“Valeur par défaut de type NULL ou UNDEFINE” Réponses codées

Valeur par défaut de type NULL ou UNDEFINE

The value undefined means value is not assigned 
  & you don’t know its value.
  It is an unintentional absence of value.
  It means that a variable has been declared but has not yet
  been assigned a value.

The value null indicates that you know that the field does not
  have a value. It is an intentional absence of value.