WebAssembly is a low level code (similar to assembly), which could be written in C/C++ or Rust, compiled (to .wasm file), and run in a browser together with JavaScript.

To create WebAssembly binaries we will use Emscripten compiler.

Installation instructions could be found here. (Note you have to have Python installed on your computer).
In a nutshell it’s very simple:

# Get the emsdk repo
git clone https://github.com/emscripten-core/emsdk.git
# Enter that directory
cd emsdk
# Fetch the latest version of the emsdk (not needed the first time you clone)
git pull
# Download and install the latest SDK tools.
./emsdk install latest
# Make the "latest" SDK "active" for the current user. (writes .emscripten file)
./emsdk activate latest
# Activate PATH and other environment variables in the current terminal:
# on Linux
source ./emsdk_env.sh
# or on Windows
emsdk_env.bat

If yor are using PowerShell on Windows you might need allow script access:
Set-ExecutionPolicy Unrestricted

To verify the installation run
emcc -v
emcc --version

Now is the time to create first Hello World. As a Hello World we will use our first C Hello World

#include <stdio.h>
int main()
{
printf("Hello, World from C\n");
return 0;
}

Save it as hello_world.cpp and to compile run:

emcc hello_world.cpp -o hello_world.html --emrun

(Note first time we will run the compiler it might take a which as it will download additional components)

-o – output we want: could be *.html, *.js, or *.wasm
–emrun – we want testable code

To quickly test our first applicaion we can use Emscripten test tool:

emrun hello_world.html

If everything went correctly you should see a browser open with out test page:

And if you upload all three new files (hello_world.html, hello_world.js, hello_world.wasm) to your web site and point your browser to hello_world.html you should see the same page.

Locally you can also use Python to run web-server: browse to the folder with your page and run:

python -m http.server

If you need just web assembly file use command line parameter -o:

emcc hello_world.cpp -o hello_world.wasm

Emscripten is a wrapper around LLVM compiler, so many it has flags coming from it. For example optimization:

emcc hello_world.cpp -o hello_world.wasm -Oz -std=c++17 //optimized for size
emcc hello_world.cpp -o hello_world.wasm -O3 -std=c++17 //optimized for speed