Build isXXX the Easy Way? Meet is-kit

Hey folks! The leaves are about to turn 🍁

I keep telling myself “just one more coffee”… but my hands are starting to shake ☕️😂

Anyway, I’m @nyaomaru, a frontend engineer.

Why I built this

If you write TypeScript a lot, you’ve probably hand-written type guards like value is Foo more times than you can count.

And keeping runtime checks perfectly aligned with types… kinda exhausting.

Enter is-kit — a lightweight, zero-dependency “guard generator kit” that slices through boring boilerplate ✂️

https://github.com/nyaomaru/is-kit

Why is-kit?

• 🖋 Stop hand-writing value is Foo
  
  
  
  
  • Centralize guard definitions with define and predicateToRefine.
  
  
  
  


• ⚙️ Compose logic with and / or / not
  
  
  
  
  • Express domain-specific conditions declaratively.
  
  
  
  


• 🧩 Combinators like struct / array / tuple
  
  
  
  
  • Validate nested objects in just a few lines.
  
  
  
  


• ✅ Runtime + type tests
  
  
  
  
  • Confidence via Jest (runtime) and tsd (type assertions).
  
  
  
  

How do I use it?

A super common case: “non-empty string” without hand-rolling the predicate signature.

import { define, isString } from 'is-kit';



const isNonEmptyString = define<string>(

  (value) => isString(value) && value.length > 0

);



isNonEmptyString('foo'); // true

isNonEmptyString(''); // false

By passing the generic <string>, you don’t need to write the value is string predicate signature yourself — clean and tidy.

Another practical one: identifying a User shape.

import { struct, isString, isNumber, safeParseWith } from 'is-kit';



const isUser = struct({ id: isNumber, name: isString });



isUser({ id: 1, name: 'A' }); // true



const parseUser = safeParseWith(isUser);



const result = parseUser({ id: 'abc', age: 42 });

// result.valid === false



if (result.valid) {

  // on success

} else {

  // on failure

}

You can check whether API responses or form inputs satisfy your expected types safely.

For more complex conditions, compose:

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



const isLongLiteral = predicateToRefine<string>((value) => value.length > 3);



const isLongString = and(isString, isLongLiteral);

isLongString('abcd'); // true

isLongString('ab'); // false

isLongString(123 as unknown); // false



or(isString, isNumber)('foo'); // true

not(isString)(123); // true

With the combinators + primitives/object guards, you can express a wide range of isXXX guards cleanly.

Install & Ecosystem

Install from npm:

pnpm add is-kit

# or

bun add is-kit

# or

npm i is-kit

# or

yarn add is-kit

Prefer JSR with Deno/Bun? Import TypeScript directly:

import { struct } from 'jsr:@nyaomaru/is-kit';

Wrap-up

is-kit is a small kit for writing safe TypeScript guards:

• 
  Keep it tiny with define
  


• 
  Model bigger shapes with struct
  


• 
  Combine complicated logic with and / or / not
  


• 
  Make ergonomics nice with safeParse / safeParseWith
  

If you’re tired of drift between types and runtime checks, drop is-kit into your project and give it a spin. 🚀

https://github.com/nyaomaru/is-kit

And if you like it, a ⭐ would make my day! 😻