in kotlin-js how do i launch a coroutine from norm...

# coroutines

[Nikky]

in kotlin-js how do i launch a coroutine from normal code without using GlobalScope ? using that exceptions get lost apparently ? is there a better thing ?

[elizarov]

You can add your own CoroutineExceptionHandler to the GlobalScope or just catch your exceptions in the coroutines you launch.

[elizarov]

But what do you mean “lost”? They should be delivered to the default exception handler, so on backend that would mean they get printed to console and on Android it means they should get logged and would crash your app. They should not be “lost”. Can you show how that is happening for you?

[Nikky]

i guess i never installed a default exception handler, i am on jvm and javascript (browser), been too long since i did deep dive into coroutines, so i ask stupid questions

[Nikky]

https://kotlinlang.org/docs/reference/coroutines/exception-handling.html#coroutineexceptionhandler
this only talks about using ServiceLoader on jvm to install the global exception handler
does that mean it is not possible for kotlin-js ?

[Fudge]

@elizarov
If you do something like this you will get no exception

fun errorer() {
    GlobalScope.launch {
        error("asdf")
    }
}
fun main() {
    errorer()
}

[Fudge]

This does error

suspend fun errorer() = coroutineScope {
    launch {
        error("asdf")
    }
}

suspend fun main() {
    errorer()
}

[Dico]

@Fudge because the global scope threads are daemon threads. You have to delay program exit.

[Fudge]

yeah I've just tried that and you are right

[Fudge]

The difference will be the way exceptions are caught then,
if you do this the exception will be caught:

suspend fun errorer() = coroutineScope {
    launch {
        error("asdf")
    }
}

suspend fun main() {
    try {
        errorer()
    } catch (e: Exception) {
        println("Caught")
    }
    Thread.sleep(1000)
}

If you do this the exception won't be caught

fun errorer() {
    GlobalScope.launch {
        error("asdf")
    }
}

fun main() {
    try {
        errorer()
    } catch (e: Exception) {
        println("Caught")
    }
    Thread.sleep(1000)
}

[Fudge]

and in JS if there's no exception handler, in the former case if there is no try/catch it will error anyway because it will travel up the stack normally, and if you use GlobalScope it will silently fail. That's my theory, feel free to correct me.

[Nikky]

i would like to install a default exception handler in js .. first thing is the main function
can i do that ? because i am surely gonna forget to pass the handler in at least a few places and be confused for a whole afternoon

[elizarov]

val myScope = GlobalScope + CoroutineExceptionHandler { ... }

then use myScope.launch { ... }

[elizarov]

Search/Replace GlobalScope usage across your project with myScope.

[elizarov]

There is a default handler on JS, too. It does console.error(exception). So it should not be “lost” either.