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Modern TypeScript rewrite of the popular `clsx` package, with type-level compilation to provide an in-editor preview of the expected results to be generated at runtime.

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This package works with Cloudflare Workers
This package works with Node.js
This package works with Deno
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JSR Score
3 weeks ago (0.2.2)


This is a utility for constructing conditional class names in JavaScript and TypeScript from a set of mixed inputs. It's based on the popular clsx package by Luke Edwards, re-written and enhanced by Nicholas Berlette in 100% TypeScript using modern language features.

In addition to being a drop-in replacement for the original, this package also introduces an advanced type-level implementation of the clsx function. It leverages nearly identical logic as the runtime function, at a purely type level.

That type-level implementation affords you with a compile-time preview of the className strings that clsx expects to generate at runtime, without ever leaving your editor. This is quite useful for catching typos and errors before they occur.


📦 Install

This package is distributed through JSR, NPM, and Deno.

It can be installed using your package manager of choice:

deno add @nick/clsx
npx jsr add @nick/clsx
bun add @nberlette/clsx
# or
pnpm add @nberlette/clsx
# or
yarn add @nberlette/clsx
# or
npm install @nberlette/clsx

📜 Import

The package exports a single function, clsx, which can be imported as follows:

import { clsx } from "@nick/clsx";

const className = clsx("foo", "bar", "baz");

If you're using the NPM package:

import { clsx } from "@nberlette/clsx";

const className = clsx("foo", "bar", "baz");

Or, if you're importing from

import { clsx } from "";

const className = clsx("foo", "bar", "baz");



Constructs a composite className string from a given set of mixed inputs.


function clsx<T extends ClassInputs>(...classes: T): Clsx<T>;


Name Info
classes The class names to compile into a string.


Type Info
Clsx<T, string> Composite className string generated from the input.

Clsx<T, Fallback>

type Clsx<T, Fallback = string> = T extends ClassInputs
  ? T["length"] extends 0 ? Fallback
  : IsValidwInput<
    MergeValues<T, Fallback> extends infer S ? S extends "" ? Fallback : S
      : Fallback,
  : Fallback;

The type-level equivalent of the clsx function, which is used to render a compile-time preview of the className string expected to be generated.

For your convenience, an optional Fallback type parameter can be specified, which will be used in an event where a suitable type cannot be inferred. The default fallback type is the generic string type, since the clsx function will always return a string at runtime.

Type Parameters

Name Extends Default
T -- --
Fallback -- string


import { clsx } from "@nick/clsx";

const cn = clsx("foo", "bar", "baz");
//     ^? const cn: "foo bar baz"

const cn2 = clsx("foo", { bar: true, baz: false });
//     ^? const cn2: "foo bar"

const cn3 = clsx("nested", ["deep", { no: null }, ["yuh"]]);
//     ^? const cn3: "nested deep yuh"
import { clsx } from "@nick/clsx";

const dark = matchMedia("(prefers-color-scheme: dark)").matches;

const bgs = ["bg-white", "bg-black"] as const;

// This type is correctly inferred as a union of two possible class names:
const cn = clsx("w-1/2", "h-full", bgs[+dark]);
//     ^? const cn: "w-1/2 h-full bg-white" | "w-1/2 h-full bg-black"

For more examples, refer to the test suite.

Further Reading

🧑🏽‍💻 Contributing

This project is open-source, and I welcome contributions of any kind. Feel free to open an issue or pull request in the GitHub Repository if you have any suggestions, bug reports, or feature requests.

🐛 Bugs and Issues

If you encounter any bugs or unexpected behavior, please open an issue in the GitHub Repository so it can be addressed promptly. Thank you!

MIT © Nicholas Berlette. All rights reserved.
GitHub · JSR · NPM · Deno · Docs
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