Cap'n Web

Overview

Cap’n Web is a JavaScript-native RPC system from the creator of Cap’n Proto, redesigned for the web stack. It has no schemas and almost no boilerplate: you define a class on the server, and clients call its methods as if they were local — over HTTP, WebSocket, or postMessage().

Under the hood it is an object-capability protocol, which makes it far more expressive than typical RPC: you can pass functions and objects by reference, call in both directions, and chain dependent calls into a single network round trip.

Key Features

• No schemas, no codegen, no build step — install the package and start calling methods
• TypeScript-friendly with typed stubs for remote objects
• Human-readable wire format — it is JSON with light pre/post-processing
• Multiple transports: HTTP batch, WebSocket, and postMessage(), extensible to others
• Runs everywhere: browsers, Cloudflare Workers, Node.js, Bun, Deno
• Promise pipelining: use the result of one call as the input of the next without waiting for a round trip
• Bidirectional calls and pass-by-reference for callbacks and capability-based security
• Tiny: under 16 kB minified and gzipped, with zero dependencies

Getting Started

npm i capnweb

A client connects and calls a remote method:

import { newWebSocketRpcSession } from "capnweb";

const api = newWebSocketRpcSession("wss://example.com/api");
console.log(await api.hello("World"));

On the server — for example a Cloudflare Worker — you expose a class that extends RpcTarget:

import { RpcTarget, newWorkersRpcResponse } from "capnweb";

class MyApi extends RpcTarget {
  hello(name) {
    return `Hello, ${name}!`;
  }
}

export default {
  fetch(request) {
    return newWorkersRpcResponse(request, new MyApi());
  },
};

Use Cases

• Typed client/server APIs without maintaining REST endpoints or GraphQL schemas
• Real-time apps over WebSockets with server-to-client callbacks
• Communication between Workers, iframes, and web workers

License

MIT License.