quickjs-emscripten

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

- Safely evaluate untrusted Javascript (supports most of ES2023).
- Create and manipulate values inside the QuickJS runtime ([more][values]).
- Expose host functions to the QuickJS runtime ([more][functions]).
- Execute synchronous code that uses asynchronous functions, with [asyncify][asyncify].
- Supports browsers, NodeJS, Deno, Bun, Cloudflare Workers, QuickJS (via [quickjs-for-quickjs][]).

[Github] | [NPM] | [API Documentation][api] | [Variants][core] | [Examples][tests]

``typescript
import { getQuickJS } from "quickjs-emscripten"

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

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

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

vm.dispose()
}

main()
`

[github]: https://github.com/componentor/quickjs-emscripten
[npm]: https://www.npmjs.com/package/quickjs-emscripten
[api]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/packages.md
[tests]: https://github.com/componentor/quickjs-emscripten/blob/main/packages/quickjs-emscripten/src/quickjs.test.ts
[values]: #interfacing-with-the-interpreter
[asyncify]: #asyncify
[functions]: #exposing-apis
[quickjs-for-quickjs]: https://github.com/componentor/quickjs-emscripten/blob/main/packages/quickjs-for-quickjs

- quickjs-emscripten
- Usage
- Safely evaluate Javascript code
- Interfacing with the interpreter
- Runtime
- EcmaScript Module Exports
- Memory Management
- using statement
- Scope
- Lifetime.consume(fn)
- Exposing APIs
- Promises
- context.getPromiseState(handle)
- context.resolvePromise(handle)
- Asyncify
- Async module loader
- Async on host, sync in QuickJS
- Worker Pool (Parallel Execution)
- Browser Setup for Multi-Threading
- WasmFS with OPFS (Native Filesystem)
- File Watching for HMR
- Testing your code
- Packaging
- Reducing package size
- WebAssembly loading
- quickjs-ng
- Using in the browser without a build step
- Debugging
- Supported Platforms
- More Documentation
- Background
- Status \& Roadmap
- Related
- Developing
- The C parts
- The Typescript parts
- pnpm run updates

Usage

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

The root entrypoint of this library is the getQuickJS function, which returns
a promise that resolves to a QuickJSWASMModule 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 in your synchronous code.

$3

See QuickJSWASMModule.evalCode

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

getQuickJS().then((QuickJS) => {
const result = QuickJS.evalCode("1 + 1", {
shouldInterrupt: shouldInterruptAfterDeadline(Date.now() + 1000),
memoryLimitBytes: 1024 * 1024,
})
console.log(result)
})
`

$3

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][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.

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

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

vm.setProp(vm.global, "nextId", fnHandle)
fnHandle.dispose()

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

nextId.dispose()
vm.dispose()
`

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.

#### Runtime

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.

`typescript
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
)
context.unwrapResult(ok).dispose()
// logs "foo.js"
console.log(context.getProp(context.global, "result").consume(context.dump))
context.dispose()
runtime.dispose()
`

#### EcmaScript Module Exports

When you evaluate code as an ES Module, the result will be a handle to the
module's exports, or a handle to a promise that resolves to the module's
exports if the module depends on a top-level await.

`typescript
const context = QuickJS.newContext()
const result = context.evalCode(

export const name = 'Jake'
export const favoriteBean = 'wax bean'
export default 'potato'
,
"jake.js",
{ type: "module" },
)
const moduleExports = context.unwrapResult(result)
console.log(context.dump(moduleExports))
// -> { name: 'Jake', favoriteBean: 'wax bean', default: 'potato' }
moduleExports.dispose()
`

$3

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.

`typescript
const vm = QuickJS.newContext()
const numberHandle = vm.newNumber(42)
// Note: numberHandle not disposed, so it leaks memory.
vm.dispose()
// 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:

#### using statement

The using statement is a Stage 3 (as of 2023-12-29) proposal for Javascript that declares a constant variable and automatically calls the [Symbol.dispose]() method of an object when it goes out of scope. Read more [in this Typescript release announcement][using]. Here's the "Interfacing with the interpreter" example re-written using using:

`typescript
using vm = QuickJS.newContext()
let state = 0

// The block here isn't needed for correctness, but it shows
// how to get a tighter bound on the lifetime of fnHandle.
{
using fnHandle = vm.newFunction("nextId", () => {
return vm.newNumber(++state)
})

vm.setProp(vm.global, "nextId", fnHandle)
// fnHandle.dispose() is called automatically when the block exits
}

using nextId = vm.unwrapResult(vm.evalCode(nextId(); nextId(); nextId()))
console.log("vm result:", vm.getNumber(nextId), "native state:", state)
// nextId.dispose() is called automatically when the block exits
// vm.dispose() is called automatically when the block exits
`

[using]: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-2.html#using-declarations-and-explicit-resource-management

#### Scope

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:

`typescript
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(vm.global, "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(fn)

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():

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

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

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

vm.dispose()
`

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

$3

To add APIs inside the QuickJS environment, you'll need to [create objects][newObject] to
define the shape of your API, and [add properties][setProp] and [functions][newFunction] to those objects
to allow code inside QuickJS to call code on the host.
The [newFunction][] documentation covers writing functions in detail.

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

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

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

[newObject]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/classes/QuickJSContext.md#newobject
[newFunction]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/classes/QuickJSContext.md#newfunction
[setProp]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/classes/QuickJSContext.md#setprop

#### Promises

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,
there are two options:

##### context.getPromiseState(handle)

You can synchronously peek into a QuickJS promise handle and get its state
without introducing asynchronous host code, described by the type [JSPromiseState][]:

`typescript
type JSPromiseState =
| { type: "pending"; error: Error }
| { type: "fulfilled"; value: QuickJSHandle; notAPromise?: boolean }
| { type: "rejected"; error: QuickJSHandle }
`

The result conforms to the SuccessOrFail type returned by context.evalCode, so you can use context.unwrapResult(context.getPromiseState(promiseHandle)) to assert a promise is settled successfully and retrieve its value. Calling context.unwrapResult on a pending or rejected promise will throw an error.

`typescript
const promiseHandle = context.evalCode(Promise.resolve(42))
const resultHandle = context.unwrapResult(context.getPromiseState(promiseHandle))
context.getNumber(resultHandle) === 42 // true
resultHandle.dispose()
promiseHandle.dispose()
`

[JSPromiseState]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/exports.md#jspromisestate

##### context.resolvePromise(handle)

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.

`typescript
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.
promise.settled.then(vm.runtime.executePendingJobs)
return promise.handle
})
readFileHandle.consume((handle) => vm.setProp(vm.global, "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)
promiseHandle.dispose()
const resolvedHandle = vm.unwrapResult(resolvedResult)
console.log("Result:", vm.getString(resolvedHandle))
resolvedHandle.dispose()
`

#### Asyncify

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:

- [newAsyncRuntime][]: create a runtime inside a new WebAssembly module.
- [newAsyncContext][]: create runtime and context together inside a new
WebAssembly module.
- [newQuickJSAsyncWASMModule][]: create an empty WebAssembly module.

[newasyncruntime]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/exports.md#newasyncruntime
[newasynccontext]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/exports.md#newasynccontext
[newquickjsasyncwasmmodule]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/exports.md#newquickjsasyncwasmmodule

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.

`typescript
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 'esm.sh/react@17'
import * as ReactDOMServer from 'esm.sh/react-dom@17/server'
const e = React.createElement
globalThis.html = ReactDOMServer.renderToStaticMarkup(
e('div', null, e('strong', null, 'Hello world!'))
)
)
context.unwrapResult(result).dispose()
const html = context.getProp(context.global, "html").consume(context.getString)
console.log(html) //


`

##### Async on host, sync in QuickJS

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

`typescript
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(context.global, "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'))
data.map(x => x.toUpperCase()).join(' ')
)
const upperCaseData = context.unwrapResult(result).consume(context.getString)
console.log(upperCaseData) // 'VERY USEFUL DATA'
`

$3

For applications that need to run many QuickJS evaluations, the @componentor/quickjs-emscripten-worker-pool package provides a worker pool that can run tasks in parallel across multiple Web Workers.

Key features:

- Parallel execution across multiple workers for CPU-bound tasks
- Graceful degradation to single-threaded mode when SharedArrayBuffer is unavailable
- Same API regardless of threading mode
- Verbose logging to verify multi-threading is working

`bash
npm install @componentor/quickjs-emscripten-worker-pool @componentor/quickjs-singlefile-cjs-release-sync
`

`typescript
import {
newWorkerPool,
isMultiThreadingSupported,
} from "@componentor/quickjs-emscripten-worker-pool"

// Check if multi-threading is available
console.log("Multi-threading:", isMultiThreadingSupported())

// Create pool with verbose logging to verify workers
const pool = await newWorkerPool({
poolSize: 4,
verbose: true, // Logs worker creation and task dispatch
})

console.log("Using real workers:", pool.isMultiThreaded)

// Execute tasks in parallel (or sequentially if SharedArrayBuffer unavailable)
const results = await Promise.all([
pool.evalCode("1 + 1"),
pool.evalCode("2 * 2"),
pool.evalCode("Math.sqrt(16)"),
])

pool.dispose()
`

#### Browser Setup for Multi-Threading

Multi-threaded mode requires Cross-Origin Isolation via HTTP headers:

`
Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp
`

Vite example:

`typescript
// vite.config.ts
export default defineConfig({
server: {
headers: {
"Cross-Origin-Opener-Policy": "same-origin",
"Cross-Origin-Embedder-Policy": "require-corp",
},
},
})
`

Without these headers, the pool automatically falls back to single-threaded mode (sequential execution on the main thread).

See the worker-pool package documentation for full API reference.

$3

For applications that need high-performance filesystem access, the @componentor/quickjs-wasmfs-release-sync variant provides native filesystem operations via WasmFS with OPFS (Origin Private File System) backend.

Key benefits:

- No JS boundary crossing - filesystem operations run entirely inside Wasm
- Persistent storage - files persist across sessions via OPFS
- Faster I/O - eliminates ~1-5ms overhead per filesystem call

`bash
npm install @componentor/quickjs-wasmfs-release-sync quickjs-emscripten-core
`

`typescript
import variant from "@componentor/quickjs-wasmfs-release-sync"
import { newQuickJSWASMModuleFromVariant } from "quickjs-emscripten-core"

const QuickJS = await newQuickJSWASMModuleFromVariant(variant)

// Mount OPFS at /opfs
await QuickJS.getWasmModule().mountOPFS("/opfs")

// Filesystem operations inside QuickJS now access /opfs/* directly
// without crossing the JS↔Wasm boundary
const vm = QuickJS.newContext()
vm.evalCode(
const fs = require('fs')
fs.writeFileSync('/opfs/hello.txt', 'Hello from Wasm!')
const content = fs.readFileSync('/opfs/hello.txt', 'utf-8')
console.log(content)
)
`

#### File Watching for HMR

WasmFS includes a polling-based directory watcher for Hot Module Replacement:

`typescript
const wasm = QuickJS.getWasmModule()

// Watch directory for changes
const stopWatching = wasm.watchDirectory(
"/opfs/src",
(changes) => {
for (const { path, type } of changes) {
console.log(${type}: ${path})
}
},
100, // poll every 100ms
)

// Stop watching when done
stopWatching()
`

Note: WasmFS with OPFS is browser-only (OPFS is a browser API). For more details, see the WasmFS variant documentation and WasmFS Integration Guide.

$3

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. You can also
enable debug logging to help diagnose failures.

`typescript
// Define your test suite in a function, so that you can test against
// different module loaders.
function myTests(moduleLoader: () => Promise) {
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.
QuickJS.assertNoMemoryAllocated()
})

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

describe("Realistic test with QuickJS RELEASE build", () => {
myTests(getQuickJS)
})
`

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

[ts]: https://github.com/componentor/quickjs-emscripten/blob/main/packages/quickjs-emscripten/src/
[debug_sync]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/exports.md#debug_sync
[testquickjswasmmodule]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/classes/TestQuickJSWASMModule.md

$3

The main quickjs-emscripten package includes several build variants of the WebAssembly module:

- RELEASE... build variants should be used in production. They offer better performance and smaller file size compared to DEBUG... build variants.
- RELEASE_SYNC: This is the default variant used when you don't explicitly provide one. It offers the fastest performance and smallest file size.
- RELEASE_ASYNC: The default variant if you need [asyncify][] magic, which comes at a performance cost. See the asyncify docs for details.
- DEBUG... build variants can be helpful during development and testing. They include source maps and assertions for catching bugs in your code. We recommend running your tests with _both_ a debug build variant and the release build variant you'll use in production.
- DEBUG_SYNC: Instrumented to detect memory leaks, in addition to assertions and source maps.
- DEBUG_ASYNC: An [asyncify][] variant with source maps.

To use a variant, call newQuickJSWASMModule or newQuickJSAsyncWASMModule with the variant object. These functions return a promise that resolves to a QuickJSWASMModule, the same as getQuickJS.

`typescript
import {
newQuickJSWASMModule,
newQuickJSAsyncWASMModule,
RELEASE_SYNC,
DEBUG_SYNC,
RELEASE_ASYNC,
DEBUG_ASYNC,
} from "quickjs-emscripten"

const QuickJSReleaseSync = await newQuickJSWASMModule(RELEASE_SYNC)
const QuickJSDebugSync = await newQuickJSWASMModule(DEBUG_SYNC)
const QuickJSReleaseAsync = await newQuickJSAsyncWASMModule(RELEASE_ASYNC)
const QuickJSDebugAsync = await newQuickJSAsyncWASMModule(DEBUG_ASYNC)

for (const quickjs of [
QuickJSReleaseSync,
QuickJSDebugSync,
QuickJSReleaseAsync,
QuickJSDebugAsync,
]) {
const vm = quickjs.newContext()
const result = vm.unwrapResult(vm.evalCode("1 + 1")).consume(vm.getNumber)
console.log(result)
vm.dispose()
quickjs.dispose()
}
`

#### Reducing package size

Including 4 different copies of the WebAssembly module in the main package gives it an install size of about 9.04mb. If you're building a CLI package or library of your own, or otherwise don't need to include 4 different variants in your node_modules, you can switch to the quickjs-emscripten-core package, which contains only the Javascript code for this library, and install one (or more) variants a-la-carte as separate packages.

The most minimal setup would be to install quickjs-emscripten-core and @componentor/quickjs-wasmfile-release-sync (1.3mb total):

`bash
pnpm run add quickjs-emscripten-core @componentor/quickjs-wasmfile-release-sync
du -h node_modules


640K node_modules/@componentor/quickjs-wasmfile-release-sync

80K node_modules/@componentor/quickjs-ffi-types

588K node_modules/quickjs-emscripten-core

1.3M node_modules

`

Then, you can use quickjs-emscripten-core's newQuickJSWASMModuleFromVariant to create a QuickJS module (see [the minimal example][minimal]):

`typescript
// src/quickjs.mjs
import { newQuickJSWASMModuleFromVariant } from "quickjs-emscripten-core"
import RELEASE_SYNC from "@componentor/quickjs-wasmfile-release-sync"
export const QuickJS = await newQuickJSWASMModuleFromVariant(RELEASE_SYNC)

// src/app.mjs
import { QuickJS } from "./quickjs.mjs"
console.log(QuickJS.evalCode("1 + 1"))
`

See the [documentation of quickjs-emscripten-core][core] for more details and the list of variant packages.

[core]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/quickjs-emscripten-core/README.md

#### WebAssembly loading

To run QuickJS, we need to load a WebAssembly module into the host Javascript runtime's memory (usually as an ArrayBuffer or TypedArray) and compile it to a WebAssembly.Module. This means we need to find the file path or URI of the WebAssembly module, and then read it using an API like fetch (browser) or fs.readFile (NodeJS). quickjs-emscripten tries to handle this automatically using patterns like new URL('./local-path', import.meta.url) that work in the browser or are handled automatically by bundlers, or __dirname in NodeJS, but you may need to configure this manually if these don't work in your environment, or you want more control about how the WebAssembly module is loaded.

To customize the loading of an existing variant, create a new variant with your loading settings using newVariant, passing [CustomizeVariantOptions][newVariant]. For example, you need to customize loading in Cloudflare Workers (see [the full example][cloudflare]).

`typescript
import { newQuickJSWASMModule, DEBUG_SYNC as baseVariant, newVariant } from "quickjs-emscripten"
import cloudflareWasmModule from "./DEBUG_SYNC.wasm"
import cloudflareWasmModuleSourceMap from "./DEBUG_SYNC.wasm.map.txt"

/**
* We need to make a new variant that directly passes the imported WebAssembly.Module
* to Emscripten. Normally we'd load the wasm file as bytes from a URL, but
* that's forbidden in Cloudflare workers.
*/
const cloudflareVariant = newVariant(baseVariant, {
wasmModule: cloudflareWasmModule,
wasmSourceMapData: cloudflareWasmModuleSourceMap,
})
`

[newVariant]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten/interfaces/CustomizeVariantOptions.md

#### quickjs-ng

quickjs-ng/quickjs (aka quickjs-ng) is a fork of the original bellard/quickjs under active development. It implements more EcmaScript standards and removes some of quickjs's custom language features like BigFloat.

There are several variants of quickjs-ng available, and quickjs-emscripten may switch to using quickjs-ng by default in the future. See [the list of variants][core].

#### Using in the browser without a build step

You can use quickjs-emscripten directly from an HTML file in two ways:

1. Import it in an ES Module script tag

`html



`

1. In edge cases, you might want to use the IIFE build which provides QuickJS as the global QJS. You should probably use the ES module though, any recent browser supports it.

`html


src="https://cdn.jsdelivr.net/npm/quickjs-emscripten@0.25.0/dist/index.global.js"
type="text/javascript"
>


`

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Debug logging can be enabled globally, or for specific runtimes. You need to use a DEBUG build variant of the WebAssembly module to see debug log messages from the C part of this library.

`typescript
import { newQuickJSWASMModule, DEBUG_SYNC } from "quickjs-emscripten"

const QuickJS = await newQuickJSWASMModule(DEBUG_SYNC)
`

With quickjs-emscripten-core:

`typescript
import { newQuickJSWASMModuleFromVariant } from "quickjs-emscripten-core"
import DEBUG_SYNC from "@componentor/quickjs-wasmfile-debug-sync"

const QuickJS = await newQuickJSWASMModuleFromVariant(DEBUG_SYNC)
`

To enable debug logging globally, call [setDebugMode][setDebugMode]. This affects global Javascript parts of the library, like the module loader and asyncify internals, and is inherited by runtimes created after the call.

`typescript
import { setDebugMode } from "quickjs-emscripten"

setDebugMode(true)
`

With quickjs-emscripten-core:

`typescript
import { setDebugMode } from "quickjs-emscripten-core"

setDebugMode(true)
`

To enable debug logging for a specific runtime, call [setDebugModeRt][setDebugModeRt]. This affects only the runtime and its associated contexts.

`typescript
const runtime = QuickJS.newRuntime()
runtime.setDebugMode(true)

const context = QuickJS.newContext()
context.runtime.setDebugMode(true)
`

[setDebugMode]: doc/@componentor/quickjs-emscripten/README.md#setdebugmode
[setDebugModeRt]: https://github.com/componentor/quickjs-emscripten/blob/main/doc/@componentor/quickjs-emscripten-core/classes/QuickJSRuntime.md#setdebugmode

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quickjs-emscripten and related packages should work in any environment that supports ES2020.

- Browsers: we estimate support for the following browser versions. See the [global-iife][iife] and [esmodule][esm-html] HTML examples.
- Chrome 63+
- Edge 79+
- Safari 11.1+
- Firefox 58+
- NodeJS: requires v16.0.0 or later for WebAssembly compatibility. Tested with node@18. See the [node-typescript][tsx-example] and [node-minimal][minimal] examples.
- Typescript: tested with typescript@4.5.5 and typescript@5.3.3. See the [node-typescript example][tsx-example].
- Vite: tested with vite@5.0.10. See the [Vite/Vue example][vite].
- Create react app: tested with react-scripts@5.0.1. See the [create-react-app example][cra].
- Webpack: tested with webpack@5.89.0 via create-react-app.
- Cloudflare Workers: tested with wrangler@3.22.1. See the [Cloudflare Workers example][cloudflare].
- Deno: tested with deno 1.39.1. See the [Deno example][deno].

[iife]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/global-iife.html
[esm-html]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/esmodule.html
[deno]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/deno
[vite]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/vite-vue
[cra]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/create-react-app
[cloudflare]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/cloudflare-workers
[tsx-example]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/node-typescript
[minimal]: https://github.com/componentor/quickjs-emscripten/blob/main/examples/node-minimal

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[Github] | [NPM] | [API Documentation][api] | [Variants][core] | [Examples][tests]

Background

This was inspired by seeing https://github.com/maple3142/duktape-eval
on Hacker News and Figma's
blogposts about using building a Javascript plugin runtime:

- How Figma built the Figma plugin system: Describes the LowLevelJavascriptVm interface.
- An update on plugin security: Figma switches to QuickJS.

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. SQLite integration.

Related

- Duktape wrapped in Wasm: https://github.com/maple3142/duktape-eval/blob/main/src/Makefile
- QuickJS wrapped in C++: https://github.com/ftk/quickjspp

Developing

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

You will need node, pnpm run, make, and emscripten to build this project.

$3

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 with emscripten (using emcc), to produce WebAssembly.
The version of Emscripten used by the project is defined in templates/Variant.mk.

- On ARM64, you should install emscripten on your machine. For example on macOS, brew install emscripten.
- If _the correct version of emcc_ is not in your PATH, compilation falls back to using Docker.
On ARM64, this is 10-50x slower than native compilation, but it's just fine on x64.

We produce multiple build variants of the C code compiled to WebAssembly using a
template script the ./packages directory. Each build variant uses its own copy of a Makefile
to build the C code. The Makefile is generated from a template in ./templates/Variant.mk.

Related NPM scripts:

- pnpm run vendor:update updates vendor/quickjs and vendor/quickjs-ng to the latest versions on Github.
- pnpm run build:codegen updates the ./packages from the template script ./prepareVariants.ts and Variant.mk.
- pnpm run build:packages builds the variant packages in parallel.

$3

The Javascript/Typescript code is also organized into several NPM packages in ./packages:

- ./packages/quickjs-ffi-types: Low-level types that define the FFI interface to the C code.
Each variant exposes an API conforming to these types that's consumed by the higher-level library.
- ./packages/quickjs-emscripten-core: The higher-level Typescript that implements the user-facing abstractions of the library.
This package doesn't link directly to the WebAssembly/C code; callers must provide a build variant.
- ./packages/quicks-emscripten: The main entrypoint of the library, which provides the getQuickJS function.
This package combines quickjs-emscripten-core with platform-appropriate WebAssembly/C code.

Related NPM scripts:

- pnpm run check runs all available checks (build, format, tests, etc).
- pnpm run build builds all the packages and generates the docs.
- pnpm run test runs the tests for all packages.
- pnpm run test:fast runs the tests using only fast build variants.
- pnpm run doc generates the docs into ./doc.
- pnpm run doc:serve previews the current ./doc in a browser.
- pnpm run prettier formats the repo.

$3

Just run pnpm run set version from sources` to upgrade the pnpm run release.