JavaScript: Difference Between null vs undefined

undefined is the value when a variable isn't assigned, property doesn't exist, value of function that doesn't have return statement, value of function args not given, out of bound array access, etc. See: undefined .

null is the value of a object's internal slot [[prototype]] when the object doesn't have a parent. [see Prototype and Inheritance]

JavaScript the language does not restrict how you use null or undefined.

In general, you can pass undefined as argument in function call, as if no argument is given. Such as in setUserData("John", undefined, 23). (But exact behavior of the function depends on the function.)

You can also use null as initial value of a variable.