A complete guide to JavaScript typechecking

JavaScript's dynamic typing is one of its most powerful features. Nonetheless, it's often the source of issues and confusion, especially for inexperienced developers. Thus, typechecking is a crucial skill for any JavaScript developer.

Value types

Before we dive into the different ways to typecheck values in JavaScript, let's take a look at the different types of values we can encounter in JavaScript.

Apart from the last two types, all other types are considered primitive types. Primitive types are immutable, meaning that their values cannot be changed. Objects and functions, on the other hand, are mutable, meaning that their values can be changed.

Typechecking primitives

Primitives are generally easier to typecheck than objects and functions. This is because primitives are immutable, meaning that their values cannot be changed. Thus, we can simply compare the type of a value to the type we want to check for. This is where the typeof operator comes in handy.

Value is undefined

Checking for an undefined value is as simple as comparing the value to undefined. This yields the exact same result as using the typeof operator.

const isUndefined = val => val === undefined;

isUndefined(undefined); // true

Value is null

Checking for a null value can only be done by comparing the value to null itself. This is because typeof null returns 'object', which is not what we want.

const isNull = val => val === null;

isNull(null); // true

Value is nil

In some other languages, a value of nil is used to represent the absence of a value. In JavaScript, however, null and undefined are used for this purpose. Thus, checking for a nil value is the same as checking for a null or undefined value.

const isNil = val => val === undefined || val === null;

isNil(null); // true
isNil(undefined); // true
isNil(''); // false

Value is boolean

Boolean values are either true or false. Checking for either value is relatively inefficient, so typeof is generally preferred.

const isBoolean = val => typeof val === 'boolean';

isBoolean(true); // true
isBoolean(false); // true
isBoolean('true'); // false
isBoolean(null); // false

Value is number

Numbers can also be typechecked using typeof. However, this will also return true for NaN, which is a special numeric value that represents the result of an operation that cannot produce a normal result.

You can read more about NaN and why it can be tricky to work with, but using Number.isNaN() to add an additional check for NaN is generally a good idea.

const isNumber = val => typeof val === 'number' && !Number.isNaN(val);

isNumber(1); // true
isNumber('1'); // false
isNumber(NaN); // false

Value is bigint

BigInts are a relatively new addition to JavaScript. They are integers with arbitrary precision. BigInts are typechecked using typeof.

const isBigInt = val => typeof val === 'bigint';

isBigInt(1n); // true
isBigInt(1); // false

Value is string

String primitives, like most other primitives, can be typechecked using typeof.

const isString = val => typeof val === 'string';

isString('Hello!'); // true
isString(1); // false

Value is symbol

Symbols are unique values that can be used as the key of an object property. They are also typechecked using typeof.

const isSymbol = val => typeof val === 'symbol';

isSymbol(Symbol('x')); // true
isSymbol('x'); // false

Value is primitive

Checking if a value is of any primitive type is a bit more tricky. We can't simply use typeof for this, as it won't work for null.

Instead, we can create an object from the value and compare it with the value itself. If the value is primitive, the object will not be equal to the value.

const isPrimitive = val => Object(val) !== val;

isPrimitive(null); // true
isPrimitive(undefined); // true
isPrimitive(50); // true
isPrimitive('Hello!'); // true
isPrimitive(false); // true
isPrimitive(Symbol()); // true
isPrimitive([]); // false
isPrimitive({}); // false

Typechecking non-primitives

Objects and functions behave slightly differently than primitives. While typeof can get us part of the way there, we might want to employ some other methods, especially for objects.

Value is object

As we've seen before typeof returns 'object' for null. This is not what we want, when typechecking for objects. Instead, we can use the Object constructor to create an object wrapper for the given value. If the value is null or undefined, the returned value will be an empty object. Otherwise, the returned object will be the same as the input object.

const isObject = obj => obj === Object(obj);

isObject([1, 2, 3, 4]); // true
isObject([]); // true
isObject(['Hello!']); // true
isObject({ a: 1 }); // true
isObject({}); // true
isObject(true); // false
isObject(null); // false
isObject(undefined); // false

Value is function

Luckily, functions are not as tricky as objects. We can simply use typeof to check if a value is a function.

const isFunction = val => typeof val === 'function';

isFunction(x => x); // true
isFunction('x'); // false

Value is plain object

An object is considered plain when it is created by the Object constructor. This is in comparison to other objects like arrays, functions, or instances of classes, such as Map.

In order to check if a value is a plain object, we need to make sure that the value is truthy (not null or undefined), that it is an object, and that its constructor is equal to Object, using Object.prototype.constructor.

const isPlainObject = val =>
  !!val && typeof val === 'object' && val.constructor === Object;

isPlainObject({ a: 1 }); // true
isPlainObject(new Map()); // false

Value is async function

A function declared with the async keyword is considered asynchronous. Typechecking for async functions requires the use of Object.prototype.toString() and Function.prototype.call() to check if the result is '[object AsyncFunction]'.

const isAsyncFunction = val =>
  Object.prototype.toString.call(val) === '[object AsyncFunction]';

isAsyncFunction(function() {}); // false
isAsyncFunction(async function() {}); // true

Value is generator function

Generator functions can be typechecked in the same manner as async functions. The expected value for them is '[object GeneratorFunction]'.

const isGeneratorFunction = val =>
  Object.prototype.toString.call(val) === '[object GeneratorFunction]';

isGeneratorFunction(function() {}); // false
isGeneratorFunction(function*() {}); // true

Type of value

If all else fails, or if you are working with classes and other custom types, you might want to get a string representation of the type of a value. This can be done using Object.prototype.constructor and Function.prototype.name.

const getType = v =>
  v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name;

getType(undefined); // 'undefined'
getType(null); // 'null'
getType(true); // 'Boolean'
getType(1); // 'Number'
getType(1n); // 'BigInt'
getType('Hello!'); // 'String'
getType(Symbol()); // 'Symbol'
getType([]); // 'Array'
getType({}); // 'Object'
getType(() => {}); // 'Function'
getType(new Set([1, 2, 3])); // 'Set'

Check if value is of type

Flipping the previous snippet around, we can also check if a value is of a specific type. Same as before, special care needs to be taken for undefined and null, as the do not have a constructor property.

const isOfType = (type, val) =>
  ([undefined, null].includes(val) && val === type) ||
  v.constructor.name === type;

isOfType(undefined, undefined); // true
isOfType(null, null); // true
isOfType('Boolean', true); // true
isOfType('Number', 1); // true
isOfType('BigInt', 1n); // true
isOfType('String', 'Hello!'); // true
isOfType('Symbol', Symbol()); // true
isOfType('Array', []); // true
isOfType('Object', {}); // true
isOfType('Function', () => {}); // true
isOfType('Set', new Set([1, 2, 3])); // true