What Debugging JavaScript on WebAssembly Looks Like

As WebAssembly (Wasm) continues to evolve from a browser-only technology into a powerful runtime for server-side and edge computing, developers are increasingly interested in running JavaScript inside Wasm environments. But what does debugging JavaScript in this context actually look like? Engineers from Microsoft and Fermyon recently demonstrated how SpiderMonkey — Mozilla’s JavaScript engine — can be integrated and debugged within Wasm runtimes.

Why JavaScript on WebAssembly?

WebAssembly offers a secure, fast, and portable execution model. While languages like Rust and C++ are commonly compiled to Wasm, JavaScript — traditionally interpreted in the browser — is now being explored as a guest language within Wasm runtimes. This opens doors for:

• Running JS in serverless environments
• Embedding JS in microservices or plugin systems
• Creating lightweight scripting layers for Wasm apps

SpiderMonkey Meets Wasm

SpiderMonkey is Mozilla’s open-source JavaScript engine, written in C++. It supports modern ECMAScript features and includes a debugger API. Microsoft and Fermyon engineers have shown how SpiderMonkey can be compiled to WebAssembly and embedded into Wasm runtimes like Spin (Fermyon’s framework for serverless apps).

Compiling SpiderMonkey to Wasm

To compile SpiderMonkey to WebAssembly, engineers use Emscripten — a toolchain that compiles C/C++ to Wasm. The process involves:

emcc -Iinclude -o spidermonkey.wasm spidermonkey.cpp \
  -s WASM=1 \
  -s EXPORTED_FUNCTIONS="['_js_eval', '_js_debug']" \
  -s ALLOW_MEMORY_GROWTH=1

This produces a Wasm module that can be loaded into a host runtime and invoked via exported functions.

Debugging JavaScript Inside Wasm

Debugging JavaScript inside a Wasm runtime is fundamentally different from debugging in the browser. Here’s what it typically involves:

1. Embedding Debug Hooks

SpiderMonkey provides a C++ API for debugging. When compiled to Wasm, these hooks can be exposed to the host runtime:

JS_SetDebuggerHandler(cx, my_debug_callback);

The host application can then receive events like breakpoints, exceptions, and stack traces.

2. Console Output and Logging

Since traditional browser consoles aren’t available, logging is redirected to the host environment:

console.log("Debug info: ", someVariable);

In practice, this might be implemented as a call from Wasm to the host via WASI or custom bindings.

3. Source Maps and Stack Traces

To make debugging easier, source maps can be embedded or referenced, allowing stack traces to point to original JS code rather than Wasm internals.

Challenges and Limitations

• Performance overhead from embedding a full JS engine
• Limited tooling compared to browser devtools
• Complexity of mapping Wasm memory to JS objects

Future Directions

As Wasm matures, we can expect better debugging support, including:

• Standardized debugging protocols for Wasm runtimes
• Improved integration with IDEs and remote debuggers
• Lightweight JS engines optimized for Wasm

Conclusion

Debugging JavaScript on WebAssembly is still an emerging practice, but the work by Microsoft and Fermyon shows promising progress. By embedding SpiderMonkey and exposing its debugging APIs, developers can begin to treat Wasm as a viable runtime for JavaScript — not just for execution, but for full development workflows.