Traditionally, JavaScript code contains some sort of asynchronous logic
either on the client side or on the server side.
This makes the `try/catch` construct non-practical for many real use cases
because it can not catch errors caused by callbacks and/or other
asynchronous operations involved. This is why we have common idioms like
the `(err,result)` of NodeJS callbacks or reject in promises
implementations and so on.
NodeJS also introduces domains which let you catch all errors that
originated from a piece of code in a certain way.
ES6 Generators recently implemented in NodeJS allow us to do concurrency
using coroutines. This allows for very nice code that's very readable in
certain cases. The async_functions proposal is also very interesting
solving the problem without needing a "runner" for the generator.
However, one issue arises from using generators for such concurrency. The
current way try/catch works with generators does not give us a way to
differentiate things like syntax errors from logic errors.
Consider the following piece of code using Bluebird promises:
```
Promise.coroutine(function *(){
try{
let db = yield DBEngine.open("northwind");
let result = yield db.query("SELECT name FROM users")
return res;
} catch (e){
//code to handle exception in DB
}
});
```
Noticed the `ReferenceError`? Not obvious at first glance. A compile time
tool _might_ find it but I may have a closure variable named `res`. This
might even happen later when making changes. A catch-all does not do a good
job here. What I'd want to do is to catch all exceptions that are _mine_. I
need a way to catch exceptions of a different type.
This is a real need in light of the new found usage for try/catch given
generators.
I've looked around and found one possible solution using catch guards (
http://wiki.ecmascript.org/doku.php?id=strawman:catch_guards ) , I know
Firefox supports catch guards but I was unable to find anything in the
specification or in the mailing list when searching relating to it.
Petka added:
Without predicated catch clauses you would need to write something like
this:
```js
Promise.coroutine(function *(){
try{
let db = yield DBEngine.open("northwind");
let result = yield db.query("SELECT name FROM users")
return res;
} catch (e) {
if( e instanceof RejectionError ) {
//code to handle exception in DB
}
else {
throw e;
}
}
});
```
The manual rethrow can easily be forgotten to be written
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