How to use Qt WebAssembly – The Complete Guide with demo

Hey, welcome back to another blog post. Today we’re going to talk about the new Qt WebAssembly. This post will help you understand the basics and get practical information and examples to get started quickly. We’ll cover what this is and why you should use it, and then we’ll build some cool demos together.

As a team with years of experience in Qt development — from cross-platform desktop apps to embedded systems — we’re always looking for new ways to push the technology further. If you’re considering Qt WebAssembly for your next project and need support, guidance or a development partner, feel free to contact us.

Enough chit-chat, let’s get started!

 

Why you should care

As a Qt developer I always felt that with this tech stack I can do projects for almost all target platforms, from bare metal hardware, microcontrollers to full desktop and mobile applications. I could even create a computer game 😎

There was one area that I always thought I had covered — web applications. Until recently I was under the impression that the only option was HTML+CSS+JS + [insert your favourite JS-based framework here].

Luckily I was wrong and I checked out how Qt Web Assembly works, and oh boy, now I rule them all! It took some trial and error to get here — and I’ll also highlight where you might go wrong if you’re just getting started. We’ll cover what to watch out for so you don’t repeat mine.

 
 

But before I start explaining the benefits of this approach, let me explain what the whole web assembly even is.

 

What is Web Assembly in general?

Before we dive in, you should learn what WebAssembly actually is — understanding this will help you make the right technical decisions later.

WebAssembly (Wasm) is a low-level, binary instruction format designed to run code in web browsers with near-native performance. It was created to address one of the key limitations of traditional web technologies: the difficulty of executing complex, computationally intensive applications efficiently within the browser environment.

Unlike JavaScript, which is interpreted and dynamically typed, WebAssembly is compiled from languages such as C, C++ or Rust into a binary format that can be loaded and executed by the browser at high speed.

 

Everything in one place – *.wasm file

At the heart of every WebAssembly application is the .wasm file. This file contains the actual code that the browser will execute. But instead of being human-readable like JavaScript or HTML, it’s a binary format—compact, efficient and designed for fast execution.

You can think of a .wasm file as the compiled version of your program, much like how C++ code is compiled into an .exe or .out file for native platforms. The difference is that .wasm is designed to run inside the browser’s WebAssembly runtime, which is built into all major modern browsers.

The .wasm file itself is produced during the build process using the Emscripten toolchain. Emscripten compiles C or C++ source code into an intermediate representation, then translates it into the WebAssembly binary format.

 
 

Why you should use Web Assembly

• Near-native performance – Compiled to binary, WebAssembly executes much faster than JavaScript, making it ideal for demanding tasks like graphics, simulations or data processing.

 

• Portability – A key advantage of WebAssembly is its portability. The same .wasm file can run in all modern browsers, regardless of the underlying operating system or hardware.

 

• Secure sandboxed environment – WebAssembly runs in a browser sandbox, with no direct access to the system, enhancing security and isolating potential risks.

 

• Works with JavaScript – WebAssembly modules can be called from JavaScript (and vice versa), enabling hybrid apps where only the performance-critical parts use Wasm.

 

• Code reuse – Existing C/C++ libraries can be compiled to .wasm, allowing reuse of proven native code without rewriting it for the web.

 

Limitations and Downsides of WebAssembly

While WebAssembly is powerful, it’s not without its drawbacks. Before you jump into using it for production apps, consider the following:

 

• Limited browser integration – Unlike JavaScript, WebAssembly doesn’t have direct access to the DOM or browser APIs. You often need to bridge functionality through JavaScript, which can add complexity.

 

• Larger initial payload size – WebAssembly modules are typically heavier than JavaScript bundles. Although they execute faster, the initial load time may be longer, especially on slow connections.

 

• Debugging can be harder – Debugging .wasm files is not as straightforward as working with JS or TypeScript. While source maps exist, the tooling is still maturing.

 

• Single-threaded unless configured properly- Without proper setup (e.g., cross-origin isolation), multithreading won’t work. Some hosting platforms don’t support the required headers by default.

 

• Not for every use case- If your app is mostly UI-driven and doesn’t rely on native code performance, a modern JS framework might be a simpler, more efficient choice.

 

• Security risks for poorly sandboxed builds- While WASM runs in a sandbox, misconfigured apps or libraries can still expose attack vectors, especially if you’re compiling large native codebases.

Getting started with Qt for WebAssembly

Now that we have the theory out of the way, let’s get to something more interesting. However, we will first prepare our environment and the necessary things.

Prerequisites & Environment setup

To use Qt for WebAssembly, you need:

 

• Qt with WebAssembly support- preferably Qt 6+
• Emscripten SDK (emsdk)- a compiler toolchain that converts C and C++ code into WebAssembly.
• Python 3- used later for running a simple local web server
• Any modern web browser- Chrome, Firefox, Safari or Edge — all current versions support WebAssembly.

The whole process of setting up the environment is well described in this article of the official documentation, so I won’t duplicate it and we will move on.

Building a sample Qt WebAssembly app

Now that we have the environment ready, let’s try WebAssembly.

 

Qt WebAssembly Demo: Brain Viewer in Action

Instead of writing an app from scratch, try opening a project you have already written before. I will open one of our demos – Brain Viewer. If you are not familiar with it, I invite you to read its case study. But in short, it’s an app I developed to display a 3D model of the human brain.

Let’s see how Web Assembly will handle this tough challenge!

 

 

Compile & deploy

Ok, so first thing is to open Qt Creator and your project. Once your app is ready, you can build it for WebAssembly directly from Qt Creator, if the Qt for WebAssembly kit is installed and configured correctly. Qt Creator simplifies the process by handling the underlying toolchain and build steps, so you can focus on development rather than low-level configuration.

 

1. In the Projects panel, select the WebAssembly kit (e.g., Qt 6.x WebAssembly).
2. Choose the desired build configuration (typically Release)
3. Click Build or press Ctrl+B to start the build process.
4. Watch how it works!

  

After a successful build, Qt Creator will generate a set of files in your build directory:

 

• .wasm – the compiled WebAssembly binary,
• .js – JavaScript runtime loader,
• .html – a basic HTML page to launch the app,
• .data – application resources, if any.

These files can be deployed to any HTTP/HTTPS server and accessed via a modern browser. By default Qt creator runs localhost server on port 30000.

 

Common Mistakes to Avoid

A frequent mistake is attempting to build without a properly configured WebAssembly kit or missing Emscripten from your PATH. Make sure all tools are installed and up to date. If your app fails to load, check the browser console for missing files or CORS errors.

 

Single-threaded vs Multi-threaded Kits

When selecting a WebAssembly kit in Qt Creator, you may see options for single-threaded and multi-threaded builds. This choice affects how the application is compiled and how it behaves at runtime.

  

• Single-threaded: This is the most compatible mode. The application runs entirely on the browser’s main thread and does not rely on WebAssembly threads or shared memory. Works in all modern browsers, even in environments with stricter security policies (e.g., without SharedArrayBuffer support). However, limits concurrency—features like QThread or background processing are not available.

 

• Multi-threaded: This build enables support for threading in WebAssembly, allows the application to use features like QThread or parallel rendering. Relies on SharedArrayBuffer, which requires cross-origin isolation and specific HTTP headers (COOP and COEP). More powerful, but requires additional configuration on the server and may not be supported in all hosting environments.

Use single-threaded for broader compatibility and simpler deployment, and multi-threaded if your application relies on concurrency and you can control the hosting environment.

 

Integrating Your WASM Build into a Web App

In simple cases, the generated HTML file can be opened directly via a local server. In production, these files are often embedded into a larger web application or custom UI.

Typical integration steps:

 

• Put the generated files in your web app’s public directory,
• Load the JavaScript runtime (.js) that initializes and runs the .wasm file,
• Embed the application in a custom <canvas> or <iframe> element in your page.

Qt WebAssembly applications are drawn into an HTML <canvas>, managed automatically by the runtime script. The generated .html file is a basic wrapper, you can use as-is or replace with your web page – just make sure the JS runtime is loaded and a canvas element is present for the app to render into.

  

Note: While the auto-generated HTML is helpful, many users choose to integrate the WebAssembly module into an existing web app for more control. Make sure to document each integration step to help others learn from your experience.

 

Running a simple server

For demonstration purposes, let’s assume we have a simple website and want to embed our demo on it. I wrote a minimal example of a Python HTTP server that supports cross-origin isolation, which is required for multithreaded WebAssembly:

 

from http.server import SimpleHTTPRequestHandler, HTTPServer

class CustomHandler(SimpleHTTPRequestHandler):
    def end_headers(self):
        # Adds necessary headers for enabling WebAssembly multithreading
        self.send_header("Cross-Origin-Opener-Policy", "same-origin")
        self.send_header("Cross-Origin-Embedder-Policy", "require-corp")
        super().end_headers()

    def do_GET(self):
        # Serves a specific HTML file as the default entry point
        if self.path == '/':
            self.path = '/appBrainViewer.html'
        return super().do_GET()

PORT = 8080

with HTTPServer(("", PORT), CustomHandler) as httpd:
    print(f"Serving at http://0.0.0.0:{PORT}")
    httpd.serve_forever()

This local server handles every incoming HTTP request and serves your WebAssembly files with the correct headers. It’s a great way for users to test their build locally without deploying to production.

What this does:

 

• Serves files over HTTP on port 8080* Forces the browser to treat the context as isolated (required for WebAssembly threads)
• Redirects / to a specific HTML file (e.g., your app’s main entry point)

This script allows you to test your app locally with full WASM features enabled, including multithreading where applicable.

 

Summary

That’s all from today’s article. If you’re curious as to what other (perhaps as yet uncreated) platforms you can run Qt application to, then keep an eye on our blog, until next time!

We hope this article helped you find the information you were looking for. If you agree that Qt WebAssembly opens exciting new possibilities, give our demo a try or request a consult from our team.