How to check the type of a variable

Short answer

The typeof operator returns the type as a string:

console.log(typeof 42);
console.log(typeof 'hello');
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof (() => {}));

Output:

number
string
boolean
undefined
function

Special cases

For null and arrays typeof says object. Check them separately:

console.log(typeof null, typeof [1, 2]);
console.log(Array.isArray([1, 2]), [1, 2] === null);

Output:

object object
true false

Objects of classes

instanceof checks which class an object was created from:

console.log(new Date() instanceof Date);
console.log(new Error('oops') instanceof Error);
console.log([] instanceof Object);

Output:

true
true
true

A universal function

Object.prototype.toString gives a precise name for any value:

const typeOf = (value) => Object.prototype.toString.call(value).slice(8, -1);
 
console.log(typeOf(null), typeOf([]), typeOf(new Date()), typeOf(/a/), typeOf(new Map()));

Output:

Null Array Date RegExp Map

Common mistake

Checking typeof value === 'object' lets null through, and reading a property of it throws an error:

const isObject = (value) => typeof value === 'object' && value !== null && !Array.isArray(value);
 
console.log(typeof null === 'object');
console.log(isObject(null), isObject({}), isObject([]));

Output:

true
false true false

Thanks for reading!

Short answer

Special cases

Objects of classes

A universal function

Common mistake

Functions used in this recipe

Thanks for reading!