is-what

library5.5.0jsnpmunverified

`is-what` is a lightweight, fully TypeScript-supported utility library providing a comprehensive set of functions for JavaScript type checking, such as `isPlainObject()`, `isArray()`, `isString()`, and many more specific checks. Currently at version `5.5.0`, the library maintains a steady release cadence, frequently adding new type-checking utilities and dependency updates. Its core differentiators include a focus on simplicity, a small footprint, robust TypeScript type inference (automatically narrowing types after checks), and a more intuitive handling of edge cases like `NaN` compared to native JavaScript functions. It explicitly distinguishes between plain objects (`{}`) and class instances, addressing a common pain point in JavaScript development. The library aims to be a straightforward and reliable alternative to more complex or less type-safe existing solutions. Its `v5.x` releases moved to ESM-only and refined its TypeScript integration.

serialization

npm install is-what

github ↗package ↗

INSTALL

IMPORT

SIG · IS-WHAT

is-what

serializationjavascriptv5.5.0

Install

—

Import

—

Disk

—

Pass rate

0/ 6

Env Coverage0 / 6

glibc

18–22

musl

18–22

Install & Compatibility

Where this runs

tested against v? · npm install

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

node 18–226 runs

build_error

glibc

node 18–226 runs

build_error

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

isPlainObject

✓ import { isPlainObject } from 'is-what'

✗ const { isPlainObject } = require('is-what')

The library is ESM-only since v5.0.0. Use `import` statements.

isString

✓ import { isString } from 'is-what'

✗ import isString from 'is-what'

All functions are named exports; there is no default export.

isNaNValue

✓ import { isNaNValue } from 'is-what'

Use `isNaNValue` for explicit NaN checks, as `isNumber(NaN)` returns `false` in this library.

Demonstrates basic type checking with `is-what`, including special handling for `NaN` and `isPlainObject`.

import { isString, isNumber, isPlainObject, isArray, isDate, isNaNValue, isHexDecimal } from 'is-what'; console.log('Is "hello" a string?', isString('hello')); // true console.log('Is 123 a number?', isNumber(123)); // true console.log('Is NaN a number (is-what)?', isNumber(NaN)); // false, by design console.log('Is NaN truly NaN?', isNaNValue(NaN)); // true console.log('Is {} a plain object?', isPlainObject({})); // true console.log('Is new Date() a plain object?', isPlainObject(new Date())); // false console.log('Is [] an array?', isArray([])); // true console.log('Is new Date() a Date object?', isDate(new Date())); // true console.log('Is an invalid date a Date object?', isDate(new Date('invalid date'))); // false console.log('Is "60adf084f0fbdcab42de841e" a hex decimal of length 24?', isHexDecimal('60adf084f0fbdcab42de841e', 24)); // true

Debug

Known issues

breaking`is-what` became an ESM-only package in version 5.0.0. CommonJS `require()` statements will no longer work.

fix

Migrate your project to use ES modules (`import`) or use a tool like `esm` if you cannot fully migrate.

affects: >=5.0.0

breakingIn `v5.0.0`, type definitions were updated to use `unknown` instead of `any` for better type safety. This might cause TypeScript errors if your code relied on `any` for inferred types.

fix

Adjust your TypeScript code to explicitly narrow types or handle `unknown` correctly after `is-what` checks.

affects: >=5.0.0

gotchaThe `isNumber()` function in `is-what` explicitly returns `false` for `NaN`, differing from JavaScript's native `typeof NaN === 'number'` or `Number.isNaN()` behavior. Use `isNaNValue()` for specific `NaN` checks.

fix

When checking for numbers, be aware that `isNumber(NaN)` will be `false`. If you specifically need to check if a value is `NaN`, use the dedicated `isNaNValue()` function.

affects: >=1.0.0

securityVersion `5.1.0` included a fix for a security issue identified via `npm audit`. Users on older `v5.x` versions should upgrade.

fix

Upgrade to `is-what@5.1.0` or newer to incorporate the security patch: `npm install is-what@latest`.

affects: <5.1.0

gotchaThe `isHexDecimal` function, introduced in `v5.2.0`, accepts an optional second argument for checking a specific string length, useful for IDs like MongoDB ObjectIds. Omitting it will only check for valid hexadecimal characters.

fix

To check for a specific length, pass the desired length as the second argument: `isHexDecimal('idstring', 24)`.

affects: >=5.2.0

Errors

Common errors & fixes

TypeError [ERR_REQUIRE_ESM]: require() of ES Module .../node_modules/is-what/dist/index.js from ... not supported.

Attempting to use `require()` with `is-what` v5 and later, which is an ESM-only package.

fix

Change `const { someFn } = require('is-what')` to `import { someFn } from 'is-what'`.

Property 'xyz' does not exist on type 'unknown'.

After checking a variable with `is-what`, TypeScript still infers it as `unknown` due to the v5 update, preventing direct property access.

fix

Explicitly narrow the type using a type assertion `(myVar as MyType).xyz` or perform further checks that TypeScript can use for inference.

console.log(isNumber(NaN)); // Expected true, got false

`isNumber(NaN)` intentionally returns `false` in `is-what`.

fix

If you need to check specifically if a value is `NaN`, use `isNaNValue(value)` instead.

Upgrade

Version history

5.5.0latest on npm

Audit

Dependencies

No dependency data recorded yet.

Agent activity

6 hits · last 30 days

node

Resources

githubhttps://github.com/mesqueeb/is-what ↗

packagehttps://www.npmjs.com/package/is-what ↗