pnpm add is-kit
# or
bun add is-kit
# or
npm i is-kit
# or
yarn add is-kit

// Deno/Bun/JSR-aware tooling
import { define, and, or } from 'jsr:@nyaomaru/is-kit';

is-kit

Lightweight, zero-dependency toolkit for building isFoo style type guards in TypeScript. Runtime-safe 🛡️, composable 🧩, and ergonomic ✨.

👉 See full document

Why?

Tired of writing the same isFoo again and again? Let is-kit do it for you:

• Less boilerplate
• Type-safe
• Composable
• Zero-dependency

☕ Grab a coffee, let is-kit do the boring work.

Install

Node via npm

pnpm add is-kit
# or
bun add is-kit
# or
npm i is-kit
# or
yarn add is-kit

ESM and CJS builds are provided for npm consumers. Types are bundled.

JSR (TypeScript source)

// Deno/Bun/JSR-aware tooling
import { define, and, or } from 'jsr:@nyaomaru/is-kit';

Quick start

Build is functions from tiny, composable pieces:

import {
define,
predicateToRefine,
and,
or,
not,
isString,
isNumber,
} from 'is-kit';

// 1. define small pieces
const isShortString = define<string>(
(value) => isString(value) && value.length <= 3
);
const isPositive = predicateToRefine<number>((num) => num > 0);

// 2. compose them
const isPositiveNumber = and(isNumber, isPositive);
const shortOrPositive = or(isShortString, isPositiveNumber);
const isOther = not(shortOrPositive);

// 3. use them
shortOrPositive('foo'); // true
shortOrPositive(42); // true
isOther(false); // true

These primitives plug into higher-level combinators like struct when you need to shape objects.

Core Ideas

• Define once: define<T>(fn) turns a plain function into a type guard.
• Upgrade predicates: predicateToRefine(fn) adds narrowing.
• Compose freely: and, or, not, oneOf, arrayOf, struct …
• Stay ergonomic: helpers like nullable, optional, equals, safeParse.

API Reference

👉 See the docs app under docs/ (API Reference section). Each helper is documented with runnable examples.

Development

Requires Node 22 and pnpm 10.12.4 (via Corepack or mise). Want to hack on is-kit? See DEVELOPER.md for setup, scripts, and contribution workflow.

Contributing

See CONTRIBUTE.md for workflow, commit style, and tool setup. 🚀