ReScript Schema 🧬

The fastest parser in the entire JavaScript ecosystem with a focus on small bundle size and top-notch DX.

⚠️ Be aware that rescript-schema uses eval for parsing. It's usually fine but might not work in some environments like Cloudflare Workers or third-party scripts used on pages with the script-src header.

Highlights:

• Works with plain JavaScript, TypeScript, and ReScript. You don't need to use any compiler.
• The fastest parsing and validation library in the entire JavaScript ecosystem (benchmark)
• Small JS footprint & tree-shakable API (Comparison with Zod and Valibot)
• Implements the Standard Schema spec
• Declarative transformations in a schema without a performance loss
• Reverse schema and convert values to the initial format (serializing)
• Detailed and easy to understand error messages
• Support for asynchronous transformations
• Immutable API with 100+ different operation combinations
• Easy to create recursive schema
• Opt-in strict mode for object schema to prevent unknown fields with ability to change it for the whole project
• Opt-in ReScript Schema codegen from type definition (ppx)

Also, you can use rescript-schema as a building block for your tools. And there are many existing ones:

• rescript-rest - RPC-like client, contract, and server implementation for a pure REST API
• rescript-envsafe - Makes sure you don't accidentally deploy apps with missing or invalid environment variables
• rescript-json-schema - Typesafe JSON schema for ReScript
• rescript-stripe - Describe and manage Stripe billing in a declarative way with code
• Internal form library at Carla
• tRPC, TanStack Form, TanStack Router, Hono and 15+ more using Standard Schema spec

Documentation

• For JS/TS users
• For ReScript users
• For PPX users
• For library maintainers

Resources

• Building and consuming REST API in ReScript with rescript-rest and Fastify (YouTube)
• ReScript Schema V9 Changes Overview (Dev.to)

Comparison

Instead of relying on a few large functions with many methods, rescript-schema follows Valibot's approach, where API design and source code is based on many small and independent functions, each with just a single task. This modular design has several advantages.

For example, this allows a bundler to use the import statements to remove code that is not needed. This way, only the code that is actually used gets into your production build. This can reduce the bundle size by up to 2 times compared to Zod.

Besides the individual bundle size, the overall size of the library is also significantly smaller.

At the same time rescript-schema is the fastest composable validation library in the entire JavaScript ecosystem. This is achieved because of the JIT approach when an ultra optimized validator is created using eval.

	rescript-schema@9.2.2	Zod@3.24.1	Valibot@0.42.1	ArkType@2.0.4
Total size (minified + gzipped)	11 kB	14.8 kB	10.5 kB	40.8 kB
Example size (minified + gzipped)	4.45 kB	13.5 kB	1.22 kB	40.7 kB
Parse with the same schema	93,491 ops/ms	1,191 ops/ms	3,540 ops/ms	84,772 ops/ms
Create schema & parse once	166 ops/ms	93 ops/ms	2,302 ops/ms	13 ops/ms
Eval-free	❌	✅	✅	❓
Codegen-free (Doesn't need compiler)	✅	✅	✅	✅
Ecosystem	⭐️⭐️	⭐️⭐️⭐️⭐️⭐️	⭐️⭐️⭐️	⭐️⭐️