[JS/TS] Improvements to Async support (#4059)

* [JS/TS] Report an error at compilation time when trying to use `Async.RunSynchronously`

* [JS/TS] Propagate non-captured exception when running `Async.Start` or `Async.StartImmediate`

* fix test for running on .NET

* adapt fcs-fable code to not use Async.RunSynchronously

* update changelogs

Author: MangelMaxime

src/Fable.Cli/CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * [JS/TS] Fix #4025: No reflection info for pojos (by @alfonsogarciacaro)
 * [JS/TS] Fix #4049: decimal/bigint to integer conversion checks (by @ncave)
 * [JS/TS] Fix `decimal` to `char` conversion checks (by @ManngelMaxime)
+* [JS/TS] Propagate non-captured exception when running `Async.Start` or `Async.StartImmediate` (by @MangelMaxime)
+* [JS/TS] Report an error at compilation time when trying to use `Async.RunSynchronously` (by @MangelMaxime)
 
 ## 5.0.0-alpha.10 - 2025-02-16
 

src/Fable.Compiler/CHANGELOG.md

@@ -16,6 +16,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * [JS/TS] Fix #4025: No reflection info for pojos (by @alfonsogarciacaro)
 * [JS/TS] Fix #4049: decimal/bigint to integer conversion checks (by @ncave)
 * [JS/TS] Fix `decimal` to `char` conversion checks (by @ManngelMaxime)
+* [JS/TS] Propagate non-captured exception when running `Async.Start` or `Async.StartImmediate` (by @MangelMaxime)
+* [JS/TS] Report an error at compilation time when trying to use `Async.RunSynchronously` (by @MangelMaxime)
 
 ## 5.0.0-alpha.10 - 2025-02-16
 

src/Fable.Transforms/Replacements.fs

@@ -3622,7 +3622,6 @@ let asyncBuilder (com: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Exp
 
 let asyncs com (ctx: Context) r t (i: CallInfo) (_: Expr option) (args: Expr list) =
     match i.CompiledName with
-    // TODO: Throw error for RunSynchronously
     | "Start" ->
         "Async.Start will behave as StartImmediate" |> addWarning com ctx.InlinePath r
 
@@ -3634,6 +3633,7 @@ let asyncs com (ctx: Context) r t (i: CallInfo) (_: Expr option) (args: Expr lis
     | "Catch" ->
         Helper.LibCall(com, "Async", "catchAsync", t, args, i.SignatureArgTypes, genArgs = i.GenericArgs, ?loc = r)
         |> Some
+    | "RunSynchronously" -> None
     // Fable.Core extensions
     | meth ->
         Helper.LibCall(

src/fable-library-ts/Async.ts

@@ -159,12 +159,14 @@ export function sleep(millisecondsDueTime: number) {
   });
 }
 
-export function runSynchronously(): never {
-  throw new Error("Asynchronous code cannot be run synchronously in JS");
-}
-
 export function start<T>(computation: Async<T>, cancellationToken?: CancellationToken) {
-  return startWithContinuations(computation, cancellationToken);
+  return startWithContinuations(
+    computation,
+    emptyContinuation,
+    function (err) { throw err },
+    emptyContinuation,
+    cancellationToken
+  );
 }
 
 export function startImmediate<T>(computation: Async<T>, cancellationToken?: CancellationToken) {
@@ -173,19 +175,16 @@ export function startImmediate<T>(computation: Async<T>, cancellationToken?: Can
 
 export function startWithContinuations<T>(
   computation: Async<T>,
-  continuation?: Continuation<T> | CancellationToken,
-  exceptionContinuation?: Continuation<any>,
-  cancellationContinuation?: Continuation<any>,
+  continuation: Continuation<T>,
+  exceptionContinuation: Continuation<any>,
+  cancellationContinuation: Continuation<any>,
   cancelToken?: CancellationToken) {
-  if (typeof continuation !== "function") {
-    cancelToken = continuation as CancellationToken;
-    continuation = undefined;
-  }
+
   const trampoline = new Trampoline();
   computation({
     onSuccess: continuation ? continuation as Continuation<T> : emptyContinuation,
-    onError: exceptionContinuation ? exceptionContinuation : emptyContinuation,
-    onCancel: cancellationContinuation ? cancellationContinuation : emptyContinuation,
+    onError: exceptionContinuation,
+    onCancel: cancellationContinuation,
     cancelToken: cancelToken ? cancelToken : defaultCancellationToken,
     trampoline,
   });

src/fable-library-ts/CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * [JS/TS] Fix #4049: decimal/bigint to integer conversion checks (by @ncave)
 * [JS/TS] Fix `decimal` to `char` conversion checks (by @ManngelMaxime)
+* [JS/TS] Propagate non-captured exception when running `Async.Start` or `Async.StartImmediate` (by @MangelMaxime)
+* [JS/TS] Remove `Async.RunSynchronously` (by @MangelMaxime)
+* [JS/TS] Change signature of `startWithContinuations` to always require all its arguments (by @MangelMaxime)
 
 ## 2.0.0-beta.1 - 2025-02-16
 

src/fcs-fable/src/Compiler/Service/FSharpCheckerResults.fs

@@ -3860,6 +3860,9 @@ type FSharpCheckProjectResults
                 |> Array.toSeq
 #endif //!FABLE_COMPILER
             | Choice2Of2 task ->
+#if FABLE_COMPILER
+                seq {}
+#else
                 Async.RunSynchronously(
                     async {
                         let! tcSymbolUses = task
@@ -3872,6 +3875,7 @@ type FSharpCheckProjectResults
                     },
                     ?cancellationToken = cancellationToken
                 )
+#endif //!FABLE_COMPILER
 
         results
         |> Seq.filter (fun symbolUse -> symbolUse.ItemOccurrence <> ItemOccurrence.RelatedText)
@@ -3890,7 +3894,7 @@ type FSharpCheckProjectResults
 
         let cenv = SymbolEnv(tcGlobals, thisCcu, Some ccuSig, tcImports)
 
-        let tcSymbolUses =
+        let tcSymbolUses : TcSymbolUses seq =
             match builderOrSymbolUses with
             | Choice1Of2 builder ->
 #if FABLE_COMPILER
@@ -3908,7 +3912,12 @@ type FSharpCheckProjectResults
                     | _ -> TcSymbolUses.Empty)
                 |> Array.toSeq
 #endif //!FABLE_COMPILER
-            | Choice2Of2 tcSymbolUses -> Async.RunSynchronously(tcSymbolUses, ?cancellationToken = cancellationToken)
+            | Choice2Of2 tcSymbolUses ->
+#if FABLE_COMPILER
+                seq {}
+#else
+                Async.RunSynchronously(tcSymbolUses, ?cancellationToken = cancellationToken)
+#endif //!FABLE_COMPILER
 
         [|
             for r in tcSymbolUses do

tests/Js/Main/AsyncTests.fs

@@ -1,6 +1,7 @@
 module Fable.Tests.Async
 
 open System
+open Util
 open Util.Testing
 
 #if FABLE_COMPILER
@@ -72,6 +73,26 @@ let tests =
             !result
         f true + f false |> equal 22
 
+    testCase "Non captured exception in async is propagated when using Async.StartImmediate" <| fun () ->
+        throwsAnyError (fun _ ->
+            async {
+                failwith "boom!"
+            } |> Async.StartImmediate
+        )
+
+    #if FABLE_COMPILER
+    // Behaviour of Async.Start, is the same as Async.StartImmediate in JS
+    // We disable this test for .NET, because it seems like we can't capture the exception
+    // This should be fine, because Fable generate a warning about the Async.Start
+    // behaviour being the same as Async.StartImmediate
+    testCase "Non captured exception in async is propagated when using Async.Start" <| fun () ->
+        throwsAnyError (fun _ ->
+            async {
+                failwith "boom!"
+            } |> Async.Start
+        )
+    #endif
+
     testCase "Simple async is executed correctly" <| fun () ->
         let result = ref false
         let x = async { return 99 }
@@ -596,4 +617,4 @@ let tests =
             let! res = parentWorkflow()
             equal 7 res
         }
-  ]
+  ]