library.js

mergeInto(LibraryManager.library, {
    js_output_result: function (symbol, data, polygon, polygon_size) {
        // function provided by Emscripten to convert WASM heap string pointers to JS strings.
        const Pointer_stringify = Module["UTF8ToString"];

        // Note: new TypedArray(someBuffer) will create a new view onto the same memory chunk, 
        // while new TypedArray(someTypedArray) will copy the data so the original can be freed.
        const resultView = new Int32Array(
            Module.HEAP32.buffer,
            polygon,
            polygon_size * 2
        );
        const coordinates = new Int32Array(resultView);

        // call the downstream processing function that should have been set by the client code
        const downstreamProcessor = Module["processResult"];
        if (downstreamProcessor == null) {
            throw new Error("No downstream processing function set");
        }
        downstreamProcessor(
            Pointer_stringify(symbol),
            Pointer_stringify(data),
            coordinates
        );
    }
});