To call WebAssembly functions from JavaScript, the WebAssembly JavaScript API makes it pretty straightforward.

The API exposes a few objects in the global namespace, such as WebAssembly.Module, WebAssembly.Instance, and WebAssembly.Memory. Here's how you can use them:

Step 1: Load your WebAssembly module. You can use JavaScript's fetch API to load your module and convert it into an ArrayBuffer.

Step 2: Convert the module into a WebAssembly.Module. Use the WebAssembly.Module constructor for this.

Step 3: Create an instance of your module. The WebAssembly.Instance constructor takes a module and optionally a set of imports.

Step 4: Call your function. The exports property of your instance contains your WebAssembly functions, now callable as JavaScript functions.

This technique of calling wasm functions from JavaScript is useful as it brings the computational speed of WebAssembly into JavaScript apps. However, interoperation between JS and wasm can sometimes lead to overhead, and debugging is more complicated since you're stepping into the WebAssembly world from the JavaScript land. That's all about calling wasm functions from JavaScript!

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#include <iostream>

using namespace std;

int main() {

  // Call from JS:

  // const wasmModule = await WebAssembly.instantiateStreaming(fetch('hello.wasm'), {});

  // wasmModule.instance.exports._Z5hellov();

  cout << 'Hello, JavaScript!';

}