quickjs-wasi

A snapshotable JavaScript runtime via WebAssembly. Runs QuickJS compiled to WASM, with the ability to snapshot the entire VM state (including pending promises) and restore it in a fresh WASM instance.

Motivation

The Workflow DevKit project implements durable function execution for TypeScript using an event-replay technique: workflow code is re-executed from the beginning on every resumption, with the full event log used as the source of truth for previously completed work. This approach has scaling limitations:

• As the event log grows, re-fetching it becomes expensive
• Replaying the full log takes increasingly longer
• There is an effective upper bound on how much work a workflow can do
• Running "forever" workflows is impractical

This project explores a fundamentally different approach: VM snapshotting. Instead of replaying from the beginning, we snapshot the JavaScript execution environment at each suspension point and restore it on resumption. The restored VM already has the correct state — only events since the last snapshot need to be fetched and applied.

Install

npm install quickjs-wasi

Usage

Basic Evaluation

Both QuickJS and JSValueHandle implement Symbol.dispose, so you can use using declarations for automatic cleanup:

import { QuickJS } from 'quickjs-wasi';

{
  using vm = await QuickJS.create(wasmBytes);

  // Evaluate code — handles are auto-disposed with `using`
  using result = vm.unwrapResult(vm.evalCode('1 + 2'));
  console.log(result.toNumber()); // 3
} // vm and result are automatically disposed here

Working with Values

using vm = await QuickJS.create(wasmBytes);

// Create values — `using` ensures they're disposed at end of scope
{
  using str = vm.newString('hello');
  using num = vm.newNumber(42);
  using big = vm.newBigInt(9007199254740993n);
  vm.setProp(vm.global, 'message', str);
}

// Read back the value
using msg = vm.unwrapResult(vm.evalCode('message'));
console.log(msg.toString()); // "hello"

// Convert host values to QuickJS handles (and back)
using handle = vm.hostToHandle({ x: 1, y: [2, 3] });
const dumped = vm.dump(handle); // { x: 1, y: [2, 3] }

// consume() is still useful for inline one-liners
const value = vm.evalCode('1 + 2').consume(h => h.toNumber()); // 3

Host Functions

Register JavaScript functions backed by host (Node.js) callbacks:

using vm = await QuickJS.create(wasmBytes);

// The first argument to the callback is always `this`
{
  using add = vm.newFunction('add', (...args) => {
    return vm.newNumber(args[0].toNumber() + args[1].toNumber());
  });
  vm.setProp(vm.global, 'add', add);
}

using result = vm.unwrapResult(vm.evalCode('add(3, 4)'));
console.log(result.toNumber()); // 7

Promises and Async Host Functions

Bridge async host operations into the QuickJS sandbox:

using vm = await QuickJS.create(wasmBytes);

// Create an async host function that returns a promise to QuickJS
{
  using dnsResolve = vm.newFunction('dnsResolve', (...args) => {
    const hostname = args[0].toString();
    const deferred = vm.newPromise();

    // Do real async work on the host side
    dns.resolve4(hostname).then(
      (addresses) => {
        deferred.resolve(vm.newString(addresses[0]));
        vm.executePendingJobs(); // drain the QuickJS job queue
      },
      (err) => {
        deferred.reject(vm.newError(err));
        vm.executePendingJobs();
      }
    );

    return deferred.handle; // return the QuickJS promise
  });
  vm.setProp(vm.global, 'dnsResolve', dnsResolve);
}

Error Handling

using vm = await QuickJS.create(wasmBytes);

// unwrapResult() throws a host Error if the eval/call produced an exception
try {
  vm.unwrapResult(vm.evalCode('throw new TypeError("bad")'));
} catch (err) {
  console.log(err.name);    // "TypeError"
  console.log(err.message); // "bad"
  console.log(err.stack);   // QuickJS stack trace
}

// Create errors from host Error objects (preserves name, message, stack)
{
  using errHandle = vm.newError(new RangeError('out of bounds'));
  vm.setProp(vm.global, 'hostError', errHandle);
}

Deterministic Execution

The wasi.now option controls Date.now(), new Date(), and — crucially — the Math.random() PRNG seed. QuickJS uses a xorshift64* PRNG that is seeded once from the clock value during context creation. The now() callback is not called on every Math.random() invocation — it seeds the PRNG at startup, and subsequent calls are purely deterministic from that seed.

This means two VMs created with the same now() value will produce identical Math.random() sequences:

const fixedTime = () => BigInt(1700000000000) * 1_000_000n; // nanoseconds

using vm1 = await QuickJS.create({ wasm: wasmBytes, wasi: { now: fixedTime } });
using vm2 = await QuickJS.create({ wasm: wasmBytes, wasi: { now: fixedTime } });

vm1.evalCode('Math.random()').consume(h => h.toNumber());
// => 0.8130834347906803

vm2.evalCode('Math.random()').consume(h => h.toNumber());
// => 0.8130834347906803 (identical)

The time can also be advanced between calls for realistic behavior:

let currentTime = 1700000000000n;
using vm = await QuickJS.create({
  wasm: wasmBytes,
  wasi: {
    now: () => currentTime * 1_000_000n,
  },
});

vm.evalCode('Date.now()').consume(h => h.toNumber()); // 1700000000000
currentTime += 1000n; // advance 1 second
vm.evalCode('Date.now()').consume(h => h.toNumber()); // 1700000001000

Memory Limits

Restrict how much memory the QuickJS runtime can allocate. When exceeded, allocations fail and surface as JS exceptions:

using vm = await QuickJS.create({
  wasm: wasmBytes,
  memoryLimit: 4 * 1024 * 1024, // 4 MB
});

vm.evalCode(`
  try {
    const huge = new Array(10000000).fill("x".repeat(1000));
  } catch (e) {
    console.log(e.message); // allocation failure
  }
`);

The limit is re-applied after QuickJS.restore(), so you can use a different limit for restored VMs than the original.

Interrupt Handler

Prevent infinite loops and enforce execution timeouts:

const start = Date.now();
using vm = await QuickJS.create({
  wasm: wasmBytes,
  interruptHandler: () => {
    // Return true to interrupt — called periodically during JS execution
    return Date.now() - start > 5000; // 5 second timeout
  },
});

const result = vm.evalCode('while (true) {}');
result.isException; // true — interrupted
result.dispose();

// VM is still usable after an interrupt
vm.evalCode('1 + 2').consume(h => h.toNumber()); // 3

The handler is called approximately once per JS bytecode instruction, so it should be fast. When it returns true, the current execution is interrupted and returns an exception result. The VM remains usable after an interrupt.

Snapshot and Restore

The key differentiator — snapshot the entire VM state and restore it later:

let snapshot: Snapshot;

{
  using vm = await QuickJS.create(wasmBytes);

  // Build up some state, including a pending promise
  vm.unwrapResult(vm.evalCode(`
    globalThis.counter = 0;

    let __resolve;
    globalThis.pendingWork = new Promise(r => { __resolve = r; });
    globalThis.__resolve = __resolve;

    globalThis.pendingWork.then(value => {
      globalThis.counter = value;
    });
  `)).dispose();
  vm.executePendingJobs();

  // Take a snapshot
  snapshot = vm.snapshot();
}

// Serialize to a binary buffer for storage (apply gzip on top for best compression)
const bytes = QuickJS.serializeSnapshot(snapshot);
await storage.put('snapshots/run-123', bytes);

// ... time passes, maybe a different process entirely ...

// Deserialize and restore
const loaded = await storage.get('snapshots/run-123');
const restored = QuickJS.deserializeSnapshot(loaded);

{
  using vm = await QuickJS.restore(restored, wasmBytes);

  // The pending promise still exists — resolve it
  using resolve = vm.global.getProp('__resolve');
  using arg = vm.newNumber(42);
  vm.callFunction(resolve, vm.undefined, arg).dispose();
  vm.executePendingJobs();

  // The .then handler ran in the restored VM
  using counter = vm.global.getProp('counter');
  console.log(counter.toNumber()); // 42
}

Host Callbacks After Restore

Host functions registered with newFunction() are assigned integer IDs that get baked into the snapshot. After restoring, re-register the callbacks:

let snapshot: Snapshot;

{
  using vm = await QuickJS.create(wasmBytes);
  // fn is assigned callback ID 1 (first registered callback)
  using fn = vm.newFunction('hostAdd', (...args) => {
    return vm.newNumber(args[0].toNumber() + args[1].toNumber());
  });
  vm.setProp(vm.global, 'hostAdd', fn);
  snapshot = vm.snapshot();
}

{
  // After restore — re-register with the same ID
  using vm = await QuickJS.restore(snapshot, wasmBytes);
  vm.registerHostCallback(1, (...args) => {
    return vm.newNumber(args[0].toNumber() + args[1].toNumber());
  });

  // hostAdd() works again
  using result = vm.unwrapResult(vm.evalCode('hostAdd(100, 200)'));
  console.log(result.toNumber()); // 300
}

Native WASM Extensions

Load C-based extensions compiled as WASM shared libraries. Extensions link directly against the QuickJS C API with zero marshalling overhead — they share the same linear memory and can register custom classes, prototypes, and globals.

import { QuickJS } from 'quickjs-wasi';
import { readFileSync } from 'fs';

const urlExt = readFileSync('./extensions/url/url.so');

using vm = await QuickJS.create({
  extensions: [{ name: 'url', wasm: urlExt }],
});

using result = vm.unwrapResult(vm.evalCode(`
  const url = new URL('https://example.com:8080/api?key=value#section');
  url.hostname // 'example.com'
`));

Extensions survive snapshot/restore — provide the same extensions when restoring:

const snapshot = vm.snapshot();

using vm2 = await QuickJS.restore(snapshot, {
  extensions: [{ name: 'url', wasm: urlExt }],
});
// URL objects created before the snapshot still work

See EXTENSIONS.md for how to build extensions, how dynamic linking works, and known limitations.

Sandboxed Execution (PAC Files)

quickjs-wasm can be used as a drop-in sandbox for running untrusted code, similar to how pac-resolver uses quickjs-emscripten:

using vm = await QuickJS.create(wasmBytes);

// Inject sandbox functions
{
  using isPlainHostName = vm.newFunction('isPlainHostName', (...args) => {
    const host = args[0].toString();
    return host.includes('.') ? vm.false : vm.true;
  });
  vm.setProp(vm.global, 'isPlainHostName', isPlainHostName);
}

// Evaluate untrusted PAC code
vm.unwrapResult(vm.evalCode(`
  function FindProxyForURL(url, host) {
    if (isPlainHostName(host)) return "DIRECT";
    return "PROXY proxy:8080";
  }
`)).dispose();

// Call it
{
  using fn = vm.global.getProp('FindProxyForURL');
  using url = vm.newString('http://intranet/');
  using host = vm.newString('intranet');
  using result = vm.unwrapResult(vm.callFunction(fn, vm.undefined, url, host));
  console.log(result.toString()); // "DIRECT"
}

API Reference

QuickJS (VM Instance)

| Method | Description | |--------|-------------| | QuickJS.create(options?) | Create a fresh VM instance | | QuickJS.restore(snapshot, options?) | Restore a VM from a snapshot | | QuickJS.serializeSnapshot(snapshot) | Serialize a snapshot to a versioned binary Uint8Array | | QuickJS.deserializeSnapshot(data) | Deserialize a snapshot from a binary Uint8Array | | vm.evalCode(code, filename?) | Evaluate JS code, returns JSValueHandle | | vm.unwrapResult(handle) | Returns the handle if not an exception, otherwise throws | | vm.callFunction(fn, this, ...args) | Call a QuickJS function | | vm.executePendingJobs() | Drain the promise microtask queue | | vm.newString(str) | Create a string value | | vm.newNumber(num) | Create a number value | | vm.newBigInt(val) | Create a BigInt value | | vm.newObject() | Create an empty object | | vm.newArray() | Create an empty array | | vm.newSymbolFor(description) | Create a global symbol (Symbol.for(description)) | | vm.newArrayBuffer(data) | Create an ArrayBuffer from host ArrayBuffer or Uint8Array | | vm.newUint8Array(data) | Create a Uint8Array from host Uint8Array | | vm.newFunction(name, callback) | Create a function backed by a host callback | | vm.newPromise() | Create a Deferred (promise + resolve/reject) | | vm.newError(messageOrError) | Create an Error from a string or native Error | | vm.resolvePromise(handle) | Await a QuickJS promise from the host side | | vm.setProp(obj, key, value) | Set a property (key: string or handle, including symbols) | | vm.getProp(obj, key) | Get a property using a handle key (including symbols) | | vm.typeof(handle) | Get the typeof as a string | | vm.dump(handle) | Convert a QuickJS value to a host value | | vm.hostToHandle(value) | Convert a host value to a QuickJS handle | | vm.snapshot() | Capture the entire VM state (including extension metadata) | | vm.registerHostCallback(id, fn) | Re-register a host callback after restore | | vm.dispose() | Free the VM | | vm[Symbol.dispose]() | Same as dispose() — enables using vm = ... |

QuickJSOptions

| Option | Description | |--------|-------------| | wasm | WASM module bytes or pre-compiled WebAssembly.Module | | wasi | Custom WASI function implementations (now, stdout) | | memoryLimit | Maximum memory the QuickJS runtime can allocate (bytes) | | interruptHandler | Callback to interrupt execution (return true to stop) | | extensions | Array of ExtensionDescriptor objects — native WASM extensions to load |

ExtensionDescriptor

| Property | Description | |----------|-------------| | name | Identifier string (used in snapshot metadata) | | wasm | WASM bytes (BufferSource) or pre-compiled WebAssembly.Module | | initFn? | Init function name (default: qjs_ext_${name}_init) |

Cached Properties

These are singleton handles — do not dispose them:

| Property | Value | |----------|-------| | vm.global | The global object | | vm.undefined | undefined | | vm.null | null | | vm.true | true | | vm.false | false |

JSValueHandle

| Method / Property | Description | |-------------------|-------------| | handle.isException | true if this is an exception result | | handle.isUndefined | true if this is undefined | | handle.isNull | true if this is null | | handle.promiseState | 0 pending, 1 fulfilled, 2 rejected | | handle.toNumber() | Extract as a number | | handle.toBigInt() | Extract as a bigint | | handle.toString() | Extract as a string | | handle.toArrayBuffer() | Extract as an ArrayBuffer (copy from WASM memory) | | handle.toUint8Array() | Extract as a Uint8Array (copy from WASM memory) | | handle.getProp(name) | Get a property by name | | handle.setProp(name, value) | Set a property by name | | handle.consume(fn) | Call fn(handle), then dispose, return result | | handle.dup() | Duplicate the handle (increment refcount) | | handle.dispose() | Free the handle | | handle[Symbol.dispose]() | Same as dispose() — enables using handle = ... |

Deferred (from vm.newPromise())

| Property / Method | Description | |--------------------|-------------| | deferred.handle | The QuickJS promise object | | deferred.settled | Host Promise<void> that resolves on settlement | | deferred.resolve(handle) | Resolve the promise with a QuickJS value | | deferred.reject(handle) | Reject the promise with a QuickJS value |

Data Marshalling

dump() and hostToHandle() automatically convert values between the host and the QuickJS VM. The following types are supported:

| Host Type | QuickJS Type | dump() returns | hostToHandle() accepts | |-----------|-------------|-----------------|------------------------| | undefined | undefined | undefined | undefined | | null | null | null | null | | boolean | boolean | boolean | boolean | | number | number | number | number | | string | string | string | string | | bigint | BigInt | bigint | bigint | | Symbol.for() | global Symbol | Symbol.for(description) | Symbol.for(description) | | Error | Error | Error (with name, message, stack) | Error | | Array | Array | Array (recursive) | Array (recursive) | | ArrayBuffer | ArrayBuffer | ArrayBuffer (copy) | ArrayBuffer | | Uint8Array | Uint8Array | Uint8Array (copy) | Uint8Array | | Other typed arrays | typed array | Corresponding typed array (copy) | ArrayBuffer (via view) | | Promise | Promise | — | QuickJS Promise (bridged via Deferred) | | Plain object | Object | Record<string, unknown> (recursive, own enumerable keys) | Object (recursive) |

Notes:

• Global symbols (Symbol.for()) round-trip as real host Symbol values via Symbol.for(description)
• Local (anonymous) symbols dump as undefined and throw if passed to hostToHandle()
• Functions dump as undefined (cannot be meaningfully serialized)
• Circular and shared references are preserved — dump() returns the same host object for the same QuickJS object pointer
• Only own enumerable string properties are included when dumping objects
• Binary data is always copied between host and WASM memory — there is no zero-copy view API
• dump() for typed arrays determines the host constructor from bytes-per-element (1 → Uint8Array, 2 → Uint16Array, 4 → Uint32Array, 8 → Float64Array)

How It Works

The Core Insight

WebAssembly linear memory is a flat byte array. Everything QuickJS allocates — the runtime struct, all contexts, all JS objects, the GC heap, the atom table, the promise job queue, pending promises — lives in this linear memory. There are no external pointers, file handles, or OS resources. When you copy the memory wholesale to a new WASM instance, all internal pointer relationships are preserved because they reference the same linear address space.

One VM = One WASM Instance

Unlike quickjs-emscripten which has a two-level model (QuickJSWASMModule → QuickJSContext), quickjs-wasm uses a simpler one-level model: each QuickJS.create() call instantiates its own WASM module with its own linear memory, runtime, and context. This gives stronger isolation (no shared memory between VMs) and makes snapshotting clean — one instance, one context, one snapshot.

Architecture

Host (Node.js / Deno / Bun / Browser)
 |
 +-- QuickJS class (ts/index.ts)
 |    |-- evalCode(), callFunction(), newFunction(), ...
 |    |-- snapshot() -> Snapshot { memory, stackPointer, runtimePtr, contextPtr }
 |    +-- restore(snapshot) -> QuickJS
 |
 +-- WASI Shim (ts/wasi-shim.ts)
 |    |-- clock_time_get, fd_write, random_get
 |    +-- fd_close, fd_fdstat_get, fd_seek (stubs)
 |
 +-- quickjs.wasm (1.4 MB)
      |-- QuickJS-NG engine
      +-- C interface layer (c/interface.c)
           |-- Lifecycle, eval, value creation/extraction
           |-- Host callback trampoline (imported host_call)
           +-- Snapshot support (get/set runtime and context pointers)

Host Callback Mechanism

When vm.newFunction() is called, an integer ID is allocated and a QuickJS C function is created via JS_NewCFunctionData2 with that ID stored as function data. When QuickJS code calls the function, the C trampoline extracts the ID and calls the imported host_call(func_id, this_ptr, argc, argv_ptr) function, which dispatches to the registered host callback by ID.

This design survives snapshot/restore: the ID is stored in QuickJS's heap (part of the snapshot), and after restore, registerHostCallback(id, fn) re-maps the ID to a new host function.

Implications for Durable Workflows

| | Event Replay (current) | VM Snapshot (this project) | |---|---|---| | Resumption cost | O(n) — replay full event log | O(1) — restore snapshot + fetch delta | | Event log growth | Unbounded, all events needed | Can be trimmed after snapshot | | Long-running workflows | Impractical at scale | No degradation over time | | State representation | Implicit (derived from log) | Explicit (WASM memory snapshot) | | Snapshot size | N/A | ~256 KB baseline, grows with JS heap | | Determinism requirement | Yes (seeded PRNG, frozen time) | No (state is captured, not re-derived) |

Development

Prerequisites

• wasi-sdk (tested with v30) — set WASI_SDK env var or defaults to /tmp/wasi-sdk
• Node.js >= 22
• pnpm

Building Locally

# Clone with submodules
git clone --recursive https://github.com/vercel-labs/quickjs-wasm.git
cd quickjs-wasm

# Install wasi-sdk (macOS arm64 — adjust URL for your platform)
curl -sL "https://github.com/WebAssembly/wasi-sdk/releases/download/wasi-sdk-30/wasi-sdk-30.0-arm64-macos.tar.gz" \
  | tar xz -C /tmp --strip-components=1 --one-top-level=wasi-sdk

# Install dependencies
pnpm install

# Build WASM binary + TypeScript
pnpm run build

# Run tests
pnpm test

Technical Details

WASM Binary

• Built from quickjs-ng (MIT license)
• Compiled with wasi-sdk targeting wasm32-wasip1 in reactor mode
• 1.4 MB uncompressed
• 7 WASM imports: 6 WASI functions + 1 env.host_call for host callbacks
• Exports memory and __stack_pointer for snapshot support

What Gets Snapshotted

The snapshot captures the entire WASM linear memory, which contains:

• The JSRuntime struct (GC state, job queue, module loader state)
• The JSContext struct (global object, intrinsics, atom table)
• All JS objects (via QuickJS's GC heap)
• The promise job queue (pending .then callbacks)
• The string intern table (atoms)
• The dlmalloc heap metadata
• The C interface's static JSRuntime *rt and static JSContext *ctx globals
• Host callback IDs stored in function data

Plus the __stack_pointer WASM global (a single i32).

Limitations and Future Work

• Snapshot size: Snapshots capture the entire WASM linear memory (~256 KB baseline, grows with heap). Use serializeSnapshot() to get a binary buffer, then apply your own compression (gzip/zstd) — the memory compresses very well due to large zero regions.
• Stack size limit: QuickJS-ng disables JS_SetMaxStackSize on WASI, so deep recursion causes a WASM trap (not a catchable exception).
• ES Modules: Only script-mode eval is supported. import/export and module loaders are not yet wired through.
• Extension ABI: Native WASM extensions use an experimental dynamic linking ABI that is not yet stabilized. All extensions must be compiled with the same wasi-sdk version as the main module. See EXTENSIONS.md for details.

Browser Usage

quickjs-wasi works in browsers — the TypeScript API uses only the standard WebAssembly API and the WASI shim is environment-agnostic. The only Node.js-specific code is the default WASM loading fallback (which uses node:fs). In the browser, pass the WASM bytes directly:

import { QuickJS } from 'quickjs-wasi';

// Fetch the .wasm file and compile it once
const response = await fetch('/quickjs.wasm');
const wasmModule = await WebAssembly.compileStreaming(response);

// Create VMs from the pre-compiled module (fast — no re-compilation)
using vm = await QuickJS.create({ wasm: wasmModule });

See examples/browser/ for a complete Vite demo app.