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* 32767 functions good, 32768 functions BAD

* change `magic` to uin16_t (avoids signed intereger overflow)

* type magic as uint32_t, add simple test

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Javascript/Typescript bindings for QuickJS, a modern Javascript interpreter, compiled to WebAssembly.

  • Safely evaluate untrusted Javascript (up to ES2020).
  • Create and manipulate values inside the QuickJS runtime (more).
  • Expose host functions to the QuickJS runtime (more).
  • Execute synchronous code that uses asynchronous functions, with asyncify.

Github | NPM | API Documentation | Examples

import { getQuickJS } from "quickjs-emscripten"

async function main() {
  const QuickJS = await getQuickJS()
  const vm = QuickJS.newContext()

  const world = vm.newString("world")
  vm.setProp(, "NAME", world)

  const result = vm.evalCode(`"Hello " + NAME + "!"`)
  if (result.error) {
    console.log("Execution failed:", vm.dump(result.error))
  } else {
    console.log("Success:", vm.dump(result.value))




Install from npm: npm install --save quickjs-emscripten or yarn add quickjs-emscripten.

The root entrypoint of this library is the getQuickJS function, which returns a promise that resolves to a QuickJS singleton when the QuickJS WASM module is ready.

Once getQuickJS has been awaited at least once, you also can use the getQuickJSSync function to directly access the singleton engine in your synchronous code.

Safely evaluate Javascript code

See QuickJS.evalCode

import { getQuickJS, shouldInterruptAfterDeadline } from "quickjs-emscripten"

getQuickJS().then((QuickJS) => {
  const result = QuickJS.evalCode("1 + 1", {
    shouldInterrupt: shouldInterruptAfterDeadline( + 1000),
    memoryLimitBytes: 1024 * 1024,

Interfacing with the interpreter

You can use QuickJSContext to build a scripting environment by modifying globals and exposing functions into the QuickJS interpreter.

Each QuickJSContext instance has its own environment -- globals, built-in classes -- and actions from one context won't leak into other contexts or runtimes (with one exception, see Asyncify).

Every context is created inside a QuickJSRuntime. A runtime represents a Javascript heap, and you can even share values between contexts in the same runtime.

const vm = QuickJS.newContext()
let state = 0

const fnHandle = vm.newFunction("nextId", () => {
  return vm.newNumber(++state)

vm.setProp(, "nextId", fnHandle)

const nextId = vm.unwrapResult(vm.evalCode(`nextId(); nextId(); nextId()`))
console.log("vm result:", vm.getNumber(nextId), "native state:", state)


When you create a context from a top-level API like in the example above, instead of by calling runtime.newContext(), a runtime is automatically created for the lifetime of the context, and disposed of when you dispose the context.


The runtime has APIs for CPU and memory limits that apply to all contexts within the runtime in aggregate. You can also use the runtime to configure EcmaScript module loading.

const runtime = QuickJS.newRuntime()
// "Should be enough for everyone" -- attributed to B. Gates
runtime.setMemoryLimit(1024 * 640)
// Limit stack size
runtime.setMaxStackSize(1024 * 320)
// Interrupt computation after 1024 calls to the interrupt handler
let interruptCycles = 0
runtime.setInterruptHandler(() => ++interruptCycles > 1024)
// Toy module system that always returns the module name
// as the default export
runtime.setModuleLoader((moduleName) => `export default '${moduleName}'`)
const context = runtime.newContext()
const ok = context.evalCode(`
import fooName from './foo.js'
globalThis.result = fooName
// logs "foo.js"
console.log(context.getProp(, "result").consume(context.dump))

Memory Management

Many methods in this library return handles to memory allocated inside the WebAssembly heap. These types cannot be garbage-collected as usual in Javascript. Instead, you must manually manage their memory by calling a .dispose() method to free the underlying resources. Once a handle has been disposed, it cannot be used anymore. Note that in the example above, we call .dispose() on each handle once it is no longer needed.

Calling QuickJSContext.dispose() will throw a RuntimeError if you've forgotten to dispose any handles associated with that VM, so it's good practice to create a new VM instance for each of your tests, and to call vm.dispose() at the end of every test.

const vm = QuickJS.newContext()
const numberHandle = vm.newNumber(42)
// Note: numberHandle not disposed, so it leaks memory.
// throws RuntimeError: abort(Assertion failed: list_empty(&rt->gc_obj_list), at: quickjs/quickjs.c,1963,JS_FreeRuntime)

Here are some strategies to reduce the toil of calling .dispose() on each handle you create:


A Scope instance manages a set of disposables and calls their .dispose() method in the reverse order in which they're added to the scope. Here's the "Interfacing with the interpreter" example re-written using Scope:

Scope.withScope((scope) => {
  const vm = scope.manage(QuickJS.newContext())
  let state = 0

  const fnHandle = scope.manage(
    vm.newFunction("nextId", () => {
      return vm.newNumber(++state)

  vm.setProp(, "nextId", fnHandle)

  const nextId = scope.manage(vm.unwrapResult(vm.evalCode(`nextId(); nextId(); nextId()`)))
  console.log("vm result:", vm.getNumber(nextId), "native state:", state)

  // When the withScope block exits, it calls scope.dispose(), which in turn calls
  // the .dispose() methods of all the disposables managed by the scope.

You can also create Scope instances with new Scope() if you want to manage calling scope.dispose() yourself.


Lifetime.consume is sugar for the common pattern of using a handle and then immediately disposing of it. Lifetime.consume takes a map function that produces a result of any type. The map fuction is called with the handle, then the handle is disposed, then the result is returned.

Here's the "Interfacing with interpreter" example re-written using .consume():

const vm = QuickJS.newContext()
let state = 0

vm.newFunction("nextId", () => {
  return vm.newNumber(++state)
}).consume((fnHandle) => vm.setProp(, "nextId", fnHandle))

vm.unwrapResult(vm.evalCode(`nextId(); nextId(); nextId()`)).consume((nextId) =>
  console.log("vm result:", vm.getNumber(nextId), "native state:", state)


Generally working with Scope leads to more straight-forward code, but Lifetime.consume can be handy sugar as part of a method call chain.

Exposing APIs

To add APIs inside the QuickJS environment, you'll need to create objects to define the shape of your API, and add properties and functions to those objects to allow code inside QuickJS to call code on the host.

By default, no host functionality is exposed to code running inside QuickJS.

const vm = QuickJS.newContext()
// `console.log`
const logHandle = vm.newFunction("log", (...args) => {
  const nativeArgs =
  console.log("QuickJS:", ...nativeArgs)
// Partially implement `console` object
const consoleHandle = vm.newObject()
vm.setProp(consoleHandle, "log", logHandle)
vm.setProp(, "console", consoleHandle)

vm.unwrapResult(vm.evalCode(`console.log("Hello from QuickJS!")`)).dispose()


To expose an asynchronous function that returns a promise to callers within QuickJS, your function can return the handle of a QuickJSDeferredPromise created via context.newPromise().

When you resolve a QuickJSDeferredPromise -- and generally whenever async behavior completes for the VM -- pending listeners inside QuickJS may not execute immediately. Your code needs to explicitly call runtime.executePendingJobs() to resume execution inside QuickJS. This API gives your code maximum control to schedule when QuickJS will block the host's event loop by resuming execution.

To work with QuickJS handles that contain a promise inside the environment, you can convert the QuickJSHandle into a native promise using context.resolvePromise(). Take care with this API to avoid 'deadlocks' where the host awaits a guest promise, but the guest cannot make progress until the host calls runtime.executePendingJobs(). The simplest way to avoid this kind of deadlock is to always schedule executePendingJobs after any promise is settled.

const vm = QuickJS.newContext()
const fakeFileSystem = new Map([["example.txt", "Example file content"]])

// Function that simulates reading data asynchronously
const readFileHandle = vm.newFunction("readFile", (pathHandle) => {
  const path = vm.getString(pathHandle)
  const promise = vm.newPromise()
  setTimeout(() => {
    const content = fakeFileSystem.get(path)
    promise.resolve(vm.newString(content || ""))
  }, 100)
  // IMPORTANT: Once you resolve an async action inside QuickJS,
  // call runtime.executePendingJobs() to run any code that was
  // waiting on the promise or callback.
  return promise.handle
readFileHandle.consume((handle) => vm.setProp(, "readFile", handle))

// Evaluate code that uses `readFile`, which returns a promise
const result = vm.evalCode(`(async () => {
  const content = await readFile('example.txt')
  return content.toUpperCase()
const promiseHandle = vm.unwrapResult(result)

// Convert the promise handle into a native promise and await it.
// If code like this deadlocks, make sure you are calling
// runtime.executePendingJobs appropriately.
const resolvedResult = await vm.resolvePromise(promiseHandle)
const resolvedHandle = vm.unwrapResult(resolvedResult)
console.log("Result:", vm.getString(resolvedHandle))


Sometimes, we want to create a function that's synchronous from the perspective of QuickJS, but prefer to implement that function asynchronously in your host code. The most obvious use-case is for EcmaScript module loading. The underlying QuickJS C library expects the module loader function to return synchronously, but loading data synchronously in the browser or server is somewhere between "a bad idea" and "impossible". QuickJS also doesn't expose an API to "pause" the execution of a runtime, and adding such an API is tricky due to the VM's implementation.

As a work-around, we provide an alternate build of QuickJS processed by Emscripten/Binaryen's ASYNCIFY compiler transform. Here's how Emscripten's documentation describes Asyncify:

Asyncify lets synchronous C or C++ code interact with asynchronous [host] JavaScript. This allows things like:

  • A synchronous call in C that yields to the event loop, which allows browser events to be handled.

  • A synchronous call in C that waits for an asynchronous operation in [host] JS to complete.

Asyncify automatically transforms ... code into a form that can be paused and resumed ..., so that it is asynchronous (hence the name “Asyncify”) even though [it is written] in a normal synchronous way.

This means we can suspend an entire WebAssembly module (which could contain multiple runtimes and contexts) while our host Javascript loads data asynchronously, and then resume execution once the data load completes. This is a very handy superpower, but it comes with a couple of major limitations:

  1. An asyncified WebAssembly module can only suspend to wait for a single asynchronous call at a time. You may call back into a suspended WebAssembly module eg. to create a QuickJS value to return a result, but the system will crash if this call tries to suspend again. Take a look at Emscripten's documentation on reentrancy.

  2. Asyncified code is bigger and runs slower. The asyncified build of Quickjs-emscripten library is 1M, 2x larger than the 500K of the default version. There may be room for further optimization Of our build in the future.

To use asyncify features, use the following functions:

These functions are asynchronous because they always create a new underlying WebAssembly module so that each instance can suspend and resume independently, and instantiating a WebAssembly module is an async operation. This also adds substantial overhead compared to creating a runtime or context inside an existing module; if you only need to wait for a single async action at a time, you can create a single top-level module and create runtimes or contexts inside of it.

Async module loader

Here's an example of valuating a script that loads React asynchronously as an ES module. In our example, we're loading from the filesystem for reproducibility, but you can use this technique to load using fetch.

const module = await newQuickJSAsyncWASMModule()
const runtime = module.newRuntime()
const path = await import("path")
const { promises: fs } = await import("fs")

const importsPath = path.join(__dirname, "../examples/imports") + "/"
// Module loaders can return promises.
// Execution will suspend until the promise resolves.
runtime.setModuleLoader((moduleName) => {
  const modulePath = path.join(importsPath, moduleName)
  if (!modulePath.startsWith(importsPath)) {
    throw new Error("out of bounds")
  console.log("loading", moduleName, "from", modulePath)
  return fs.readFile(modulePath, "utf-8")

// evalCodeAsync is required when execution may suspend.
const context = runtime.newContext()
const result = await context.evalCodeAsync(`
import * as React from ''
import * as ReactDOMServer from ''
const e = React.createElement
globalThis.html = ReactDOMServer.renderToStaticMarkup(
  e('div', null, e('strong', null, 'Hello world!'))
const html = context.getProp(, "html").consume(context.getString)
console.log(html) // <div><strong>Hello world!</strong></div>
Async on host, sync in QuickJS

Here's an example of turning an async function into a sync function inside the VM.

const context = await newAsyncContext()
const path = await import("path")
const { promises: fs } = await import("fs")

const importsPath = path.join(__dirname, "../examples/imports") + "/"
const readFileHandle = context.newAsyncifiedFunction("readFile", async (pathHandle) => {
  const pathString = path.join(importsPath, context.getString(pathHandle))
  if (!pathString.startsWith(importsPath)) {
    throw new Error("out of bounds")
  const data = await fs.readFile(pathString, "utf-8")
  return context.newString(data)
readFileHandle.consume((fn) => context.setProp(, "readFile", fn))

// evalCodeAsync is required when execution may suspend.
const result = await context.evalCodeAsync(`
// Not a promise! Sync! vvvvvvvvvvvvvvvvvvvv 
const data = JSON.parse(readFile('data.json')) => x.toUpperCase()).join(' ')
const upperCaseData = context.unwrapResult(result).consume(context.getString)
console.log(upperCaseData) // 'VERY USEFUL DATA'

Testing your code

This library is complicated to use, so please consider automated testing your implementation. We highly writing your test suite to run with both the "release" build variant of quickjs-emscripten, and also the DEBUG_SYNC build variant. The debug sync build variant has extra instrumentation code for detecting memory leaks.

The class TestQuickJSWASMModule exposes the memory leak detection API, although this API is only accurate when using DEBUG_SYNC variant.

// Define your test suite in a function, so that you can test against
// different module loaders.
function myTests(moduleLoader: () => Promise<QuickJSWASMModule>) {
  let QuickJS: TestQuickJSWASMModule
  beforeEach(async () => {
    // Get a unique TestQuickJSWASMModule instance for each test.
    const wasmModule = await moduleLoader()
    QuickJS = new TestQuickJSWASMModule(wasmModule)
  afterEach(() => {
    // Assert that the test disposed all handles. The DEBUG_SYNC build
    // variant will show detailed traces for each leak.

  it("works well", () => {
    // TODO: write a test using QuickJS
    const context = QuickJS.newContext()
    context.unwrapResult(context.evalCode("1 + 1")).dispose()

// Run the test suite against a matrix of module loaders.
describe("Check for memory leaks with QuickJS DEBUG build", () => {
  const moduleLoader = memoizePromiseFactory(() => newQuickJSWASMModule(DEBUG_SYNC))

describe("Realistic test with QuickJS RELEASE build", () => {

For more testing examples, please explore the typescript source of quickjs-emscripten repository.


  • Switch to a DEBUG build variant of the WebAssembly module to see debug log messages from the C part of this library.
  • Set process.env.QTS_DEBUG to see debug log messages from the Javascript part of this library.

More Documentation

Github | NPM | API Documentation | Examples


This was inspired by seeing on Hacker News and Figma's blogposts about using building a Javascript plugin runtime:

Status & Roadmap

Stability: Because the version number of this project is below 1.0.0, *expect occasional breaking API changes.

Security: This project makes every effort to be secure, but has not been audited. Please use with care in production settings.

Roadmap: I work on this project in my free time, for fun. Here's I'm thinking comes next. Last updated 2022-03-18.

  1. Further work on module loading APIs:

    • Create modules via Javascript, instead of source text.
    • Scan source text for imports, for ahead of time or concurrent loading. (This is possible with third-party tools, so lower priority.)
  2. Higher-level tools for reading QuickJS values:

    • Type guard functions: context.isArray(handle), context.isPromise(handle), etc.
    • Iteration utilities: context.getIterable(handle), context.iterateObjectEntries(handle). This better supports user-level code to deserialize complex handle objects.
  3. Higher-level tools for creating QuickJS values:

    • Devise a way to avoid needing to mess around with handles when setting up the environment.
    • Consider integrating quickjs-emscripten-sync for automatic translation.
    • Consider class-based or interface-type-based marshalling.
  4. EcmaScript Modules / WebAssembly files / Deno support. This requires me to learn a lot of new things, but should be interesting for modern browser usage.

  5. SQLite integration.



This library is implemented in two languages: C (compiled to WASM with Emscripten), and Typescript.

The C parts

The ./c directory contains C code that wraps the QuickJS C library (in ./quickjs). Public functions (those starting with QTS_) in ./c/interface.c are automatically exported to native code (via a generated header) and to Typescript (via a generated FFI class). See ./generate.ts for how this works.

The C code builds as both with emscripten (using emcc), to produce WASM (or ASM.js) and with clang. Build outputs are checked in, so you can iterate on the Javascript parts of the library without setting up the Emscripten toolchain.

Intermediate object files from QuickJS end up in ./build/quickjs/.

This project uses emscripten 3.1.7 via Docker. You will need a working docker install to build the Emscripten artifacts.

Related NPM scripts:

  • yarn update-quickjs will sync the ./quickjs folder with a github repo tracking the upstream QuickJS.
  • yarn make-debug will rebuild C outputs into ./build/wrapper
  • yarn make-release will rebuild C outputs in release mode, which is the mode that should be checked into the repo.

The Typescript parts

The ./ts directory contains Typescript types and wraps the generated Emscripten FFI in a more usable interface.

You'll need node and npm or yarn. Install dependencies with npm install or yarn install.

  • yarn build produces ./dist.
  • yarn test runs the tests.
  • yarn test --watch watches for changes and re-runs the tests.

Yarn updates

Just run yarn set version from sources to upgrade the Yarn release.


Javascript/Typescript bindings for QuickJS, a modern Javascript interpreter written in C by Fabrice Bellard.








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