WebAssembly

WebAssembly (Wasm) lets you run compiled code inside JavaScript. With jsrun, you can load Wasm modules and call them from JavaScript, all within your Python application.

Why Use WebAssembly?

WebAssembly offers portable, near-native performance. Use it when:

• Performance matters: CPU-intensive tasks (image processing, crypto, compression)
• Porting native code: Reuse C/C++/Rust libraries without rewriting in JavaScript
• Sandboxing: Run untrusted code with strict memory isolation
• Cross-platform: Same compiled code runs on various Wasm runtimes

Loading WebAssembly

JavaScript's WebAssembly API works in jsrun:

from jsrun import Runtime

with Runtime() as runtime:
    # Load a .wasm file from Python
    # https://webassembly.js.org/example-add.wasm
    with open("add.wasm", "rb") as f:
        wasm_bytes = f.read()

    # Pass to JavaScript as Uint8Array
    runtime.bind_object("wasm", {"bytes": wasm_bytes})

    # Instantiate and use the module
    result = runtime.eval("""
        const module = new WebAssembly.Module(wasm.bytes);
        const instance = new WebAssembly.Instance(module);
        instance.exports.add(10, 20);
    """)
    print(result)  # 30

The Wasm module's exported functions are available on instance.exports.

Async Instantiation

For better performance, use async instantiation:

import asyncio

async def main():
    with Runtime() as runtime:
        with open("add.wasm", "rb") as f:
            runtime.bind_object("wasm", {"bytes": f.read()})

        result = await runtime.eval_async("""
            (async () => {
                const module = await WebAssembly.compile(wasm.bytes);
                const instance = await WebAssembly.instantiate(module);
                return instance.exports.add(5, 7);
            })()
        """)
        print(result)  # 12

asyncio.run(main())

This is non-blocking and recommended for large Wasm modules.

Limitations

• No file system: Wasm can't access files directly. Pass data via JavaScript bindings.
• No threads: V8's Wasm doesn't support threads (SharedArrayBuffer-based parallelism). Use multiple runtimes instead.
• Binary portability: Wasm is portable, but some features (SIMD, etc.) depend on V8 version.

Next Steps

• Learn about Inspector to debug Wasm modules with DevTools
• Explore Bindings to pass data between Python and Wasm via JavaScript
• Check out WebAssembly.org for tools and specifications