is-kit Docs — TypeScript Type Guard Toolkit

A lightweight, zero-dependency toolkit for building and composing reusable TypeScript type guards while preserving useful narrowing.

Installation

pnpm add is-kit

Usage

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

const isShort = predicateToRefine<string>((s) => s.length <= 3);
const isShortString = and(isString, isShort); // isShortString: (value: unknown) => value is string

const evaluations = ['ok', 'toolong', 123].map((value) => isShortString(value));
// evaluations: [true, false, false]
or(isString, isNumber)('x'); // true
not(isString)(42); // true

Features

Type-safe predicates

Author and compose predicates that refine types precisely.

API Reference

define

Wrap a predicate as a typed guard.

struct

Shape guard for objects; supports exact key checking.

logic

Logical combinators: and, andAll, or, not, guardIn.

parse

Guard values or decode and validate JSON with tagged results.

primitive

Primitive guards: string, number, boolean, nullish, etc.

predicate

Convert boolean predicates to refinements.

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