Jint

Asynchronous execution ​

Use the async entry points when the evaluated JavaScript may return a promise:

csharp
var engine = new Engine();

var value = await engine.EvaluateAsync("""
    (async () => {
        await Promise.resolve();
        return 42;
    })()
    """);

Console.WriteLine(value.AsNumber());

ExecuteAsync and InvokeAsync follow the same pattern. They await promise settlement without holding a thread and accept cancellation tokens:

csharp
using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
var value = await engine.EvaluateAsync(
    "(async () => compute())()",
    cancellationToken: cts.Token);

The engine's Options.Constraints.PromiseTimeout also bounds waits.

Existing promise values ​

When a synchronous call returns a JsValue that may be a promise, unwrap it asynchronously:

csharp
var pending = engine.Invoke("computeAsync");
var result = await pending.UnwrapIfPromiseAsync(cts.Token);

A non-promise is returned immediately. A rejected promise produces PromiseRejectedException.

Failure channel ​

Parsing errors, script errors, rejected promises, and execution-limit failures arrive through the returned Task; put the try/catch around await. Only usage errors throw before a task represents the operation: null or invalid prepared arguments, or an InvalidOperationException because the engine is already in use.

Always await an async entry before returning an engine to a pool or disposing it.

Host-created asynchronous work ​

engine.Tasks.RegisterPromise() returns a promise plus resolver and rejecter functions. They accept CLR values and may be called from any thread; settlement is queued and conversion occurs on the engine's thread. Bound a promise that might never settle with a cancellation token, promise timeout, or operation deadline.

Automatic conversion of CLR Task and ValueTask return values into JavaScript promises is experimental:

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

For timers, network operations, and host-controlled pumping, see Web APIs.