Data Types

Seven primitives and one object type

console.log(typeof 'hello');
console.log(typeof 42);
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof 10n);
console.log(typeof Symbol('id'));

string
number
boolean
undefined
bigint
symbol

The difference that matters

A primitive is a single value, copied when assigned. An object is a reference, shared when assigned. That one difference explains a lot of confusing behaviour later.

let a = 1;
let b = a;
b += 1;

const first = { n: 1 };
const second = first;
second.n += 1;

console.log(a, b);
console.log(first.n, second.n);

1 2
2 2

Copying a number gave two independent values. Copying an object gave two names for one object.

null and undefined

Both mean absent, and the distinction is about who caused it. undefined is what JavaScript uses when nothing has been set. null is what you write to say deliberately empty.

let notSet;
const cleared = null;

console.log(notSet);
console.log(cleared);
console.log(notSet == cleared);
console.log(notSet === cleared);

undefined
null
true
false

Everything else is an object

Arrays, functions, dates and regular expressions are all objects underneath, which is why typeof is unhelpful for telling them apart.

console.log(typeof []);
console.log(typeof {});
console.log(typeof function () {});
console.log(Array.isArray([]));

object
object
function
true

Types are attached to values

A variable has no type. The value it currently holds does, and the same name can hold a different type a line later. That flexibility is convenient and is also why careful comparison matters.