Very simple & small JS type check functions. It's fully TypeScript supported!
npm i is-what
Or for deno available at: "deno.land/x/is_what"
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'- First I'll go over the simple functions available. Only
isNumberandisDatehave special treatment. - After that I'll talk about working with Objects (plain objects vs class instances etc.).
- Lastly I'll talk about TypeScript implementation
// 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, symbolisNaN 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) // trueChecking 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:
isPlainObjectwill only returntrueon plain JavaScript objects and not on classes or othersisAnyObjectwill be more loose and returntrueon 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
isPlainObjectwill only returntruefor normal plain JavaScript objects.
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 trueIf 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 falseis-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) && payload.id) return payload.id
// if isPlainObject() would return `payload is object` then it would give an error at `payload.id`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>`
// WE LOOSE THE TYPE!
}
// 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)
}- is-what 🙉
- is-where 🙈
- merge-anything 🥡
- check-anything 👁
- remove-anything ✂️
- getorset-anything 🐊
- map-anything 🗺
- filter-anything ⚔️
- copy-anything 🎭
- case-anything 🐫
- flatten-anything 🏏
- nestify-anything 🧅
It'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.
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