This article shows C# developers the equivalent concepts of asynchronous programming in JavaScript.

Task (C#) vs Promise (JavaScript)

• Task - Promise
• Task.CompletedTask - Promise.resolve()
• Task.FromResult(value) - Promise.resolve(value)
• Task.FromException() - Promise.reject()
• Task.FromCanceled() - Promise.reject()
• Task.WhenAny() - Promise.race()
• Task.WhenAll() - Promise.all()
• Task.ContinueWith() - Promise.then()
• Task.ContinueWith(task => task.Exception != null) - Promise.catch()
• Task.ContinueWith() - Promise.catch()

CancellationToken (C#) vs AbortSignal (JavaScript)

• CancellationToken - AbortSignal
• CancellationToken.IsCancellationRequested - AbortSignal.aborted
• CancellationToken.Register() - AbortSignal.addEventListener('abort')

CancellationTokenSource (C#) vs AbortController (JavaScript)

• CancellationTokenSource - AbortController
• new CancellationTokenSource(TimeSpan delay) - AbortSignal.timeout(time)
• CancellationTokenSource.Token - AbortController.signal
• CancellationTokenSource.Cancel() - AbortController.abort()

TaskCompletionSource (C#) vs Promise (JavaScript)

var tcs = new TaskCompletionSource<DateTime>(); 
var timer = new Timer();
obj.Elapsed += (sender, e) => {
  tcs.SetResult(e.SignalTime);
  timer.Stop();
};
timer.Enabled = true;

await tcs.Task;

// JavaScript
const delay = ms => new Promise((resolve, reject) => {
  setTimeout(resolve, ms);
});

await delay(1_000);

TaskCompletionSource in C# is only used in rare cases (WaitForAppStartup) or legecy code base (e.g. event based API).

async and await in C# and JavaScript

C# and JavaScript share the same syntax using the async and await keywords.

// C#
async Task<string> TestAsync()
{
    return await Task.FromResult("Hello");
}

// JavaScript
async function test() {
  return await Promise.resolve('Hello');
}

await - async vs sync

In C# a Task may execute synchronous. In JavaScript a Promise (even Promise.resolve()) is always executed asynchronous.

.NET 8 / C# added the ability to force a Task to execute asynchronous (pull request).

await Task.CompletedTask.ConfigureAwait(ConfigureAwaitOptions.ForceYielding);