is What? ๐Ÿ™‰

Very simple & small JS type check functions. Itโ€™s fully TypeScript supported!

npm i is-what

Or for deno available at: "deno.land/x/is_what"

Motivation

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) โ€ผ๏ธ
  • Let TypeScript automatically know what type a value is when checking

And thatโ€™s exactly what is-what is! (what a great wordplay ๐Ÿ˜ƒ)

Usage

is-what is really easy to use, and most functions work just like youโ€™d expect.

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

TypeScript

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)) {
    // โ†‘ 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) && payload.id) return payload.id
// if isPlainObject() would return `payload is object` then it would give an error at `payload.id`

isObjectLike

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)
isObjectLike<object>(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

Itโ€™s litterally just these functions:

function getType (payload) {
  return Object.prototype.toString.call(payload).slice(8, -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.