WebAssembly - Debugging WASM in Firefox

Introduction to WebAssembly Debugging

Hello there, aspiring programmers! Today, we're going to embark on an exciting journey into the world of WebAssembly debugging. Don't worry if you're new to this – we'll start from the very basics and work our way up. By the end of this tutorial, you'll be debugging WebAssembly like a pro in Firefox!

What is WebAssembly?

Before we dive into debugging, let's understand what WebAssembly (WASM) actually is. WebAssembly is a binary instruction format designed for efficient execution in web browsers. It's like a secret language that computers understand, allowing programs to run faster on web pages.

Imagine you're trying to teach your dog a new trick. You could explain it in long, complicated sentences (like JavaScript), or you could use short, simple commands (like WebAssembly). That's the difference – WebAssembly is more direct and faster for computers to understand.

Setting Up Your Environment

Installing Firefox Developer Edition

To debug WebAssembly, we'll be using Firefox Developer Edition. It's like Firefox's cooler, more tech-savvy cousin. Let's get it installed:

Visit the Firefox Developer Edition website
Click the download button
Follow the installation instructions

Once installed, you'll have a powerful tool at your fingertips for debugging WASM.

Creating a Simple WebAssembly Project

Now, let's create a basic WebAssembly project to work with. We'll start with a simple "Hello, World!" example.

<!DOCTYPE html>
<html>
<head>
    <title>WASM Debug Test</title>
</head>
<body>
    <h1 id="greeting">Loading...</h1>
    <script>
        WebAssembly.instantiateStreaming(fetch('hello.wasm'))
            .then(obj => {
                document.getElementById('greeting').textContent = 
                    obj.instance.exports.hello();
            });
    </script>
</body>
</html>

This HTML file loads a WebAssembly module (hello.wasm) and calls its hello function to set the text of our heading.

Now, let's create our WebAssembly module. We'll use a language called AssemblyScript, which is like TypeScript but compiles to WebAssembly.

// hello.ts
export function hello(): string {
    return "Hello, WebAssembly World!";
}

To compile this to WebAssembly, you'll need to set up AssemblyScript, but for now, let's assume we have our hello.wasm file ready.

Debugging WASM in Firefox

Opening Developer Tools

First things first, let's open our developer tools in Firefox:

Right-click anywhere on your web page
Select "Inspect Element"
Click on the "Debugger" tab

You should now see a panel with your source code and various debugging tools.

Setting Breakpoints

Breakpoints are like pause buttons for your code. They let you stop execution at specific points to examine what's happening. Here's how to set one:

In the Sources panel, find your WebAssembly file (it might be named something like "wasm://wasm/00a3f...")
Click on the line number where you want to pause execution
A blue marker will appear, indicating your breakpoint

Stepping Through Code

Once you've set a breakpoint, refresh your page. When the code hits your breakpoint, it will pause. Now you can step through your code:

Button	Action	Description
Step Over	F10	Executes the current line and moves to the next one
Step Into	F11	Enters into a function call
Step Out	Shift + F11	Completes the current function and steps out

Examining Variables

While stepping through your code, you can examine the values of variables:

Look at the right-hand panel in the Debugger
You'll see sections for "Scopes" and "Variables"
Expand these to see the current values of your variables

This is incredibly useful for understanding what's happening in your code at each step.

Advanced Debugging Techniques

Using the Console

The console is your best friend when debugging. You can use it to log information or even execute code on the fly.

Try adding this to your HTML file:

<script>
    console.log("WASM module loaded!");
</script>

Now, when you open the Console tab in Developer Tools, you'll see this message when your WASM module loads.

Debugging Memory

WebAssembly has direct access to memory, which can be both powerful and tricky. Firefox allows you to inspect this memory:

In the Debugger, look for a section called "Wasm Memory"
You'll see a hexadecimal view of your module's memory
You can even edit this memory directly!

Remember, with great power comes great responsibility. Be careful when modifying memory directly!

Conclusion

Congratulations! You've taken your first steps into the world of WebAssembly debugging. We've covered setting up your environment, creating a simple WASM project, and using Firefox's powerful debugging tools to examine and step through your code.

Remember, debugging is as much an art as it is a science. The more you practice, the better you'll become at tracking down those elusive bugs. Don't be discouraged if it seems challenging at first – even experienced developers spend a good chunk of their time debugging.

Keep experimenting, keep learning, and most importantly, have fun! WebAssembly opens up a whole new world of possibilities for web development, and you're now equipped to explore it. Happy coding!

Credits: Image by storyset

Previous Tutorial:

WebAssembly - Javascript API

Next Tutorial:

WebAssembly - “Hello World”