WebAssembly: A Friendly Introduction

Introduction to WebAssembly

What is WebAssembly?

WebAssembly, often abbreviated as “wasm”, is a binary instruction format designed as a portable target for compiling high-level languages like C, C++, and Rust. In addition, WebAssembly was intended as a low-level virtual machine that runs code at near-native speed in web browsers using standard hardware capabilities.

Why WebAssembly is important

WebAssembly provides a new level of performance and flexibility for web developers. It allows for more efficient code execution than JavaScript, making it an ideal choice for computationally heavy tasks or real-time applications. Moreover, WebAssembly enables developers to use their preferred languages for web development, opening up new possibilities for code reuse and cross-platform development.

Table of Contents:

WebAssembly: The Basics

WebAssembly Concepts

WebAssembly consists of a few core concepts:

  • Modules: A WebAssembly module is a binary file containing compiled code and data.
  • Instances: An instance represents a module’s code, memory, and other resources loaded into a specific runtime environment.
  • Functions: Functions are the basic building blocks of WebAssembly code, and they can be exported and called from JavaScript or other WebAssembly modules.
  • Linear Memory: WebAssembly uses a linear memory model, where memory is represented as a contiguous array of bytes with a fixed size.

WebAssembly vs JavaScript

While WebAssembly and JavaScript can coexist and complement each other, there are some key differences:

  • Performance: WebAssembly code executes faster than JavaScript, making it suitable for performance-critical tasks.
  • Language Support: WebAssembly allows developers to use languages other than JavaScript, enabling them to leverage existing codebases or use languages better suited for specific tasks.
  • Binary Format: WebAssembly uses a compact binary format faster to decode and execute than JavaScript’s text-based format.
  • Strong Typing: WebAssembly is a strongly typed language, which can lead to more predictable performance and improved optimization by the compiler.

WebAssembly Features

Some of the critical features of WebAssembly include:

  • Portability: WebAssembly is designed to be a portable target for compiling high-level languages.
  • Security: WebAssembly runs inside a sandboxed environment, providing isolation and security guarantees similar to JavaScript.
  • Integration: WebAssembly is designed to integrate seamlessly with the existing web platform, allowing developers to leverage JavaScript APIs and interact with the DOM.
  • Compactness: The binary format of WebAssembly is designed to be compact and efficient, both in terms of file size and execution speed.

Setting Up Your WebAssembly Development Environment

Required Tools and Libraries

To get started with WebAssembly development, you’ll need the following tools:

  • A WebAssembly-capable browser (e.g., Chrome, Firefox, Safari, or Edge)
  • A integrated development environment (IDE) for writing code
  • The Emscripten SDK, which includes tools for compiling C/C++ code to WebAssembly
  • Use Node.js and npm, if you want additional development tools or libraries

Installing the WebAssembly Binary Toolkit (WABT)

The WebAssembly Binary Toolkit (WABT) is a set of tools for working with WebAssembly binary files. To install WABT, follow the instructions on the official GitHub repository.

Configuring Your Browser for WebAssembly

WebAssembly has been supported by all major browsers since 2017, including Chrome, Firefox, Safari, and Edge. In most cases, browsers are configured to enable WebAssembly by default, but there may be instances where you need to allow WebAssembly support manually. However, in this section, we’ll discuss some steps you can take to ensure your browser is set up correctly to run WebAssembly code and to troubleshoot any issues you may encounter.

Verifying WebAssembly Support

To check if your browser supports WebAssembly, you can visit the official WebAssembly website or use the following JavaScript code snippet:

if (typeof WebAssembly === 'object') {
  console.log('WebAssembly is supported by your browser.');
} else {
  console.log('WebAssembly is not supported by your browser.');

If your browser does not support WebAssembly, consider updating it to the latest version or switching to another browser that supports WebAssembly.

Enabling Experimental Features

Although WebAssembly is supported by default in modern browsers, some experimental features may be hidden behind feature flags. To enable these features, you can follow these steps for each browser:

  • Chrome: Navigate to chrome://flags, search for “WebAssembly”, and enable any relevant features.
  • Firefox: Navigate to about:config, search for “WebAssembly”, and toggle any relevant features.
  • Safari: Enable the “Develop” menu in the “Advanced” tab of the “Preferences” window, and then navigate to “Develop” > “Experimental Features” to enable any relevant WebAssembly features.
  • Edge: Navigate to edge://flags, search for “WebAssembly”, and enable any relevant features.

Remember that enabling experimental features may cause instability or other issues in your browser. So use caution and only allow the features you need.

Debugging WebAssembly Issues

If you encounter issues when running WebAssembly code in your browser, you can use the browser’s built-in developer tools to help diagnose and fix the problem. In addition, most browsers provide debugging and profiling tools designed explicitly for WebAssembly, such as support for source maps, which allow you to debug your WebAssembly code at the source level.

To access the developer tools, right-click on the page and select “Inspect” or “Inspect Element”. This will open the developer tools panel in a separate window or panel. From there, you can navigate to the “Console” tab to view any error messages or warnings related to WebAssembly, and to the “Sources” or “Debugger” tab to set breakpoints and step through your WebAssembly code.

Creating a Simple WebAssembly Module

Writing a C Function

To create a simple WebAssembly module, let’s start by writing a C function that we’ll compile to WebAssembly. First, create a file called example.c with the following content:

#include <emscripten.h>

void call_showAlert() {
    showAlert('Hello from WebAssembly!');

int main() {
  return 0;

Compiling to WebAssembly

To compile the C function to WebAssembly, we’ll use the Emscripten compiler. Run the following command:

emcc example.c -s WASM=1 -O3 -s "EXPORTED_RUNTIME_METHODS=['cwrap']" -o example.js

This command will generate two files: example.wasm (the WebAssembly binary) and example.js (a JavaScript loader for the WebAssembly module).

Loading and Running the WebAssembly Module

Now, let’s create an HTML file called index.html to load and run our WebAssembly module:

<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>WebAssembly Example</title>
  <h1>WebAssembly: A Friendly Introduction</h1>

  <button onclick="callWebAssemblyFunction()">Call WebAssembly Function</button>

  <!-- Include the generated JavaScript file -->
  <script src="main.js"></script>
    // showAlert function called from C code
    function showAlert(message) {

    // Load the WebAssembly module
    Module.onRuntimeInitialized = function() {
      console.log('WebAssembly module initialized');

    // Optional: Function to call the WebAssembly function from the button
    function callWebAssemblyFunction() {
      // Get the exported function from the WebAssembly module
      const call_showAlert = Module.cwrap('call_showAlert', 'void', []);
      // Call the WebAssembly function

Running the Web Server

Serve the files using a local web server. For our example, we are using Python’s built-in HTTP server. So, for instance, in the same directory as your index.html, example.js, and example.wasm files, run the following command:

python -m http.server 8080

Navigate to http://localhost:8080 in your browser. The index.html file should load, executing the WebAssembly code and showing the alert dialogue with the message ‘Hello from WebAssembly!’.

Interacting with JavaScript and the DOM

Calling JavaScript Functions from WebAssembly

To call a JavaScript function from your WebAssembly code, you can use the EM_ASM macro provided by Emscripten. For example, if you want to call a JavaScript function showAlert from your C code, you can do the following:

#include <emscripten.h>

void call_showAlert() {
     showAlert('Hello from WebAssembly!');

Calling WebAssembly Functions from JavaScript

As we saw earlier, you can use the Module.cwrap function provided by Emscripten to call WebAssembly functions from JavaScript. This function takes three arguments: the function’s name to call, the return type, and an array of argument types.

Manipulating the DOM with WebAssembly

Although WebAssembly does not have direct access to the DOM, you can still manipulate the DOM indirectly by calling JavaScript functions that interact with the DOM. For example, you can create a JavaScript function that updates the DOM and then call this function from your WebAssembly code using the EM_ASM macro.

Working with WebAssembly Memory

Understanding Linear Memory

WebAssembly uses a linear memory model, where memory is represented as a contiguous array of bytes with a fixed size. This memory can be accessed and modified using WebAssembly instructions or JavaScript.

Sharing Memory Between WebAssembly and JavaScript

To share memory between WebAssembly and JavaScript, you can use the Module.HEAP* arrays provided by Emscripten. These arrays represent different views of the WebAssembly memory, allowing you to read and write values using different data types (e.g., HEAP8 for 8-bit integers, HEAP32 for 32-bit integers).

Optimizing WebAssembly Performance

WebAssembly Optimization Techniques

Some optimization techniques for WebAssembly include:

  • Using Web Workers to run WebAssembly code off the main thread.
  • Minimizing the size of the WebAssembly binary by removing unused code and compressing the binary.
  • Using WebAssembly-specific optimization flags during compilation.

Debugging and Profiling WebAssembly Code

To debug and profile WebAssembly code, you can use the browser’s built-in developer tools. These tools support source maps, allowing you to debug and profile your WebAssembly code at the source level.

Real-World Applications of WebAssembly

Use Cases and Examples

Some use cases for WebAssembly include:

  • High-performance web applications, such as games and multimedia editors.
  • Running existing desktop applications in the browser.
  • Implementing performance-critical parts of web frameworks and libraries.

Many popular web frameworks and libraries are starting to use WebAssembly to improve performance and enable new features. For example, the popular JavaScript framework Blazor uses WebAssembly to run .NET code in the browser.


The Future of WebAssembly

WebAssembly is an exciting technology that has the potential to revolutionize web development. As it continues to evolve and gain support from browser vendors and the developer community, we can expect to see more powerful and flexible web applications built using WebAssembly.

Getting Involved in the WebAssembly Community

If you happen to be interested in learning more about WebAssembly or contributing to its development, you can join the WebAssembly community on GitHub, participate in online forums and mailing lists, and attend conferences and meetups dedicated to WebAssembly.

If you’re interested in further exploring the world of web development and cloud technologies, I invite you to read our previous articles. For those looking to dive deeper into specific WASM technologies, check out our comprehensive guide on Python in WebAssembly with Pyodide or our hands-on tutorial on Golang WebAssembly. These articles cover the basics and provide practical code examples to enhance your understanding and help you start your journey toward mastering web development and cloud technologies.


Faizan Bashir

Principal Engineer | Architecting and building distributed applications in the Cloud | Adventurer

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