jty - Just a Tiny Runtime Type Checker

A minimalistic, ergonomic, AI friendly, zero-dependency library for runtime type checking and defensive programming in JavaScript and TypeScript.

It helps you write safer, more reliable code by verifying types before usage and failing gracefully instead of bringing your application down with cryptic errors. Fail early with a good error rather than continue with the wrong assumption.

🤔 Why jty? Didn't TypeScript Solve This?

JavaScript is a highly dynamic language with famous quirks around implicit type conversions. TypeScript helps a lot at compile-time. In fact the illusion of type-safety can create more fragile code when interacting with APIs or external JS code at runtime:

• An API responds with an unexpected payload structure or values.
• A user ot AI agent submits a form with invalid input.
• Parsed JSON/YAML have drifted from the code that depends on them.
• You are interoperating with untyped 3rd-party JavaScript libraries.
• You are dealing with Typescript code that uses various escape hatches (any, unknown, or casting as SomeType).

If you don't validate your data at the boundaries, your application is vulnerable to cascading failures and extremely hard-to-debug behaviors.

This is where jty comes in. jty enables Defensive Programming, empowering you to validate shapes and types at runtime, failing early right at the boundary.

✨ Developer Ergonomics & Features

jty was designed to be a joy to write and read, focusing heavily on developer experience and AI Agent effectiveness. Written in the latest TypeScript, it provides:

• 🛡️ TypeScript Type Guards: Functions act as type guards. Once jty verifies a type, your IDE language server instantly recognizes the narrowed type structure. No more any!
• 🗣️ Expressive Error Messages: When a validation fails, jty empowers you to throw highly expressive exceptions explicitly logging what went wrong, what was expected, and exactly what was received. This also helps AI agents identify exactly what went wrong, what was expected and what was received, accelerating debugging and reducing token usage.
• 🧪 Thoroughly Tested: Fully tested against all the infamous JavaScript edge cases (like NaN, null vs undefined, object prototypes, inheritance, arrays, etc.).
• 📦 Batteries Included: Comes with TypeScript types out of the box, eliminating the need to install and update a separate @types/... package to work with jty in TypeScript repos. Zero external runtime dependencies.
• 🔌 Universal Compatibility: Works seamlessly with both ESM (import) and CommonJS (require()), natively supporting Node.js, Deno, Bun, and modern browsers.

🤖 An AI-First Library

jty is built with modern AI-driven development in mind. It is an AI-first library where an embedded SKILL.md file is shipped alongside the codebase. This allows LLM-powered coding assistants to learn exactly how to use this library efficiently.

Plus the function names and accompanying Typedocs gives an expressive token-language to LLM and makes it easy for AI agents to discover how to use it.

How to use our AI Skill:

• From node_modules: Once installed locally, your agent can read node_modules/jty/SKILL.md directly to understand the API standards, function signatures, and best practices.
• Using Skills.sh: You can reference this skill in external AI tools and workflows using platforms like skills.sh to seamlessly inject context into your favorite agentic tools with zero manual setup.
• Using a pointer file: Put a basic .agent/skills/jty/SKILL.md which references node_modules/jty/SKILL.md.

---
name: jty
description: 'Defensive programming patterns to ensure runtime type safety using the jty library'
---
Look up [jty SKILL.md](node_modules/jty/SKILL.md)
Copy

---

🚀 Quick Start

npm install jty
Copy

Basic Usage

Use jty to easily validate variables and surface descriptive errors.

import { isStr } from 'jty'

function greet(name) {
    if (!isStr(name)) {
        throw new TypeError(`Expected "name" to be a string. Got ${name} (${typeof name})`)
    }
    console.log(`Hello, ${name}!`)
}

greet('Alice') // Hello, Alice!
greet(13) // TypeError: Expected "name" to be a string. Got 13 (number)
Copy

Advanced Schema Validation with Type Guarding

Let's validate a complex API response payload. jty gracefully scopes properties so the TypeScript compiler can perfectly infer them.

import { isArr, hasProp, isInt, isStr, isStrLen } from 'jty'

function verifyResponseShape(responseJson: unknown) {
    if (!isArr(responseJson)) {
        throw new TypeError(`Expected an array. Got ${responseJson} (${typeof responseJson})`)
    }

    for (let i = 0; i < responseJson.length; i++) {
        const post = responseJson[i]

        // Ensure the base properties exist!
        if (!hasProp(post, 'userId', 'id', 'title', 'body')) {
            throw new Error(`Post ${i} is missing required properties`)
        }

        // Type Guards active! TS now knows post has 'userId', 'id', 'title', and 'body'

        if (!isInt(post.userId) || post.userId < 0) {
            throw new TypeError(`Post ${i} does not have a positive integer userId`)
        }
        if (!isInt(post.id) || post.id < 0) {
            throw new TypeError(`Post ${i} does not have a positive integer id`)
        }
        if (!isStr(post.title)) {
            throw new TypeError(`Post ${i} is missing a valid title string`)
        }
        if (!isStrLen(post.body, 10, 200)) {
            throw new RangeError(`Post ${i} has an invalid body length. Got: ${post.body}`)
        }
    }

    // Everything is verified.
    return responseJson
}
Copy

📖 API Documentation

Explore all available methods and exhaustive examples on our interactive documentation.

Read the API Docs

🎓 Best Practices

Discover patterns for resilient JavaScript and TypeScript structures on our Wiki.

---

Made in Sweden 🇸🇪 by Alex Ewerlöf