Hako compiles down to WebAssembly, a memory-safe, sandboxed execution environment. This means even though Hako is written in C/C++, programs it is embedded in have an extra layer of security from memory corruption attacks. Hako also has a built-in sandboxing mechanism in the form of VMContext which allows you to restrict the capabilities of JavaScript code.

A number of APIs are exposed to limit the amount of memory and 'gas' a script can consume before it is terminated, and development follows an opinionated 'fail-fast' design for bailing out of potentially unstable code that could consume or block I/O. Combining all of these, you can run hundreds of VMs in parallel on a single machine.

Hako does not use Emscripten to compile down to WebAssembly; so long as your language of choice has a WebAssembly runtime, you can embed Hako in it by implementing the necessary imports. You can see an example of embedding Hako in Go here.

It is also incredibly tiny. The release build is ~800KB.

Hako is a fork of PrimJS, which in sythentic benchmarks shows performance gains of 28% over QuickJS. Compiling to WebAssembly has no noticeable impact on performance as the amazing JIT compilers of JavaScriptCore/Bun, V8/NodeJS, and Wasmtime allow code to run at near-native speeds. You can enable profiling in Hako to see how your code is performing.

Here's a simple string concatenation benchmark comparing Hako to QuickJS and QuickJS-NG:

const t1 = Date.now()
let str = '';
for (let i = 0; i < 1000_000; i++) {
  str += 'a';
}
const t2 = Date.now()
console.log(t2 - t1);

// Results:
// quickjs-ng: 6854ms
// quickjs: 112ms
// hako: 92ms

As you can see, Hako outperforms both QuickJS and QuickJS-NG in this common operation, with QuickJS-NG being particularly slow at string concatenation.​​​​​​​​​​​​​​​​

This project is still in early access - documentation is a work in progress, and the API/ABI should not be considered stable. If you wish to contribute, please do so by opening an issue or a pull request for further discussion. Your feedback is greatly appreciated.

You can find the intitial reference implementation of Hako in TypeScript here.

For further reading and to get a sense of the roadmap see the initial releasse blog post here.