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.

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.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.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.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);

// evalCode() throws a JSException if the evaluated code throws
try {
  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);
}

WASI Overrides

The wasi option lets you override any wasi_snapshot_preview1 host function. It's a factory that receives the WASM linear memory and returns an object of override functions. Overrides apply to both the main module and all loaded extensions.

This is useful for deterministic execution — QuickJS uses a xorshift64* PRNG that is seeded once from the clock value during context creation. Override clock_time_get to control both Date.now() and the Math.random() seed:

const fixedClock = (memory: WebAssembly.Memory) => ({
  clock_time_get(_clockId: number, _precision: bigint, resultPtr: number) {
    new DataView(memory.buffer).setBigUint64(resultPtr, 1700000000000n * 1_000_000n, true);
    return 0;
  },
});

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

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

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

Override random_get to control the crypto extension's RNG:

using vm = await QuickJS.create({
  wasm: wasmBytes,
  wasi: (memory) => ({
    random_get(bufPtr: number, bufLen: number) {
      new Uint8Array(memory.buffer, bufPtr, bufLen).fill(0x42); // deterministic
      return 0;
    },
  }),
  extensions: [cryptoExtension],
});

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

let currentTime = 1700000000000n;
using vm = await QuickJS.create({
  wasm: wasmBytes,
  wasi: (memory) => ({
    clock_time_get(_clockId: number, _precision: bigint, resultPtr: number) {
      new DataView(memory.buffer).setBigUint64(resultPtr, currentTime * 1_000_000n, true);
      return 0;
    },
  }),
});

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
  },
});

try {
  vm.evalCode('while (true) {}');
} catch (err) {
  // JSException — interrupted
  err.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 throws a JSException. The VM remains usable after an interrupt.

Timezone Offset

By default, Date inside the sandbox mirrors the host environment's timezone. You can override this with a fixed offset or a dynamic callback:

// Fixed offset: UTC-8 (480 minutes west of UTC)
using vm = await QuickJS.create({
  wasm: wasmBytes,
  timezoneOffset: 480,
});
vm.evalCode('new Date().getTimezoneOffset()').consume(h => h.toNumber()); // 480

// Force UTC (offset 0)
using vm = await QuickJS.create({
  wasm: wasmBytes,
  timezoneOffset: 0,
});

// Dynamic callback for custom DST-aware logic
using vm = await QuickJS.create({
  wasm: wasmBytes,
  timezoneOffset: (timeSecs) => {
    // Return offset in minutes (getTimezoneOffset convention: positive = west of UTC)
    return new Date(timeSecs * 1000).getTimezoneOffset();
  },
});

The timezoneOffset option accepts:

• 'host' (default) — mirrors the host's timezone, including DST transitions.
• A number — fixed UTC offset in minutes using the getTimezoneOffset() sign convention (positive values are west of UTC, e.g. 480 for UTC-8).
• A callback (timeSecs: number) => number — called with seconds since epoch, must return the offset in minutes. Useful for custom timezone logic. The callback is invoked whenever QuickJS needs to convert between UTC and local time (e.g. getHours(), toString(), new Date(year, month, ...), getTimezoneOffset()), so it may be called multiple times per Date operation.

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.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 keyed by their name, which gets baked into the snapshot. After restoring, re-register the callbacks by name:

let snapshot: Snapshot;

{
  using vm = await QuickJS.create(wasmBytes);
  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 by name
  using vm = await QuickJS.restore(snapshot, wasmBytes);
  vm.registerHostCallback('hostAdd', (...args) => {
    return vm.newNumber(args[0].toNumber() + args[1].toNumber());
  });

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

Note: each call to newFunction() must use a unique name. Attempting to register two host functions with the same name will throw an error.

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.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.

API Reference

QuickJS (VM Instance)

QuickJSOptions

ExtensionDescriptor

Cached Properties

These are singleton handles — do not dispose them:

JSValueHandle

Deferred (from vm.newPromise())

Data Marshalling

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

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(name, fn) is called, a QuickJS C function is created via JS_NewCFunctionData2 with the function name stored as a JS string in func_data[0]. When QuickJS code calls the function, the C trampoline extracts the name and calls the imported host_call(name_ptr, name_len, this_ptr, argc, argv_ptr) function, which dispatches to the registered host callback by name.

This design survives snapshot/restore: the name string is stored in QuickJS's heap (part of the snapshot), and after restore, registerHostCallback(name, fn) re-maps the name to a new host function. Because callbacks are keyed by name rather than sequential integer IDs, the registration order doesn't matter and adding or removing host functions won't silently break restore.

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.

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