abc.js

            var path = string.ptr2str_len(MOD.memory.buffer, offset, len);
            var req = new XMLHttpRequest();
            req.open("GET", path, true);
            req.responseType = "arraybuffer";

            req.onload = function (evt) {
                var arrayBuffer = req.response; // Note: not req.responseText
                if (arrayBuffer) 
                {
                    var byteArray = new Uint8Array(arrayBuffer);
                    var fileSize = byteArray.byteLength;
                    
                    // alloc on D side
                    var ptr = MOD.WA.exports.malloc(fileSize);
                    
                    // copy file content to our buffer
                    var mem = new Uint8Array(MOD.memory.buffer, ptr, arrayBuffer.length);
                    mem.set(byteArray);

                    // notify D side
                    MOD.WA.exports.js_cb_load_file(ctx,func, id, ptr, fileSize);
                }
            };

tmp.d

struct Handle
{
	bool used;
	bool loaded;
	void* data;
    int data_len;
	int id = -1;
	void* user;
}
struct Manager
{
	Handle[64] handles;

	Handle* next()
	{
		for (int i = 0; i < handles.length; i++)
		{
			Handle* handle = &handles[i];
			if (handle.used == false) 
			{
				handle.id = i;
				return handle;
			}
		}
		return null;
	}

	void free(Handle* handle)
	{
		// unload on JS side?
		handle.loaded = false;

		handle.used = false;
		handle.data = null;
        handle.data_len = 0;
        handle.user = null;
	}

	Handle* load(string path)
	{
		Handle* handle = next();
		if (handle) 
		{
			handle.used = true;
			load_file(path, handle.id, &on_cb);
			return handle;
		}
		return null;
	}

	extern(C)void on_cb(int id, void* ptr, int len)
	{
		Handle* handle = &handles[id];
		handle.loaded = true;
		handle.data = ptr;
        handle.data_len = len;


		writeln("Id: {}:{} PTR: {} L: {}", handle.id, id, ptr, len);

        string str = cast(string) ptr[0 .. len];
        writeln("Data content:\n{}", str);
	}	
}