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is What? 🙉

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Very simple & small JS type check functions. It’s fully TypeScript supported!

npm i is-what

Or for deno available at: ""

Also check out is-where 🙈


I built is-what because the existing solutions were all too complex or too poorly built.

I was looking for:

  • A simple way to check any kind of type (including non-primitives)
  • Be able to check if an object is a plain object {} or a special object (like a class instance) ‼️
  • Let TypeScript automatically know what type a value is when checking

And that’s exactly what is-what is! (what a great wordplay 😃)


is-what is really easy to use, and most functions work just like you’d expect.

// import functions you want to use like so:
import { isString, isDate, isPlainObject } from 'is-what'
  1. First I’ll go over the simple functions available. Only isNumber and isDate have special treatment.
  2. After that I’ll talk about working with Objects (plain objects vs class instances etc.).
  3. Lastly I’ll talk about TypeScript implementation

Simple type check functions

// basics
isBoolean(true) // true
isBoolean(false) // true
isUndefined(undefined) // true
isNull(null) // true

// strings
isString('') // true
isEmptyString('') // true
isFullString('') // false

// numbers
isNumber(0) // true
isNumber('0') // false
isNumber(NaN) // false *
isPositiveNumber(1) // true
isNegativeNumber(-1) // true
// * see below for special NaN use cases!

// arrays
isArray([]) // true
isEmptyArray([]) // true
isFullArray([1]) // true

// objects
isPlainObject({}) // true *
isEmptyObject({}) // true
isFullObject({ a: 1 }) // true
// * see below for special object (& class instance) use cases!

// functions
isFunction(function () {}) // true
isFunction(() => {}) // true

// dates
isDate(new Date()) // true
isDate(new Date('invalid date')) // false

// maps & sets
isMap(new Map()) // true
isSet(new Set()) // true
isWeakMap(new WeakMap()) // true
isWeakSet(new WeakSet()) // true

// others
isRegExp(/\s/gi) // true
isSymbol(Symbol()) // true
isBlob(new Blob()) // true
isFile(new File([''], '', { type: 'text/html' })) // true
isError(new Error('')) // true
isPromise(new Promise((resolve) => {})) // true

// primitives
isPrimitive('') // true
// true for any of: boolean, null, undefined, number, string, symbol

Let’s talk about NaN

isNaN is a built-in JS Function but it really makes no sense:

// 1)
typeof NaN === 'number' // true
// 🤔 ("not a number" is a "number"...)

// 2)
isNaN('1') // false
// 🤔 the string '1' is not-"not a number"... so it's a number??

// 3)
isNaN('one') // true
// 🤔 'one' is NaN but `NaN === 'one'` is false...

With is-what the way we treat NaN makes a little bit more sense:

import { isNumber, isNaNValue } from 'is-what'

// 1)
isNumber(NaN) // false!
// let's not treat NaN as a number

// 2)
isNaNValue('1') // false
// if it's not NaN, it's not NaN!!

// 3)
isNaNValue('one') // false
// if it's not NaN, it's not NaN!!

isNaNValue(NaN) // true

isPlainObject vs isAnyObject

Checking for a JavaScript object can be really difficult. In JavaScript you can create classes that will behave just like JavaScript objects but might have completely different prototypes. With is-what I went for this classification:

  • isPlainObject will only return true on plain JavaScript objects and not on classes or others
  • isAnyObject will be more loose and return true on regular objects, classes, etc.
// define a plain object
const plainObject = { hello: 'I am a good old object.' }

// define a special object
class SpecialObject {
  constructor(somethingSpecial) {
    this.speciality = somethingSpecial
const specialObject = new SpecialObject('I am a special object! I am a class instance!!!')

// check the plain object
isPlainObject(plainObject) // returns true
isAnyObject(plainObject) // returns true
getType(plainObject) // returns 'Object'

// check the special object
isPlainObject(specialObject) // returns false !!!!!!!!!
isAnyObject(specialObject) // returns true
getType(specialObject) // returns 'Object'

Please note that isPlainObject will only return true for normal plain JavaScript objects.

Getting and checking for specific types

You can check for specific types with getType and isType:

import { getType, isType } from 'is-what'

getType('') // returns 'String'
// pass a Type as second param:
isType('', String) // returns true

If you just want to make sure your object inherits from a particular class or toStringTag value, you can use isInstanceOf() like this:

import { isInstanceOf } from 'is-what'

isInstanceOf(new XMLHttpRequest(), 'EventTarget')
// returns true
isInstanceOf(globalThis, ReadableStream)
// returns false


is-what makes TypeScript know the type during if statements. This means that a check returns the type of the payload for TypeScript users.

function isNumber(payload: unknown): payload is number {
  // return boolean
// As you can see above, all functions return a boolean for JavaScript, but pass the payload type to TypeScript.

// usage example:
function fn(payload: string | number): number {
  if (isNumber(payload)) {
    // ↑ TypeScript already knows payload is a number here!
    return payload
  return 0

isPlainObject and isAnyObject with TypeScript will declare the payload to be an object type with any props:

function isPlainObject(payload: unknown): payload is { [key: string]: unknown }
function isAnyObject(payload: unknown): payload is { [key: string]: unknown }
// The reason to return `{[key: string]: unknown}` is to be able to do
if (isPlainObject(payload) && return
// if isPlainObject() would return `payload is object` then it would give an error at ``


If you want more control over what kind of interface/type is casted when checking for objects.

To cast to a specific type while checking for isAnyObject, can use isObjectLike<T>:

import { isObjectLike } from 'is-what'

const payload = { name: 'Mesqueeb' } // current type: `{ name: string }`

// Without casting:
if (isAnyObject(payload)) {
  // in here `payload` is casted to: `Record<string | number | symbol, unknown>`

// With casting:
// you can pass a specific type for TS that will be casted when the function returns
if (isObjectLike<{ name: string }>(payload)) {
  // in here `payload` is casted to: `{ name: string }`

Please note: this library will not actually check the shape of the object, you need to do that yourself.

isObjectLike<T> works like this under the hood:

function isObjectLike<T extends object>(payload: unknown): payload is T {
  return isAnyObject(payload)

Meet the family (more tiny utils with TS support)

Source code

It’s litterally just these functions:

function getType(payload) {
  return, -1)
function isUndefined(payload) {
  return getType(payload) === 'Undefined'
function isString(payload) {
  return getType(payload) === 'String'
function isAnyObject(payload) {
  return getType(payload) === 'Object'
// etc...

See the full source code here.