typeof

Asking the question

typeof returns a string naming the type of a value. It takes one operand and needs no brackets, though they do no harm.

console.log(typeof 'text');
console.log(typeof 3.14);
console.log(typeof false);
console.log(typeof {});

string
number
boolean
object

The three answers that mislead

Three results are wrong, or at least unhelpful, and each has caught out a lot of people.

console.log(typeof null);
console.log(typeof []);
console.log(typeof NaN);

object
object
number

• typeof null is "object". This is a bug from 1995 that can never be fixed without breaking the web.
• typeof [] is "object", because an array is one. Use Array.isArray instead.
• typeof NaN is "number", which is technically correct and rarely what you wanted to know.

Checking properly

const value = null;

console.log(value === null);
console.log(Array.isArray([1, 2]));
console.log(Number.isNaN(NaN));
console.log(Number.isInteger(4.0));

true
true
true
true

It is safe on names that do not exist

typeof is the one operator that does not throw for an undeclared name, which is why it turns up in feature checks.

console.log(typeof neverDeclared);

if (typeof fetch === 'function') {
  console.log('fetch is available');
}

undefined
fetch is available