Value jsPromise = context.eval(ID, "Promise.resolve(42);");
Consumer<Object> javaThen = (value)
 -> System.out.println("Resolved from JavaScript: " + value);
jsPromise.invokeMember("then", javaThen);

from asyncio import run,sleep
from typing import TYPE_CHECKING
if TYPE_CHECKING:
 # only for static analysis / editor
 from promise_types import Promise # will not execute at runtime
async def read1():
 print('[🥇 py]', 'reading! test/fixture/index.js')
 r = fs.read('test/fixture/index.js')
 print("awaiting on",r)
 res = await r
 print('[🥇 py]', 'read test/fixture/index.js')
 return res
async def read2():
 print('[🥇 py]', 'reading test1/fixture/hello-world.js')
 r = fs.read('test1/fixture/hello-world.js')
 print("awaiting on",r)
 res = await r
 print('[🥇 py]', 'read test1/fixture/hello-world.js')
 return res
async def main():
 try:
 R1=await read1()
 await sleep(0.5)
 R2=await read2()
 except Exception as er:
 print('[🥇 py] ❌', str(er))
 raise
run(main())

def new_future():
 loop,running=ensure_main_loop()
 fut=loop.create_future()
 return fut

public <T> Value newPromise(BiConsumer<Consumer<T>, Consumer<Throwable>> callback) {
 var pyBindings = context.getBindings("python");
 var new_future = pyBindings.getMember("new_future");
 var future = new_future.execute();
 // create pure Java callbacks that complete the Python future
 Consumer<T> resolve = result -> future.invokeMember("set_result", context.asValue(result));
 Consumer<Throwable> reject = error -> future.invokeMember("set_exception", error);
 // call user lambda with pure Java callbacks
 callback.accept(resolve, reject);
 return future;
 }

 private Object read(Value... arguments) {
 if (arguments.length == 0) {
 throw new RuntimeException("Filename required");
 }
 var filename = arguments[0].asString();
 var encoding = arguments.length > 1 ? arguments[1].asString() : "utf8";
 if (program.DEBUG) {
 System.out.println("{🗿 graal} Starting read " + filename);
 }
 var p = program.Promise.<String>newPromise((BiConsumer<Consumer<String>, Consumer<Throwable>>) (resolve, reject) -> {
 CompletableFuture
 .supplyAsync(() -> {
 try {
 var path = Paths.get(filename);
 if (!Files.exists(path)) {
 var error = new RuntimeException("File not found: " + filename);
 throw error;
 }
 var bytes = Files.readAllBytes(path);
 var content = new String(bytes, getCharset(encoding));
 return content;
 } catch (Exception e) {
 throw new RuntimeException("Failed to read file: " + e.getMessage(), e);
 }
 }, program.virtualThreadExecutor)
 .whenComplete((content, error) -> {
 if (program.DEBUG) {
 if (error != null) {
 System.out.println("{🗿 graal} 📕 Failed read " + filename);
 } else {
 System.out.println("{🗿 graal} 📖 Completed read " + filename);
 }
 }
 final var c = content;
 final var e = error;
 program.loop(() -> {
 if (e != null) {
 var cause = (e.getCause() != null) ? e.getCause() : e;
 reject.accept(e);
 // program.error(cause.getMessage())
 } else {
 resolve.accept(c);
 }
 });
 });
 // return null;
 });
 if (program.DEBUG) {
 System.out.println("{🗿 graal} returning future");
 }
 return p;
 }
 private ProxyObject createFSObject() {
 return ProxyObject.fromMap(new java.util.HashMap<String, Object>() {
 {
 put("read", (ProxyExecutable) MyFileSystem.this::read);
 put("readBuffer", (ProxyExecutable) MyFileSystem.this::readBuffer);
 }
 });
 }

[🥇 py] reading! test/fixture/index.js
{🗿 graal} Starting read test/fixture/index.js
{🗿 graal} 📖 Completed read test/fixture/index.js
{🔁 loop} completed task
{🗿 graal} returning promise
awaiting on <Future finished result='// some js file'>
^ <<< fine on finished futures
[🥇 py] read test/fixture/index.js
[🥇 py] reading test1/fixture/hello-world.js
{🗿 graal} Starting read test1/fixture/hello-world.js
{🗿 graal} 📖 Completed read test1/fixture/hello-world.js
{🔁 loop} completed task
{🗿 graal} returning promise
awaiting on <Future finished result=''>
[🥇 py] read test1/fixture/hello-world.js

[🥇 py] reading! test/fixture/index.js
{🗿 graal} Starting read test/fixture/index.js
{🗿 graal} returning promise
awaiting on <Future pending>
{🗿 graal} 📖 Completed read test/fixture/index.js
... <<<<< does not return to where the promise was awaited