entre una variable undefined y una variable null? cómo puedo saber si una variable está null, undefined o ambos? también quisiera saber si son lo mismo?
To put it in other words, every NULL in an SQL expression can be treated as a distinct mathematical variable. So an expression NULL = NULL should be treated as x = y, where x and y are unbound variables. Now if someone asks you, what is the value of x = y? The only reasonable answer is, "some z ". So we have (x = y) = z - or, transcribing it back to SQL, (NULL = NULL) = NULL.
The rules for == and != explicitly include a clause that stipulates that null and undefined are the same. Personally — that is, in my code — that fact is a reason for using != (or ==) when checking for null in cases where undefined should be treated the same way (which is a pretty common situation).
The main difference between e != null and e is not null is the way the the compiler executes the comparison. Microsoft: "The compiler guarantees that no user-overloaded equality operator == is invoked when expression x is null is evaluated." Bottom Line: If you are writing code that you don't want to depend on someone's implementation of the != and == operators, use is null and is not null ...
It can be used on a type to control Nullability, it is then called the "Null Forgiving Operator". Basically, null! applies the ! operator to the value null. This overrides the nullability of the value null to non-nullable, telling the compiler that null is a "non-null" type.