DO NOT MERGE: JSG_TRY, JSG_TRY_CATCH, KJ_TRY, KJ_CATCH proof of concept macros #5802

harrishancock · 2025-12-30T18:53:54Z

This PR contains a proof of concept macro which allows us to write code like:

JSG_TRY {
  // code that might throw either a JS or KJ exception
} catch (...) {
  jsg::Value exception = getCaughtExceptionAsJsg();
  // code that handles the exception
}

In the snippet above, getCaughtExceptionAsJsg is a function object which is declared in the JSG_TRY macro. The JSG_TRY macro uses the same trick that KJ_IF_SOME pioneered to make a declaration available to only one single statement without requiring extra braces. getCaughtExceptionAsJsg(), when called, performs the same exception conversion that jsg::Lock::tryCatch() performs. Instead of passing the resulting value to a callback, however, it returns it.

Some questions you might be asking:

Q: Does getCaughtExceptionAsJsg() remember to destroy the relevant v8::TryCatch object before returning?
A: Yes. It holds it in a Maybe, and nullifies the Maybe.

Q: Can I name getCaughtExceptionAsJsg() something else?
A: Yes. I included a JSG_TRY_CATCH macro to show how that might look. I like that version better, in fact, but it's more verbose.

Q: If it uses the same trick as KJ_IF_SOME, does that mean this thing is actually an if statement?
A: Yes, but it is a complete if-else statement, which avoids some pitfalls. KJ_IF_SOME expands to if (decl1) if (decl2; false) {} else, whereas JSG_TRY expands to the simpler if (decl; false) {} else try. The else already binds to the if, so it is a compile error to add another else, and constructs like if JSG_TRY { ... } catch (...) { ... } else ... work as you would expect.

Q: Does this produce any weird compiler warnings about dangling elses, like KJ_IF_SOME sometimes does?
A: I haven't seen any, but I'm not 100% sure.

Q: What's wrong with jsg::Lock::tryCatch()?
A: I want to implement co_await syntax for JSG promises, and that syntax is somewhat useless without real try-catch syntax to go with it. That said, this is not by itself sufficient for handling JS exceptions in coroutines -- we would need to arrange to destroy the v8::TryCatch on every suspension, and recreate it on every resumption.

This PR also includes a proof of concept set of macros, KJ_TRY and KJ_CATCH, which allow us to write code like:

KJ_TRY {
  // code that might throw any exception
} KJ_CATCH (kj::Exception& exception) {
  // code that handles the exception as a KJ exception
}

That is, it is syntax sugar for our normal catch (...) pattern:

try {
  // code that might throw any exception
} catch (...) {
  auto exception = kj::getCaughtExceptionAsKj();
  // code that handles the exception as a KJ exception
}

That said, I don't like it very much, because it uses a nested try-catch. I developed it because the JSG_TRY macro doubles down on our catch (...) pattern, so I wanted to explore if it was possible to avoid that. Turns out it is, but I'm not sure it's worth the cost.

I'm working on `co_await` syntax for jsg::Promises, and `co_await` syntax isn't particularly useful if you can't use it in a try block. But, JSG doesn't support raw try-catch blocks. This is an experiment to see if we can fix that reasonably elegantly. Claude helped me with the boring parts.

Claude did all of this.

I was curious if we could get rid of the `catch (...) { auto e = kj::getExceptionAsKj()` pattern, since I doubled down on it in the JSG_TRY macro implementation. Turns out we can. But I'm not sure that we should. Claude again helped me with the boring parts.

Claude did all of this.

harrishancock · 2025-12-30T18:56:34Z

src/workerd/jsg/function-test.c++

+  kj::String testTryCatch2(Lock& js, jsg::Function<int()> thrower) {
+    // Here we prove that the macro is if-else friendly.
+    // Note that clang-format doesn't recognize it as a `try`, so we get wonky formatting. Oh well.
+    if (true) JSG_TRY {


An example of how JSG_TRY looks like in practice. Note that clang-format doesn't recognize JSG_TRY as a try keyword, so the formatting is messed up -- the catch is always formatted as if starting a new statement.

Hmm.. can clang-format be extended/updated to account for this I wonder?

harrishancock · 2025-12-30T18:58:48Z

src/workerd/api/messagechannel.c++


 void MessagePort::dispatchMessage(jsg::Lock& js, const jsg::JsValue& value) {
-  js.tryCatch([&] {
+  JSG_TRY_CATCH(getMessageException) try {


An example of how JSG_TRY_CATCH looks in practice. I like this better than JSG_TRY, because clang-format can see the full try-catch statement.

A version of JSG_TRY which includes the default name (getCaughtExceptionAsJsg) but doesn't include the try keyword could be a third option:

JSG_TRY try { ... } catch (...) { jsg::Value exception = getCaughtExceptionAsJsg(); }

harrishancock · 2025-12-30T19:01:31Z

src/workerd/api/util.c++

 template <typename Func>
 auto translateTeeErrors(Func&& f) -> decltype(kj::fwd<Func>(f)()) {
-  try {
+  KJ_TRY {


An example of KJ_TRY and KJ_CATCH at work.

Ultimately, I think I prefer the raw syntax, myself.

agreed on prefering the raw syntax.

jasnell · 2025-12-30T19:18:10Z

src/workerd/api/crypto/crypto.c++


 void DigestStream::dispose(jsg::Lock& js) {
-  js.tryCatch([&] {
+  JSG_TRY {


Nit: I appreciate that the structure is similar to try/catch syntax but the catch(...) there just feels.. odd.. for some reason. Feel free to ignore but given that the macro is a bit special I'd almost prefer something like...

JSG_TRY( { // Block to try }, { // Catch block } )

But even that feels bleh, lol. So feel free to ignore this 😆

jasnell · 2025-12-30T19:19:20Z

src/workerd/api/crypto/crypto.c++

+  catch (...) {
+    jsg::Value exception = getCaughtExceptionAsJsg();


This part is likely to be a common pattern... perhaps even something like...

JSG_CATCH(exception) { js.throwException(kj::mv(exception)); }

To boilerplate this particular piece might be handy?

jasnell · 2025-12-30T19:22:27Z

src/workerd/api/streams/standard.c++

+          }
+          catch (...) {
+            jsg::Value exception = getCaughtExceptionAsJsg();
+            return js.rejectedPromise<void>(kj::mv(exception));


there are going to be some common patterns like:

Convert the error and throw it

Convert the error and return a rejected promise

Maybe these can be boilerplated with their own macros?

JSG_CATCH_THROW() // short cut for rethrowing JSG_CATCH_RETURN_REJECTED() // short cut for returning rejected

But also just a nit.. feel free to ignore.

jasnell · 2025-12-30T19:27:06Z

src/workerd/jsg/jsg.h

+  // Handle any in-flight JsExceptionThrown or kj::Exception. If a JsExceptionThrown is found, our
+  // v8::TryCatch's currently held exception value is returned. If a kj::Exception is found, it is
+  // converted to a JS value by passing it and `options` to `Lock::exceptionToJs()`.
+  Value operator()(ExceptionToJsOptions options = {}) {


This is a fairly non-trivial function in a non-templated class. Can it perhaps be moved into jsg.c++?

jasnell · 2025-12-30T19:31:17Z

src/workerd/server/server.c++

-            } catch (...) {
-              auto exception = kj::getCaughtExceptionAsKj();
+            }
+            KJ_CATCH(kj::Exception & exception) {


Could this just be KJ_CATCH(exception) { ... the kj::Exception& part is likely a given.

harrishancock added 4 commits December 30, 2025 18:30

Use JSG_TRY and JSG_TRY_CATCH in some examples

7448a84

Claude did all of this.

Use KJ_TRY and KJ_CATCH in some examples

ebf1417

Claude did all of this.

harrishancock commented Dec 30, 2025

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DO NOT MERGE: JSG_TRY, JSG_TRY_CATCH, KJ_TRY, KJ_CATCH proof of concept macros #5802

DO NOT MERGE: JSG_TRY, JSG_TRY_CATCH, KJ_TRY, KJ_CATCH proof of concept macros #5802

harrishancock commented Dec 30, 2025

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		catch (...) {
		jsg::Value exception = getCaughtExceptionAsJsg();

DO NOT MERGE: JSG_TRY, JSG_TRY_CATCH, KJ_TRY, KJ_CATCH proof of concept macros #5802

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DO NOT MERGE: JSG_TRY, JSG_TRY_CATCH, KJ_TRY, KJ_CATCH proof of concept macros #5802

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