TypeScript icon, indicating that this package has built-in type declarations

    4.1.1 • Public • Published

    is What? 🙉

    Total Downloads Latest Stable Version

    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

    // 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


    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 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, any>`
    // 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: 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.


    npm i is-what

    DownloadsWeekly Downloads






    Unpacked Size

    40.1 kB

    Total Files


    Last publish


    • mesqueeb