Why does typeof null return "object"?

typeof null is "object" because of a historical mistake that was never fixed. MDN's account of the first JavaScript implementation: values were a type tag plus a value, the tag for objects was 0, and null was the NULL pointer (0x00 on most platforms), so null carried the object tag. A change to make typeof null === "null" was proposed for ECMAScript as an opt-in, but was rejected.

Today it is simply specified behaviour. The typeof algorithm tests undefined first, then explicitly says "if value is null, return "object"", and only afterwards looks at strings, symbols, booleans, numbers and BigInts.

null is still a primitive: the Null type has exactly one value. It doesn't behave like an object. Reading a property throws a TypeError, null instanceof Object is false, and it has no wrapper.

Consequences for real code:

• typeof value === 'object' is true for null, arrays, dates and plain objects alike. To test for "a non-null object", write value !== null && typeof value === 'object'.
• To test for null itself, use value === null. Don't use typeof.
• null as the end of the prototype chain is the one place the language itself produces it: Object.getPrototypeOf(Object.prototype) is null.

console.log(typeof null, typeof undefined, typeof {}, typeof []);
// → object undefined object object

console.log(null instanceof Object, null === null); // → false true

try {
  null.x;
} catch (error) {
  console.log(error.name); // → TypeError
}

const isObjectLike = (value) => value !== null && typeof value === 'object';
console.log(isObjectLike(null), isObjectLike({}), isObjectLike(() => {}));
// → false true false

console.log(Object.getPrototypeOf(Object.prototype)); // → null

• Explaining it as "null is an object". It's a primitive; only the typeof result is historical.
• Guarding with typeof x === 'object' and then reading x.prop. null passes the guard and throws.
• Forgetting that typeof of a function is "function", so isObjectLike above is false for functions. Add || typeof value === 'function' if you need them.
• Expecting typeof of an undeclared variable to throw. It returns "undefined", but a let or const still in its temporal dead zone throws a ReferenceError.