JavaScript Type Checking Utilities

Common errors

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

  cause 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'.

  cause 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

  cause `isNumber(NaN)` intentionally returns `false` in `is-what`.
  fix If you need to check specifically if a value is `NaN`, use `isNaNValue(value)` instead.

Warnings

• 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.
• breaking In `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.
• gotcha The `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.
• security Version `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`.
• gotcha The `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)`.

Install

• npm install is-what npm
• yarn add is-what yarn
• pnpm add is-what pnpm

Imports

• isPlainObject
  wrong:

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

  correct:

  import { isPlainObject } from 'is-what'

  The library is ESM-only since v5.0.0. Use `import` statements.
• isString
  wrong:

  import isString from 'is-what'

  correct:

  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.

Quickstart

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