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    0.15.0 • Public • Published


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

    • Safely evaluate untrusted Javascript (up to ES2020).
    • Create and manipulate values inside the QuickJS runtime.
    • Expose host functions to the QuickJS runtime.
    import { getQuickJS } from 'quickjs-emscripten'
    async function main() {
      const QuickJS = await getQuickJS()
      const vm = QuickJS.createVm()
      const world = vm.newString('world')
      vm.setProp(vm.global, '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 Emscripten 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(Date.now() + 1000),
        memoryLimitBytes: 1024 * 1024,

    Interfacing with the interpreter

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

    Each QuickJSVm instance has its own environment, CPU limit, and memory limit. See the documentation for details.

    const vm = QuickJS.createVm()
    let state = 0
    const fnHandle = vm.newFunction('nextId', () => {
      return vm.newNumber(++state)
    vm.setProp(vm.global, 'nextId', fnHandle)
    const nextId = vm.unwrapResult(vm.evalCode(`nextId(); nextId(); nextId()`))
    console.log('vm result:', vm.getNumber(nextId), 'native state:', state)

    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 QuickJSVm.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.createVm()
    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.createVm())
      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 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.createVm()
    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)

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

    More Documentation


    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:

    Status & TODOs

    Both the original project quickjs and this project are still in the early stage of development. There are tests, but I haven't built anything on top of this. Please use this project carefully in a production environment.

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

    Ideas for future work:

    • quickjs-emscripten only exposes a small subset of the QuickJS APIs. Add more QuickJS bindings!
      • Expose tools for object and array iteration and creation.
      • Stretch goals: class support, an event emitter bridge implementation
    • Higher-level abstractions for translating values into (and out of) QuickJS.
    • Remove the singleton limitations. Each QuickJS class instance could create its own copy of the emscripten module.
    • Run quickjs-emscripten inside quickjs-emscripten.
    • Remove the LowLevelJavascriptVm interface and definition. Those types provide no value, since there is no other implementations, and complicate the types and documentation for quickjs-emscripten.
    • Improve our testing strategy by running the tests with each of the Emscripten santizers, as well as with the SAFE_HEAP. This should catch more bugs in the C code. See the Emscripten docs for more details



    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 Intermediate object files from QuickJS end up in ./build/quickjs/{wasm,native}.

    This project uses emscripten 1.39.19. The install should be handled automatically if you're working from Linux or OSX (if using Windows, the best is to use WSL to work on this repository). If everything is right, running yarn embin emcc -v should print something like this:

    emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 1.39.18
    clang version 11.0.0 (/b/s/w/ir/cache/git/chromium.googlesource.com-external-github.com-llvm-llvm--project 613c4a87ba9bb39d1927402f4dd4c1ef1f9a02f7)

    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 run-n builds and runs ./c/test.c

    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.


    npm i quickjs-emscripten

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