How to call a JavaScript function from C++?

Calling a JavaScript function directly from C++ depends on the environment and the system; for this, make sure you have embedded a JavaScript engine or integrated C++ with JavaScript.

In this article, we will be using Emscripten (C++ to JavaScript in WebAssembly) to call a JavaScript function from C++. For this, you have to compile the C++ program to WebAssembly using Emscripten and then call JavaScript functions from C++. So, first, create the C++ file with the header <emscripten.h>.

How Emscripten Works

Emscripten compiles C++ code to WebAssembly, which runs in browsers. The compiled WebAssembly module can interact with JavaScript through defined interfaces, allowing bidirectional function calls between C++ and JavaScript.

C++ File

Let's consider that this file name is saved as main.cpp.

#include <emscripten.h>

// here declared an external JS function
extern "C" {
    extern void jsFunction();
}

int main() {
    // calling the JS function from C++
    jsFunction();
    return 0;
}

JavaScript Function File

This is a JS function that is called from a C++ file. Consider it to be saved as the name lib.js.

mergeInto(LibraryManager.library, {
  jsFunction: function() {
    console.log('Hello from JavaScript!');
  }
});

Compilation Process

Now, to compile this, open the command prompt and go to your project folder.

cd path\to\your\project

Then run this command:

emcc main.cpp -s EXPORTED_FUNCTIONS="['_main']" -s EXTRA_EXPORTED_RUNTIME_METHODS="['ccall', 'cwrap']" --js-library lib.js -o output.html

Output

After compilation, open the generated output.html file in a browser. The output will be displayed in the browser's developer console:

Hello from JavaScript!

Alternative Approaches

Key Points

• Emscripten requires the extern "C" declaration to prevent C++ name mangling
• JavaScript functions must be defined in a library file using mergeInto
• The compiled output includes HTML, JavaScript, and WebAssembly files
• Function calls are synchronous within the WebAssembly environment

Conclusion

Emscripten provides an effective way to call JavaScript functions from C++ in web environments. For server-side applications, consider Node.js Native Addons or embedded JavaScript engines like V8.