Question

I came across a design that overrides [Scriptable.put][1] in a subclass of ScriptableObject to do some conversion. If the conversion fails the code is throwing an exception. Which means that property assignments like following code can cause a runtime exception to be thrown

    aScriptable.dateOfArrival = aVar;

By default rhino wouldn’t let the script catch a runtime exception thrown during [Scriptable.put][1]. So catch block in following code will never run:

    try{    
      aScriptable.dateOfArrival = aVar;
    }catch(e){
    //will not run even if above assignment generates an exception. Script will be terminated instead
    }

Overriding ContextFactory.hasFeature() with following code makes the above catch block work:

    protected boolean hasFeature(Context cx, int featureIndex) {
      if(featureIndex == Context.FEATURE_ENHANCED_JAVA_ACCESS){
        return true;
      }
      return super.hasFeature(cx, featureIndex);
    }

My question is that whether the design decision to make property assignment throw exception is correct or property assignments are never supposed to throw exceptions?

[1]: http://www.mozilla.org/rhino/apidocs/org/mozilla/javascript/Scriptable.html#put(java.lang.String, org.mozilla.javascript.Scriptable, java.lang.Object)

Answer 1

IMO it doesn’t make sense to throw an exception by design from the put method that JS code can’t catch. I think throwing an exception on setting a property is fine, although not that common. Note that JS code can easily reconfigure a property (in ECMAScript 5) with a custom setter that throws.

On the other hand, I think it would be quite surprising if a property getter throws an exception.