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is What? 🙉

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

npm i is-what

Or for deno available at: ""


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

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

// numbers
isNumber(0) // true
isNumber(NaN) // false

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

// others
isBoolean(false) // true
isFunction(function () {}) // true
isArray([]) // true
isUndefined(undefined) // true
isNull(null) // true
isRegExp(/\s/gi) // true
isSymbol(Symbol()) // true
isBlob(new Blob()) // true
isFile(new File([''], '', { type: 'text/html' })) // true

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

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

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.


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: any): 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: any): payload is {[key: string]: any}
function isAnyObject (payload: any): payload is {[key: string]: any}
// The reason to return `{[key: string]: any}` 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 which kind of objects are allowed you can use isObjectLike<T>:

import { isObjectLike } from 'is-what'
// usage examples:
isObjectLike<{specificKey: string}>(payload)
// you can pass a specific type for TS to check on.

isObjectLike<T> works like this under the hood:

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

Meet the family

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.