Conditional catch clause

David Bruant bruant.d at
Wed Dec 19 06:32:04 PST 2012

Le 19/12/2012 14:13, Claude Pache a écrit :
> Hello,
> In SpiderMonkey (and perhaps other JS engines?), there are conditional catch clauses:
> 	catch (exception if condition)
> Could such a feature added to ECMAScript?
> Rationale: Although try/catch mechanism was initially intended for treating errors, it could be used in normal control flow with non-error condition.
Why not just use normal control flow when you want to express non-error 
throw/try/catch was indeed intented for treating errors and when I read 
code, I expect an error path when encountering a try/catch.

> sanction this pattern with StopIteration. In this case, it would be nice to have a catch clause listening only what is wanted, without having to add code for filtering out and rethrowing. Here is an example of use:
> 	Map.prototype.some = function(f, o) {
> 		var ok = {} // an arbitrary private object
> 		try {
> 			this.forEach(function(v, k, m) {
> 				if (, v, k, m))
> 					throw ok
> 			})
> 		}
> 		catch (e if e === ok) {
> 			return true
> 		}
> 		return false
> 	}
> instead of, e.g.,
> 		catch (e) {
> 			if (e !== ok)
> 				throw e
> 			return true
> 		}
I don't see the benefit from your example. You're basically removing one 
or 2 lines of code in an hardly more expressive way. Your above example 
could even be rewritten as:

         catch (e) { if (e !== ok) throw e
             return true

That could be a code convention without needing to be part of the language.

Do you have example of code in the wild that would benefit (performance, 
readability, etc.) from such an addition?


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