JavaScript undefined vs null

In JavaScript both undefined and null primitive types.

let foo;

console.log(foo);

// undefined

A variable is undefined when it has been declared but not assigned a value.

let bar = null;

console.log(bar);

// null

For a variable to be null it must be directly assigned the value.

You can check for equality for both primitives using === like so:

let foo;
let bar = null;

console.log(foo === undefined);

//true

console.log(bar === null);

//true

And as they are both falsy values they will be false when coalesced to a boolean.

let foo;
if (!foo) {
  console.log('foo is False');
}

// Foo is False

let bar = null;
if (!bar) {
  console.log('bar is False');
}

// bar is False

However there's some slightly unexpected behaviour when we use the typeof operator.

let foo;
typeof foo;

// undefined

let bar = null;
typeof bar;

// object

Despite being a primitive type null evaluates to an object.