https://stackoverflow.com/questions/801032/why-is-null-an-object-and-whats-the-difference-between-null-and-undefined

The difference between null and undefined is as follows:

undefined

undefined: used by JavaScript and means “no value”.

– Uninitialized variables
– missing parameters
– unknown variables have that value.

> var noValueYet;
> console.log(noValueYet);
undefined

> function foo(x) { console.log(x) }
> foo()
undefined

> var obj = {};
> console.log(obj.unknownProperty)
undefined

null

null: used by programmers to indicate “no value”, e.g. as a parameter to a function.
Examining a variable:

console.log(typeof unknownVariable === “undefined”); // true

var foo;
console.log(typeof foo === “undefined”); // true
console.log(foo === undefined); // true

var bar = null;
console.log(bar === null); // true