WebAssembly
WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable compilation target for programming languages, enabling deployment on the web for client and server applications.
What detecting WebAssembly tells you about a site
WebAssembly is a binary instruction format that runs near-native compute in the browser, so the detection signal is concrete: a `.wasm` module loaded in the network tab plus the JavaScript glue that instantiates it. Finding it tells you the site offloads heavy work the JavaScript engine handles poorly — image/video processing, 3D, parsers, crypto, emulators, or a whole app compiled from Rust/C++/Go. Its presence flags a deliberate performance decision rather than a default framework choice.
The detail that matters for WebAssembly
The toolchain that produced the module is often readable from the glue code: `wasm-bindgen` points to Rust, an Emscripten runtime points to C/C++, and a large `.wasm` with a Go-specific loader points to TinyGo or Go itself.
WebAssembly in a real-world stack
When you find WebAssembly, it rarely travels alone. A source language toolchain (Rust + wasm-bindgen, Emscripten for C/C++, TinyGo) plus a JavaScript host application around it.
About
WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable compilation target for programming languages, enabling deployment on the web for client and server applications.
Categories: Programming languages
Quick facts
Detection methodology for Programming languages
Programming languages reveal themselves through extension fingerprints (`.php` URLs for PHP, `.aspx` for ASP.NET), framework-specific patterns and (for client-side runtime languages like TypeScript and Sass) the `node_modules/<lang>/package.json` entry inside the sourcemap. Bun, Node.js and Deno on the server side identify themselves via response headers when emitted (`X-Powered-By: Bun`, `X-Bun-Version`).
How we detect WebAssembly
Sourcemap Explorer carries 4 fingerprint signals for WebAssembly, spread across 3 channels — response header, javascript global and script src url. The exact patterns are listed below, and you can replay each one in Chrome DevTools to confirm a match by hand.
Each signal alone is rarely conclusive — Sourcemap Explorer cross-references all of them and weights by confidence. You can reproduce any of these checks yourself in Chrome DevTools.
Server-side fingerprint: the response header reveals the technology behind the page. Visible in DevTools → Network → response headers.
Content-Type: application/wasm
Window-level global the technology installs on page. Reproducible by typing the path into the DevTools console.
window.wasmBinary
window.wasmBinaryFile
Script URL pattern. Typically a CDN host or chunk path that ships with the technology.
/wasm_exec\.js
FAQ
How do I check if a website is using WebAssembly?
Open the page in Chrome, click the Sourcemap Explorer toolbar icon, and read the Stack tab. WebAssembly's specific fingerprints here are response header, javascript global and script src url, and the popup flags WebAssembly whenever any combination of them is found. The same checks can be reproduced manually in DevTools — see the "How we detect" section above.
What WebAssembly version can Sourcemap Explorer detect?
WebAssembly ships as a hosted programming languages rather than a bundled npm package, so version-specific detection isn't always possible. Where the platform leaks a version in response headers (`X-Powered-By`, `Server`, generator meta tags) we surface it; otherwise we report presence only.
Is WebAssembly open source?
Yes, WebAssembly is open source. The license details are listed on the official site (https://webassembly.org/), and the codebase typically lives on a public source-control host.
Where can I read more about WebAssembly?
Official site: https://webassembly.org/. For Sourcemap Explorer's detection guide, see the deep-dive link below or the related guides in the cross-link section.
Keep reading on Sourcemap Explorer
Practical guides
Alternative tools
Detected by Sourcemap Explorer
Open the popup on any page running WebAssemblyand you'll see the exact version pulled from the bundled package.json when sourcemaps are exposed.