Capture error message at JSException construction

Sources/JavaScriptKit/JSException.swift

@@ -12,7 +12,7 @@
 ///     let jsErrorValue = error.thrownValue
 /// }
 /// ```
-public struct JSException: Error, Equatable {
+public struct JSException: Error, Equatable, CustomStringConvertible {
     /// The value thrown from JavaScript.
     /// This can be any JavaScript value (error object, string, number, etc.).
     public var thrownValue: JSValue {
@@ -25,10 +25,25 @@ public struct JSException: Error, Equatable {
     /// from `Error` protocol.
     private nonisolated(unsafe) let _thrownValue: JSValue
 
+    /// A description of the exception.
+    public let description: String
+
+    /// The stack trace of the exception.
+    public let stack: String?
+
     /// Initializes a new JSException instance with a value thrown from JavaScript.
     ///
-    /// Only available within the package.
+    /// Only available within the package. This must be called on the thread where the exception object created.
     package init(_ thrownValue: JSValue) {
         self._thrownValue = thrownValue
+        // Capture the stringified representation on the object owner thread
+        // to bring useful info to the catching thread even if they are different threads.
+        if let errorObject = thrownValue.object, let stack = errorObject.stack.string {
+            self.description = "JSException(\(stack))"
+            self.stack = stack
+        } else {
+            self.description = "JSException(\(thrownValue))"
+            self.stack = nil
+        }
     }
 }

Tests/JavaScriptEventLoopTests/WebWorkerTaskExecutorTests.swift

@@ -620,6 +620,20 @@ final class WebWorkerTaskExecutorTests: XCTestCase {
         XCTAssertEqual(object["test"].string!, "Hello, World!")
     }
 
+    func testThrowJSExceptionAcrossThreads() async throws {
+        let executor = try await WebWorkerTaskExecutor(numberOfThreads: 1)
+        let task = Task(executorPreference: executor) {
+            _ = try JSObject.global.eval.function!.throws("throw new Error()")
+        }
+        do {
+            try await task.value
+            XCTFail()
+        } catch let error as JSException {
+            // Stringify JSException coming from worker should be allowed
+            _ = String(describing: error)
+        }
+    }
+
     // func testDeinitJSObjectOnDifferentThread() async throws {
     //     let executor = try await WebWorkerTaskExecutor(numberOfThreads: 1)
     //