---
title: "Cap'n Web"
description: "A JavaScript-native, object-capability RPC system with promise pipelining. No schemas, no boilerplate, under 16 kB."
---

> Documentation Index
> Fetch the complete documentation index at: https://255.pr.capnweb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Cap'n Web

import { examples } from '../../examples.ts';
import bundleSize from '../../generated/bundle-size.json';
import CanvasFigure from '../../components/canvas-hero/CanvasFigure.astro';
import Features from '../../components/Features.astro';
import NavList from '../../components/NavList.astro';
import { HERO_TITLE, HERO_FIGURE_CAPTION, heroTagline } from '../../lib/hero-copy.ts';

Cap'n Web is a spiritual sibling to [Cap'n Proto](https://capnproto.org), created by the same
author, but designed to play nice in the web stack. Like Cap'n Proto it is an **object-capability**
protocol: "Cap'n" is short for "capabilities and". Unlike Cap'n Proto, it has no schemas, almost no
boilerplate, and its serialization is just JSON with a little pre- and post-processing.

## Why it's different

## The magic trick

Every call returns an `RpcPromise`. You can use that promise, or a property of it, as the input to
another call *before it has resolved*. The server substitutes the real value on arrival, so a chain
of dependent calls costs exactly one round trip:

```ts
// Authenticate, get the user's ID, fetch their profile, and
// fetch every friend's profile too. One request, one response.
let authed = api.authenticate(apiToken);
let profile = api.getUserProfile(authed.getUserId());
let friends = authed.getFriendIds().map(id => api.getUserProfile(id));

let [me, myFriends] = await Promise.all([profile, friends]);
```

[See how promise pipelining works →](/start/pipelining-tour/) — a step-by-step tour, and the
record-replay trick behind `.map()`.

## See it running

Both examples run right here in the docs, with the source alongside them. They make the same point
from opposite ends of the stack: the pipelined version issues one HTTP request where the ordinary
version issues three.

<NavList items={examples.map((example) => ({
	title: example.title,
	description: example.description,
	href: example.docsPath,
}))} />

## Get going

Source: https://255.pr.capnweb.com/index.mdx