Documentation

Strictly typed, ergonomic, and lightweight (13kb gzipped) RPC library in Typescript. Send complex types and call functions across contexts with inferred typing, pluggable transports.

TL;DR: Osra makes your multi-context code looks like normal code. Zero boilerplate and gives you the best error messages you've ever seen.

worker.ts

import { expose } from 'osra'

const payload = {
  hash: crypto.getRandomValues(new Uint8Array(10)),
  add: (a: number, b: number) => a + b,
  makeCounter: () => {
    let count = 0
    return () => ++count
  },
  streamData: async function* () { yield* [0, 1, 2] }
}
export type Payload = typeof payload

expose(payload, { transport: globalThis })

main.ts

import type { Payload } from './worker'
import { expose } from 'osra'

const worker = new Worker(new URL('./worker.ts', import.meta.url), { type: 'module' })

export const {
  hash, // Uint8Array
  add, // (a: number, b: number) => Promise<number>
  makeCounter, // () => Promise<() => Promise<number>>,
  streamData, // () => Promise<AsyncIterableIterator<number>>
} = await expose<Payload>({}, { transport: worker })

hash.byteLength // 10

await add(40, 2) // 42

const counter = await makeCounter()
await counter() // 1
await counter() // 2

for await (const n of await streamData()) {
  console.log(n) // 0, 1, 2
}

Features

• Efficient transport modes:

  • Structured-clone (default for Window, Worker, etc...) is the fastest transport mode, being able to clone and transfer values to other contexts efficiently.
  • JSON (default for WebSocket, WebExtensions, etc...) is slower but supports more transport targets (e.g WebSocket, WebExtensions, etc...).

• Wide type support: Support all of the native platform types like Function, Promise, ReadableStream, Response, Map, Uint8Array, and many more...

• Explicit typescript errors: The codebase is entirely and extensively strictly typed. Anything that CAN cause issues at runtime will throw compile time errors.

As an example, trying to transfer a File value over a JSON transport, like so, will throw a compile time error:

┌─────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ ... {                                                                                                   │
│   [ErrorMessage]: "Value type is only supported on structured-clone transports, not on JSON transports";│
│   [BadValue]: File;                                                                                     │
│   [Path]: "foo";                                                                                        │
│   [ParentObject]: { ...; };                                                                             │
│ }'.                                                                                                     │
│   Type '{ foo: File; }' ...                                                                             │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────┘
       ^^^^^^^^^^^^^^^^^^^^^^^^^
expose({ foo: new File([], '') }, { transport: new WebSocket('') })

• Extensive automated test suite on Chromium, Firefox, and WebKit via Playwright

Transport modes

• Structured-clone ( Window, Worker, SharedWorker, ServiceWorker, MessagePort, custom transports)
• JSON ( WebSocket, WebExtension runtime connect() and onMessage, WebExtension port, custom transports with isJson: true)

Supported types

Transports are either structured-clone (Worker, Window, MessagePort, SharedWorker) or JSON (WebSocket, web extension messaging, custom transports with isJson: true).

| Type | Clone | JSON | Notes | |---|---|---|---| | JSON primitives, plain objects, arrays | ✅ | ✅ | | | undefined, NaN, ±Infinity | ✅ | ✅ | | | Date, BigInt, Map, Set | ✅ | ✅ | | | ArrayBuffer, Int8Array, Uint8Array, Uint8ClampedArray, Int16Array, Uint16Array, Int32Array, Uint32Array, Float16Array, Float32Array, Float64Array, BigInt64Array, BigUint64Array | ✅ | ✅ | | | Error + subclasses | ✅ | ✅ | built-ins errors properly preserve their subclass; custom error classes becomes generic Error | | Symbol | ✅ | ✅ | Symbol.for properly preserves the Symbol's key; Symbol() is automatically wrapped with identity() | | RegExp | ✅ | ❌ | | | SharedArrayBuffer | ✅ | ❌ | | | Function | ✅ | ✅ | becomes (...args) => Promise<result>; arguments and results are properly handled too | | Promise | ✅ | ✅ | | | Async generators / async iterables | ✅ | ✅ | | | ReadableStream | ✅ | ✅ | | | WritableStream | ✅ | ✅ | | | MessagePort | ✅ | ✅ | | | AbortSignal | ✅ | ✅ | | | File / FileList / Blob | ✅ | ❌ | | | Request / Response / Headers | ✅ | ✅ | | | Event / CustomEvent | ✅ | ✅ | Event subclass is not preserved | | EventTarget | ✅ | ✅ | revives as a listener-only façade: add/removeEventListener proxy to the source; you can't dispatch through it | | Structured-clonables (ImageData, DOMRect, CryptoKey, …) | ✅ | ❌ | | | Transfer-only host objects (OffscreenCanvas, MediaStreamTrack, RTCDataChannel, …) | ✅ | ❌ | | | ImageBitmap, VideoFrame, AudioData | ✅ | ❌ | | | WeakMap / WeakSet, other unclonables | ❌ | ❌ | |

Identity

identity(value) preserves reference equality across contexts, sending the same identity wrapped value twice results in the same object reference on the peer.

worker.ts

import { expose, identity } from 'osra'

const value = { foo: 'bar' }
const payload = { value, ref1: identity(value), ref2: identity(value) }

expose(payload, { transport: globalThis })
export type Payload = typeof payload

main.ts

import type { Payload } from './worker'
import { expose } from 'osra'

const worker = new Worker(new URL('./worker.ts', import.meta.url), { type: 'module' })
const { value, ref1, ref2 } = await expose<Payload>({}, { transport: worker })

value === ref1 // false
ref1 === ref2 // true

Transfer

By default, osra will always copy values, if the value you want to send is a transferable, wrapping it with transfer(value) will properly transfer it to the other context. Transfer behavior is preserved, which means the value can no longer be used in the sender context once it has been transferred.

import { transfer } from 'osra'

const buffer = new ArrayBuffer(16_000_000)
await remote.transferBuffer(transfer(buffer)) // moved - buffer is detached locally

Options

| Option | Default | Description | |---|---|---| | transport | required | The channel to communicate over (see Transport modes), should be equal to the place where addEventListener('message') and postMessage() calls target the remote context you want to communicate with | | key | '__OSRA_DEFAULT_KEY__' | Namespacing tag that lets multiple independent osra connections share one channel | | origin | '*' | Similar to postMessage's origin, It restricts the remote origin | | name | - | Defines the name that will be used for the announcement | | remoteName | - | Filters any incoming messages that are not equal to the name of the remote peer | | unregisterSignal | - | AbortSignal that will tear down the connection when aborted | | uuid / remoteUuid | random / - | Same as name and remoteName, but automatically generated at announce time | | revivableModules | - | defaults => modules function to add, drop, reorder, or override revivable modules | | connection | ({ value }) => value | What one connection resolves to, for the await and for iteration alike (see Connections) |

Connections

expose() is awaitable and async-iterable. Awaiting gives the first peer, iterating gives every peer as it connects:

import { expose } from 'osra'

type PeerApi = { version: () => string }

const api = { log: (line: string) => console.log(line) }

// the first peer to connect
const remote = await expose<PeerApi>(api, { transport: window })
await remote.version()

// every peer, as each one arrives
for await (const peer of expose<PeerApi>(api, { transport: window })) {
  console.log('peer connected, running', await peer.version())
}

Each loop is one peer, so a page embedding several iframes serves them all from one expose(). Several loops over the same expose() each see every peer, and peers that connect before anything iterates are buffered and replayed.

Pass connection to decide what a peer resolves to, which is also how you reach its origin and its per-peer abort:

for await (const peer of expose({}, {
  transport: window,
  connection: ({ value, context }) => ({ value, context })
})) {
  if (!allowed(peer.context.origin)) peer.context.abort?.()
}

The context holds only what the transport observed, plus abort. A window or iframe gives origin and source; a WebExtension gives port and sender; a WebSocket gives the socket URL as origin; a MessagePort or Worker observes nothing at all.

Wrap a value in context() to build it once per connection, so one server can answer each realm differently instead of sharing one object with all of them:

import { expose, context } from 'osra'

expose(context(({ origin }) => ({ read: readFor(origin) })), { transport: window })

It runs before your value is sent, so calling ctx.abort() inside it refuses that peer outright.

Limitations

• Circular structures throw a TypeError at send time; break the cycle or restructure.
• Classes/prototypes are not preserved: Classes and their instances are not preserved, please use plain objects and functions instead.
• Synchronous functions become asynchronous: () => number will become () => Promise<number>.