quickjs-ng

Python wrapper around quickjs-ng, the actively maintained fork of the QuickJS JavaScript engine.

Drop-in replacement for the archived quickjs package — import quickjs works unchanged.

Installation

pip install quickjs-ng

Requires Python ≥ 3.10. Pre-built wheels are available for Linux (x86_64, i686, aarch64), Windows (AMD64, x86), and macOS (arm64), for CPython 3.10+ and PyPy 3.10/3.11.

Usage

import quickjs

# Evaluate expressions
ctx = quickjs.Context()
ctx.eval("1 + 2")  # => 3

# Call JS functions from Python
ctx.eval("function add(a, b) { return a + b; }")
add = ctx.get("add")
add(3, 4)  # => 7

# Call Python functions from JS
ctx.add_callable("py_add", lambda a, b: a + b)
ctx.eval("py_add(3, 4)")  # => 7

# Use the Function helper for thread-safe execution
f = quickjs.Function("f", "function f(x) { return x * 2; }")
f(21)  # => 42

# Resource limits
ctx.set_memory_limit(1024 * 1024)  # 1 MB
ctx.set_time_limit(5)              # 5 seconds of CPU time
ctx.set_max_stack_size(512 * 1024) # 512 KB stack

Threading

Each Context owns an isolated QuickJS runtime — there is no shared state between contexts. A single Context is not thread-safe and must not be used from multiple threads.

Recommended patterns:

Context per thread — create a separate Context in each thread. No locking, no overhead, full parallelism:

import threading, quickjs

def worker():
    ctx = quickjs.Context()
    print(ctx.eval("1 + 1"))

threads = [threading.Thread(target=worker) for _ in range(4)]
for t in threads: t.start()

Function helper — wraps a Context with a dedicated worker thread and lock, safe to call from any thread:

f = quickjs.Function("f", "function f(x) { return x * 2; }")
# safe to call f(21) from any thread

Pattern	Thread-safe	Overhead
Context per thread	✅	None
`Function` helper	✅	Small (executor dispatch)
Shared `Context`	❌	—

Free-threaded Python (3.13t / 3.14t): Both patterns remain safe. The GIL was never relied upon for thread-safety.

musl / Alpine: Worker threads are automatically created with an 8 MB stack (matching glibc defaults) so set_max_stack_size and deep recursion work correctly on musl-based systems.

Versioning

Version format is X.Y.Z.P where X.Y.Z matches the upstream quickjs-ng tag and P is the wrapper patch (starts at 1 per upstream release). Wheels are built automatically when a new upstream tag is detected.

Development

Requires a C compiler and uv.

git clone --recurse-submodules https://github.com/genotrance/quickjs-ng.git
cd quickjs-ng
make install
make test

Target	Description
`make install`	Create venv, build C extension, install pre-commit
`make test`	Run tests with coverage
`make check`	Run linters and type checking
`make build`	Build sdist and wheel
`make clean`	Remove build artifacts
`make publish`	Publish to PyPI

Contributing

Bug reports and pull requests are welcome at https://github.com/genotrance/quickjs-ng/issues.

Fork and clone with --recurse-submodules.
Run make install to set up the venv, build the C extension, and install pre-commit hooks.
Create a feature branch, make changes, add tests in tests/.
Run make check && make test — all checks must pass.
Open a pull request. CI runs on Ubuntu, Windows, and macOS across Python 3.10–3.14 (including free-threaded 3.13t/3.14t) and PyPy 3.10/3.11.

Documentation

Full technical documentation is in the docs/ folder:

File	Contents
docs/porting.md	Porting history from PetterS/quickjs
docs/c-extension.md	C extension design and stable ABI migration
docs/build.md	Build system: setup.py, pyproject.toml, wheels
docs/ci.md	GitHub Actions workflows
docs/threading.md	Threading model and musl stack fix
docs/testing.md	Test suite layout and memory leak tests
docs/versioning.md	Version scheme and automated stamping
docs/typing.md	Type annotations and mypy configuration

Acknowledgments

This project is a fork of quickjs by Petter Strandmark. The original design of the C bindings, the Function thread-safety wrapper, and the overall API shape are all his work.

The porting to quickjs-ng, the stable ABI migration, the expanded test suite, CICD and all documentation were developed with the assistance of LLMs.

License

MIT

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quickjs-ng 0.15.1.1

quickjs-ng

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