WebAssembly (WASM) for High Performance Apps · Lesson

Debugging WebAssembly Modules

Utilize browser developer tools and other debugging techniques to inspect and troubleshoot your WASM applications.

Debugging WebAssembly Modules is a free WebAssembly (WASM) for High Performance Apps lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebAssembly (WASM) for High Performance Apps learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Debugging WASM: An Overview

Ever wondered how to peek inside your WebAssembly code while it runs? Debugging WASM lets you find and fix issues in your high-performance modules.

It's a bit different from debugging pure JavaScript, but modern browser developer tools offer powerful capabilities to help you out.

Browser DevTools: Sources Tab

Your browser's developer tools are your best friend for WASM debugging. In Chrome, open DevTools (F12 or Cmd+Option+I).

Navigate to the Sources tab.
Look for your WASM module in the file tree, often under wasm:// or similar.
Without source maps, you'll see raw WebAssembly text format – which is very hard to read!

Source Maps: From WASM to Source

Debugging raw WebAssembly text is incredibly tough! Source maps are special files that link the compiled WASM code back to your original C, C++, or Rust source code.

When present, your browser can display your original source files, allowing you to debug them directly, just like JavaScript.

Rust & wasm-pack Source Maps

When compiling Rust to WASM, tools like wasm-pack can automatically generate source maps. The key is to compile in debug mode.

A simple Rust function that adds one to a number:

// src/lib.rs
#[no_mangle]
pub extern "C" fn add_one(num: i32) -> i32 {
    // This function adds 1 to the input number.
    // It will be exported from the WASM module.
    num + 1
}

Loading WASM & Source Maps

Let's see how a browser loads a WASM module. When compiled with source maps (e.g., wasm-pack build --target web --dev), the browser uses them to show your original Rust/C/C++ files in the Sources tab.

Here's how JavaScript loads a tiny, conceptual WASM module:

async function loadAndRunWasm() {
  // This is a conceptual WASM binary for 'add(a, b) => a + b'
  const wasmCode = new Uint8Array([
    0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00, 0x01, 0x07, 0x01, 0x60,
    0x02, 0x7f, 0x7f, 0x01, 0x7f, 0x03, 0x02, 0x01, 0x00, 0x07, 0x07, 0x01,
    0x03, 0x61, 0x64, 0x64, 0x00, 0x00, 0x0a, 0x09, 0x01, 0x07, 0x00, 0x20,
    0x00, 0x20, 0x01, 0x6a, 0x0b
  ]);

  const wasmModule = await WebAssembly.instantiate(wasmCode, {});
  const add = wasmModule.instance.exports.add;
  console.log("WASM add(5, 7):", add(5, 7)); // Expected: 12
}
loadAndRunWasm();

Pausing Execution with Breakpoints

Just like JavaScript, you can set breakpoints directly in your original C/C++/Rust source files shown in the Sources tab of your browser's DevTools.

Click on the line number in the gutter where you want to pause execution.
When your WASM code executes that line, the browser will pause, letting you inspect variables and the call stack.

Inspect Variables, Memory & Stack

When execution pauses at a breakpoint, you gain powerful insights:

Scope pane: View local variables and function arguments, often with their original names if source maps are good.
Memory Inspector: Use the dedicated 'Memory' tab or right-click a memory address to inspect WASM's linear memory buffer.
Call Stack pane: See the sequence of function calls that led to the current point, including both JavaScript and WASM frames.

Step-by-Step Execution

Once paused at a breakpoint, use the debugger controls to navigate your code line by line:

Step Over (F10): Execute the current line and move to the next.
Step Into (F11): Dive into a function call on the current line.
Step Out (Shift+F11): Finish executing the current function and return to where it was called.
Resume (F8): Continue execution until the next breakpoint or the program ends.

Debugging JS-WASM Interaction

Often, bugs occur at the boundary where JavaScript calls WASM functions or vice-versa. Set breakpoints in both your JavaScript and WASM code.

You can step seamlessly between JS and WASM execution frames in the DevTools call stack, making it easy to trace data flow and identify issues in the interop layer.

Debugging Knowledge Check

Test your understanding of WebAssembly debugging!

Debugging WASM: Recap

Today, you learned how to leverage browser developer tools to debug your WebAssembly applications. We covered:

Using source maps to see original source code.
Setting breakpoints and stepping through execution.
Inspecting variables, memory, and the call stack.
Debugging interactions between JavaScript and WASM.

Mastering these techniques is crucial for building robust and performant WASM applications. Keep practicing!

Frequently asked questions

Is the “Debugging WebAssembly Modules” lesson free?

Yes — the full text of “Debugging WebAssembly Modules” is free to read here on the web, and the WebAssembly (WASM) for High Performance Apps course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebAssembly (WASM) for High Performance Apps course, upgrade to CoddyKit PRO.

What will I learn in “Debugging WebAssembly Modules”?

Utilize browser developer tools and other debugging techniques to inspect and troubleshoot your WASM applications. You practise WebAssembly (WASM) for High Performance Apps with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start WebAssembly (WASM) for High Performance Apps?

No prior experience is required. WebAssembly (WASM) for High Performance Apps on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Debugging WebAssembly Modules” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this WebAssembly (WASM) for High Performance Apps lesson?

Yes. Every WebAssembly (WASM) for High Performance Apps lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Benchmarking WASM Performance
Optimizing Rust Code for WASM
Debugging WebAssembly Modules
SIMD & Multithreading for Maximum Throughput

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