Write a function in C, compile it to WebAssembly, and call it from Node.js (or Bun) and the browser. No Emscripten, no bundlers, just clang and pure WASM.

This guide works on macOS Tahoe as of 2026, and on modern Linux distros.

You need a clang that supports the wasm32 target and the LLVM linker (lld).

On macOS, Apple's built-in clang does not support wasm32, so install Homebrew's LLVM and lld:

brew install llvm lld

On Linux, the distro clang should be fine. Install clang and lld packages.

Here's a basic C function (in fib.c):

int fib(int n) {
  int a = 0, b = 1, c = 0;

  for (int i = 1; i <= n; i++) {
    c = a + b;
    a = b;
    b = c;
  }

  return a;
}

make

The Makefile picks Homebrew's clang on macOS and the distro clang on Linux. You can also run the command directly:

clang fib.c -o fib.wasm \
  --target=wasm32 -nostdlib -fvisibility=default -fuse-ld=lld \
  -Xlinker --no-entry -Xlinker -export-dynamic

On macOS, replace clang with $(brew --prefix llvm)/bin/clang.

node node.js

Serve over HTTP (needed for WebAssembly.instantiateStreaming):

python3 -m http.server 3000

Then open http://localhost:3000/

If you want to go beyond basic C, things get complicated fast which is why lots of tooling dedicated to this task exists. Good luck!