# capnp-es
> TypeScript implementation of the Cap'n Proto serialization.
Latest version **0.0.16** (published 2026-08-14) · MIT license · 0 weekly downloads
## Install
```sh
npm install capnp-es
pnpm add capnp-es
yarn add capnp-es
bun add capnp-es
```
Provides the commands `capnp-es`, `capnpc-js`, `capnpc-ts`, `capnpc-dts`.
## Health
**Score 70/100 (B)** — status: active.
Positive: has types; esm support; no vulnerabilities; recently updated; high maintenance score; high quality score.
Warnings: low downloads; pre 1.0.
## Facts
| | |
|---|---|
| Version | 0.0.16 |
| Published | 2026-08-14 |
| First published | 2024-08-16 |
| Weekly downloads | 0 |
| License | MIT |
| TypeScript types | bundled |
| Module format | ESM + CommonJS |
| Dependencies | 0 |
| Unpacked size | 803.1 KB |
| Known vulnerabilities | 0 |
| Install scripts | no |
| GitHub stars | 178 |
| Maintainers | pi0, victorb, penalosa |
## Links
- npm: https://www.npmjs.com/package/capnp-es
- Repository: https://github.com/unjs/capnp-es
- Homepage: https://github.com/unjs/capnp-es#readme
- Issues: https://github.com/unjs/capnp-es/issues
- npm.io page: https://npm.io/package/capnp-es
## Recent versions
- 0.0.16 (latest) — 2026-08-14
- 0.0.15 — 2026-08-11
- 0.0.14 — 2026-01-15
- 0.0.13 — 2026-01-15
- 0.0.12 — 2026-01-15
- 0.0.11 — 2025-06-06
- 0.0.10 — 2025-06-05
- 0.0.9 — 2025-04-11
- 0.0.7 — 2025-01-19
- 0.0.6 — 2025-01-16
- 0.0.5 — 2025-01-16
- 0.0.4 — 2024-08-27
- 0.0.3 — 2024-08-20
- 0.0.2 — 2024-08-16
- 0.0.1 — 2024-08-16
- … 1 more at https://npm.io/package/capnp-es/versions
## README
# 🔥 capnp-es
[](https://npmjs.com/package/capnp-es)
[](https://npm.chart.dev/capnp-es)
[](https://bundlephobia.com/package/capnp-es)
[](https://codecov.io/gh/unjs/capnp-es)
> [!WARNING]
> This is an alpha-quality software. please use at your own risk ([project status](#status)).
TypeScript implementation of the [Cap'n Proto](https://capnproto.org) serialization protocol.
[Cap’n Proto](https://capnproto.org/) is an insanely fast data interchange format and capability-based RPC system. Think JSON, except binary. Or think [Protocol Buffers](https://github.com/protocolbuffers/protobuf), except faster. Cap’n Proto was built by [Kenton Varda](https://github.com/kentonv) to be used in [Sandstorm](https://capnproto.org/faq.html#sandstorm) and is now heavily used in [Cloudflare](https://capnproto.org/faq.html#cloudflare).
## Usage
### Compiling schema
> [!NOTE]
> Make sure `capnpc` command is available. You can find install instructions [here](https://capnproto.org/install.html) to install it.
Install `capnp-es` dependency:
```sh
npx nypm install capnp-es
```
You can use `capnp-es` to compile a schema file into typeScript/javascript source code:
```shell
npx capnp-es path/to/myschema.capnp -ojs,ts,dts
```
This will generate `path/to/myschema.{js,ts,dts}`.
Use `npx capnp-es --help` for full usage info.
See [playground](./playground/) for examples and learn more about `.capnp` schema in [language docs](https://capnproto.org/language.html).
### Reading Messages
Here's a quick usage example:
```ts
import * as capnp from "capnp-es";
import { MyStruct } from "./myschema.js";
const message = capnp.Message.fromArrayBuffer(buffer);
const struct = message.getRoot(MyStruct);
```
### RPC Protocol
Experimental [RPC protocol](https://capnproto.org/rpc.html) is supported ([level 1](https://capnproto.org/rpc.html#protocol-features)).
See [tests](./test/integration/rpc.spec.ts) for some examples.
### TypeScript compatibility
TypeScript `5.7.3` and later, including `6.x`, are supported and covered by CI. That range applies to:
- the schema compiler (`capnp-es/compiler` and the `capnpc-*` binaries), which uses the TypeScript compiler API to format the generated code and to transpile the `.js` and `.d.ts` output;
- the TypeScript you use to type check the generated `.ts` files and the declarations shipped with this package.
TypeScript is declared as an optional peer dependency because the runtime (`capnp-es`) never imports it, only the schema compiler does. Generated `.ts` files can also be run with a strip-only loader, such as Node.js type stripping or esbuild, which does not need the `typescript` package at all.
> [!NOTE]
> TypeScript `7.x` is not supported yet. It is a native (Go) compiler and its npm package no longer exposes the compiler API that the schema compiler relies on. It will be re-introduced from version 7.1.
## Status
This project is a rework of [jdiaz5513/capnp-ts](https://github.com/jdiaz5513/capnp-ts/) by [Julián Díaz](https://github.com/jdiaz5513) and is under development.
Changes from capnp-ts
- Internal refactors and simplifications as was playing around.
- Compiler, runtime, and std lib published via a single and compact ESM-only package with subpath exports.
- Compiler updated to use Typescript v5 API
- Output files can be `.ts` (new), `.js` (ESM instead of CJS), and `.d.ts` and has no `.capnp` suffix.
- Compiler API can be used via the `capnp-es/compiler` subpath export programmatically.
- Use native `TextEncoder` and `TextDecoder` for utf8 encoding
- Enums are typed plain JS objects (this way `.ts` files work with strip-only ts loaders without enum support.)
- Compiler CLI can directly accept a path to `.capnp` files and internally use `capnpc`
- Built-in schemas are compiled from source (compiler, compiles itself. so cool right?)
- Use reflection (getter setters) to access structs.
- RPC level-1 merged from [jdiaz5513/capnp-ts#169](https://github.com/jdiaz5513/capnp-ts/pull/169).
- List interface implements [`Array` object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array) (custom methods removed).
- Pointers had been improved to feel (inspected and serialized) like native JS values as much as possible.
- Basic JSDocs generated for class and getter
## Contribution
Feedback and PRs are more than welcome. 🙏
Local development
- Clone this repository
- Install the latest LTS version of [Node.js](https://nodejs.org/en/)
- Enable [Corepack](https://github.com/nodejs/corepack) using `corepack enable`
- Install dependencies using `pnpm install`
- Run interactive tests using `pnpm dev`
## License
🔀 Forked from [jdiaz5513/capnp-ts](https://github.com/jdiaz5513/capnp-ts/) by [Julián Díaz](https://github.com/jdiaz5513).
💛 Published under the [MIT](https://github.com/unjs/capnp-es/blob/main/LICENSE) license.
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