Calling async function without await

[mspertus]

If I call an async function without await, I am used to immediately getting a promise back as illustrated when I call f() with the following JavaScript on playcode.

var x;
async function f() {
  await new Promise(resolve => setTimeout(resolve, 10000));
  x = 'p resolved';
}

console.info(f()); // Prints immediately

However, when I call Engine.evaluate(f()), it doesn't return the promise until it has been resolved many seconds later

        var engine = new Engine();

        var p = engine.RegisterPromise();
        var prom = p.Promise as JsPromise;
        var result = engine.SetValue("p", p.Promise);
        engine.SetValue("setTimeout",
            (Action action, int ms) =>
            {
                Task.Delay(ms).ContinueWith(_ => action());
            });
        engine.Execute(Engine.PrepareScript(@"
            var x;
            async function f() {
                await new Promise(resolve => setTimeout(resolve, 10000));
                x = 'p resolved';
            }"));
        var v = engine.Evaluate("f()"); // Doesn't return for 10 seconds

Is there any way I can call f() in Jint and immediately get the promise back? My program is single threaded, and I can't block.

As more context, my actual use case uses ManualPromise rather than Task.Delay, in which case I can make analogous examples work with then() but not await, because the call to engine.Evaluate() never returns, so I can't resolve the promise in my thread. I did look at some of the discussions of this, but wasn't able to figure out how to reproduce the behavior of immediately evaluating an async function to a pending promise. I apologize if that was covered and I missed it.

Thanks, Mike

[lahma]

You could check if task wrapping would help here:

jint/Jint.Tests/Runtime/AsyncTests.cs

Line 38 in 8750190

Engine engine = new(options => options.ExperimentalFeatures = ExperimentalFeature.TaskInterop);

[mspertus]

Thanks for the response, Lahma. Unfortunately, setting that has no effect.

Here is an illustration that calling an async function without await blocks until the promise is settled in Jint instead of returning a pending promise.

In the following code, x ends up with the value p resolved - f() called - in Jint but gets the value f() called - p resolved - in equivalent code on Playcode, so I'm inclined to consider this a bug. Do you agree? Again, the setting of the option has no effect

            var x='';
            async function f() {
                await new Promise(resolve => setTimeout(resolve, 10000));
                x += 'p resolved -';
            };
            f();
            x += 'f() called - ';

[lahma]

Might very well be a bug, there a couple of open issues in the tracker here related to async resolving.

[mspertus]

Thanks, Lahma. While it may be related to an existing issue, it feels distinct from the ones I found. I'll create a new issue and work on a fix.

[mspertus]

Created #2069