Build your first webassembly project

Hey folks,

This post is part of a series exploring WebAssembly. Check out the other parts here [link to other parts of the series].

Ever heard of WebAssembly and thought it was too complicated? Today, you'll build a simple WebAssembly project from scratch. We're keeping it super basic: a random hexadecimal color code generator. Think of it as the "Hello, World!" of WebAssembly, but with a colorful twist. You'll learn to write, compile, and use simple WebAssembly code with JavaScript to generate random colors in your browser.

Setting Up WebAssembly

This series uses C and C examples, so we'll use Emscripten to compile C/C to WASM. You can use other languages with their respective compilers.

Emscripten setup is easy. Follow these instructions: https://www.php.cn/link/b1fb3726a5a825732f9a25e210426c4a

macOS users can use Homebrew:

brew install emscripten

Linux users can follow these instructions: https://www.php.cn/link/cdcc686a434d09ed24e4b736d593858f

After installation, verify with:

emcc -v

A Simple Addition Example

Before building our color generator, let's create a simple addition application to understand the WASM workflow. We'll build it in JavaScript first, then convert it to WASM. Create index.html:

<!-- index.html -->
<input type="number" id="num1"> + <input type="number" id="num2"> = <p id="result"></p>
<button onclick="addNumbers()">Add</button>
<script src="add.js"></script>

Now create add.c:

// add.c
int addTwoNumbers(int num1, int num2) {
    return num1 + num2;
}

Compile the C code to WASM:

emcc add.c -o add.js -s EXPORTED_FUNCTIONS="['_addTwoNumbers']" -s "EXTRA_EXPORTED_RUNTIME_METHODS=['ccall', 'cwrap']"

This command:

• Compiles add.c to JavaScript (add.js).
• Exports the addTwoNumbers function for JavaScript use (the underscore is part of the command).
• Exports runtime functions (ccall, cwrap) needed for JavaScript interaction.

This creates add.wasm (the WASM binary) and add.js (the glue code). Modify index.html to include the WASM functionality:

<!-- index.html (modified) -->
<input type="number" id="num1"> + <input type="number" id="num2"> = <p id="result"></p>
<button onclick="addNumbers()">Add</button>
<script>
  const Module = {
    onRuntimeInitialized: () => {
      const addTwoNumbers = Module.cwrap('addTwoNumbers', 'number', ['number', 'number']);
      function addNumbers() {
        const num1 = parseInt(document.getElementById('num1').value);
        const num2 = parseInt(document.getElementById('num2').value);
        document.getElementById('result').innerText = addTwoNumbers(num1, num2);
      }
    }
  };
</script>
<script src="add.js"></script>

Run this using a local server (e.g., python -m http.server, python3 -m http.server, or live-server). Enter two numbers and click "Add" to see the result.

JavaScript handles DOM manipulation, while WASM (C code) performs the addition. The complete code is available here [link to code].

Understanding Hexadecimal

Hexadecimal (base-16) uses 16 symbols (0-9, A-F, where A=10, etc.). Six-digit hex codes represent colors (e.g., #FF5733, #42A5F5). Learn more about hexadecimal colors here [link to explanation].

The Random Color Generator

Create random_color.c:

// random_color.c
#include <stdlib.h>
#include <time.h>

char* intToHex(int num) {
    static char hexStr[3];
    const char hexDigits[] = "0123456789ABCDEF";
    hexStr[0] = hexDigits[(num >> 4) & 0xF];
    hexStr[1] = hexDigits[num & 0xF];
    hexStr[2] = '<pre class="brush:php;toolbar:false"><code class="language-bash">emcc random_color.c -o random_color.js -s EXPORTED_FUNCTIONS="['_generateRandomHexColor']" -s "EXTRA_EXPORTED_RUNTIME_METHODS=['ccall', 'cwrap', 'UTF8ToString']"

'; return hexStr; } char* generateRandomHexColor() { static char color[8]; srand(time(NULL)); color[0] = '#'; char* r = intToHex(rand() % 256); color[1] = r[0]; color[2] = r[1]; char* g = intToHex(rand() % 256); color[3] = g[0]; color[4] = g[1]; char* b = intToHex(rand() % 256); color[5] = b[0]; color[6] = b[1]; color[7] = ''; return color; }

Compile:

brew install emscripten

Create (or modify) your HTML to use this function: [HTML code to call the WASM function and update the background color].

Clicking a button will change the page's background color.

Complete code: [link to code]

Conclusion

We've built a simple WebAssembly application! This demonstrates WebAssembly's power to bring high-performance code to the web. Future posts will explore more advanced applications. Stay tuned!

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