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Working with try/catch

medini_1024169437

In one of my recent posts (Better Excel Scripts : Ansys medini analyze Support) I proposed to use try/catch to intercept an error during an excel import, enrich the error message and rethrow or show an alert dialog. Thats a nice thing, however, it somewhat is a problem if you want t knwo the exact location of the original error.

An example. Assume I have this simple errorneous script:

// $EXPERIMENTAL$ $STRICT_MODE$ $WARNING_AS_ERROR$ 
var a = [["foo"], undefined];
console.log("{0}", a[0][0]);
console.log("{1}", a[1][2]);

On execution I get: Failed to [...]: Cannot read property [...] (sample.js#4).

So I basically see the file name and line number where I easily can spot the problem and fix the error. If I now wrap this with a try/catch like this:

// $EXPERIMENTAL$ $STRICT_MODE$ $WARNING_AS_ERROR$ 
try {
	var a = [["foo"], undefined];
	console.log("{0}", a[0][0]);
	console.log("{1}", a[1][2]);
}
catch (e) {
	throw e + " with sugar";
}

I get: Failed to [...]: Cannot read property [...] (sample.js#8).

So in line 8 I have the throw statement - no information on the original problematic error. Luckily the exception object has all the information for me:

// $EXPERIMENTAL$ $STRICT_MODE$ $WARNING_AS_ERROR$ 
try {
	var a = [["foo"], undefined];
	console.log("{0}", a[0][0]);
	console.log("{1}", a[1][2]);
}
catch (e) {
	console.log("Exception: {0} {1}#{2}", e.message, e.fileName, e.lineNumber);
	throw e + " with sugar";
}

will print to the console: Exception: Cannot read property "2" from undefined sample.js#5

Happy Scripting

Jan

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medini_1024169437

An update to this article: I heard that often the world is a bit more complicated, especially when using libraries from other people, for example the standard libraries we have posted here in the forum. They for example throw a string message as "an exception". JS is quit flexible but that makes exception handling a bit harder. So it is difficult to distinguish between, an error that happened in a used library, an error that our own script has thrown intentionally and an error that happened because our own script has flaws. I tried to come up with a simply solution, and that could be like this:

In case the script is wrong, a real JS Error is thrown, treat this as a sever problem, stopping the script
In case the library throws a string, we interpret this as a problem, but not a critical one, we may continue
For our own exception handling, we invent an own object that holds a severity label as well as a message to distinguish it from the other 2 and also to encode the severity, "warning" means continue, error means full stop.

I created a simple script that show cases all 4 cases and some way to play with it, should be self explanatory:

try {
	var a;
	var choice = prompt("Give me a 1, 2, 3 or 4", "1");
	switch ('' + choice) {
		case "1": a.foo = 12; break; 
		case "2": throw "Houston, we have a problem"; 
		case "3": throw { severity: "warning", message: "Houston, we have a minor problem" };
		case "4": throw { severity: "error", message: "Houston, we have a major problem" };
		default: alert("??? You gave me: " + choice);
	}
}
catch (e) {
	console.error(e);
	if (e instanceof Error) {
		throw "A real JS error was catched - this script is terminated now - error was: " + e; 
	}
	if (typeof e === "string") {
		alert("A string was catched - the script can continue - error was: " + e); 
	}
	if (typeof e === "object" && "severity" in e) {
		alert("A custom error object was catched - the script can continue - error message was: " + e.message);
		if (e.severity == "error") throw e.message;
		// TODO record the error and show it at the end 
	}
}

Happy Scripting

Jan

medini_1063548292

Jan,

If I may to ask a question.

the lineNumber and fileName is only available form the JavaException right?

Is there any way how we can use throws and still keep the lineNumber & fileName ?
I tired throwing string, Error, RangeError, RefError JS objects but they only have a message attribute.

try {
throw "Message for you";
} 
catch (e) {
console.log("Thrown:\n\t{0}\n\t{1}",e,["message","lineNumber","fileName"].map(k=>e[k]).join("-|-").toString());
//only message is in exception
}

try {
throw new Error("Message for you in Error");
} 
catch (e) {
console.log("Thrown:\n\t{0}\n\t{1}",e,["message","lineNumber","fileName"].map(k=>e[k]).join("-|-").toString());
//only message is in exception
}

Danke,

Marian

medini_1024169437

Hm, good question. The JS engine is assembling the Error object and filling it with line number and filename, I never investigated how to do that on my own. Le me check...

Jan

medini_1024169437

Marian,

I played a bit with medini and had a chat with our new best friend (AI). As usual it gave valuable hints but could not get me to 100%. The facts:

Rhino supports an Error constructor where you can pass file name and line number yourself:

throw new Error("Something went wrong", "custom_script.js", 42);

Well, this is nice but who wants to pass hard coded file names or line numbers? The AI kept saying that if I throw an error like throw new Error("foo"); Rhino would automatically attach file name and number. It was bubbling something about a mandatory context parameter which I need to play with but I think it is halucinating.

Anyway, it gave me a crazy idea to catch filename and line number like this:

var loc = (function() { try { null.foo(); } catch (e) { return e; } })();
throw new Error("Something went wrong", loc.fileName, loc.lineNumber );

So while this looks awkward in the first place, it may be an idea to continue.

At least I was told that throwing a plain string is not good practice, so I guess we will update our libraries to something better.

Jan

medini_1063548292

Jan,

thankx for the trials!

I did not use the AI friend ( I usally end up fighting with him as he is a good & tricky lier

But , I have checked the Context class and found one interesting stuff
public method EvaluationError reportRuntimeError(String message)

//need a currentContext
var t = org.mozilla.javascript.Context.getCurrentContext();
//report runtime error , will contain java stack trace and also script stack trace
//the Rhino will automatically fill in the filename, and lineNumber
//er is org.mozilla.javascript.EvaluationException class
var er = t.reportRuntimeError("Problem encountered in script, why are you doing this to me ....")
//java stack trace contain all java path until the script evaluation 
console.log(er.getStackTrace().map(a=>a).join("\n"));
//contains the array of script stack trace
console.log(er.getScriptStackTrace().map(a=>a).join("\n"));
//get localize message or message 
console.log(er.getLocalizedMessage());
//print the stacktrace ( java + rhino localized message), default pirnted itnt System.out
console.log(er.printStackTrace());

ant outputof printStackTrace may look like :

the getScriptStack()
TypeError: Cannot find function map in object at :5

at CFTCreateModel.js:1668

So there seem to be away via Context to create and eventually stack the script errors.

Marian

medini_1063548292

As usual I have to correct something.

I was trying to catch a bus but did I have missed it anyway

er.getScriptStack() returns and Array while the er.getScriptStackTrace() returns the String !!

so the corrected script snippet:

//need a currentContext
var t = org.mozilla.javascript.Context.getCurrentContext();
//report runtime error , will contain java stack trace and also script stack trace
//the Rhino will automatically fill in the filename, and lineNumber
//er is org.mozilla.javascript.EvaluationException class
var er = t.reportRuntimeError("Problem encountered in script, why are you doing this to me ....");
//java stack trace contain all java path until the script evaluation (available at RuntimeException)
console.log(er.getStackTrace().map(a=>a).join("\n"));
//returns stack trace as String , avialable at RhinoException
//returns String
console.log(er.getScriptStackTrace().map(a=>a).join("\n"));
//returns stack race array With all records, available at RhinoException
console.log(er.getScriptStack().map(a=>a).join("\n"));
//get localize message or message 
console.log(er.getLocalizedMessage());
//print the stacktrace ( java + rhino localized message), default pirnted itnt System.out
console.log(er.printStackTrace());

Note: Creating EvaluatorException dos does not stop the script execution.
Only when a throw er; is used then it starts to break the execution and propagate through the try catch to the top. And of course, preserving a hierarchy path so if such exception is raised within the loaded ui file it will be correctly indicated at the script stack trace ...

The Exception can be also opened & reviewed through the Error View with all the details.

Happy throwing (not up),

Marian

medini_1024169437

Nice! Need to play with this. Maybe we can add some more supporting functions to make throwing errors more fun and easier.

Jan

medini_1063548292

Hi Jan,

You could maybe start and test one thing.

It seems like there is possibility to enable the automatic attachement of script file & line# into an Error if the ContextFactory implenetation has a feature 10 ( returns true);

(Context.FEATURE_LOCATION_INFORMATION_IN_ERROR)

This should eventually extend new Error("message here") with lineNumber, fileName.
I did not try this one because it requires modification of teh ContextFactory...

In Addition, there are also other Context methods that are attaching the location info and creationg and error:

Context.reportError (create and throws it) -> calling the Context's errorReporter error(...)
Context.reportWarning (create warning and stores it nowhere-> calls errorReporter's warning(..)
Context.reportRuntimeError (create an EvaluatorException object, no automatic throwing)

Alternatively, there is another way how to create a new Exception (EcmaError), this time residing at the ScriptRuntime:

.ScriptRuntime.constructError(String typeName,String message)
Only create an EcmaError with custom type(string) and message + attaching the location info. no automatic throwing

Finally, to complete the story, all of those methods are interacting with the Context's errorReporter. If not set it is using default implementation ( DefaultErrorReporter).

So, if you want to fiddle around and create own implementation, you can do it by onfiguring it to the Context (set or get possible).

Marian

medini_1024169437

Last update on this: I did some code investigation and tests and can happy report the following:

Enabling the context flag Context.FEATURE_LOCATION_INFORMATION_IN_ERROR indeed will fill file and script into an Error object that is created by new Error("foo");
There is still the possibility to overwrite that using a second and third parameter
In addition I found out that Error has a nice function named capureStackTrace(); which can be called already now without the flag and doe sthe same
So we will go ahead and enable the context parameter in future version, till then you may use the fill method

My final test script:

try {
	var a;
	var choice = prompt("Give me a 1 to 6", "1");
	switch ('' + choice) {
		case "1": a.foo = 12; break; 
		case "2": throw "Houston, we have a problem"; 
		case "3": throw { severity: "warning", message: "Houston, we have a minor problem" };
		case "4": throw { severity: "error", message: "Houston, we have a major problem" };
		case "5": throw new Error("Houston!!");
		case "6": throw new Error("Houston!!", "anyscript.js", 27);
		case "7": e = new Error("Houston!!"); e.capureStackTrace(); throw e;
		default: alert("??? You gave me: " + choice);
	}
}
catch (e) {
	console.error("{0} file={1} line={2}", e, e.fileName, e.lineNumber);
	if (e instanceof Error) {
		throw "A real JS error was catched - this script is terminated now - error was: " + e; 
	}
	if (typeof e === "string") {
		alert("A string was catched - the script can continue - error was: " + e); 
	}
	if (typeof e === "object" && "severity" in e) {
		alert("A custom error object was catched - the script can continue - error message was: " + e.message);
		if (e.severity == "error") throw e.message;
		// TODO record the error and show it at the end 
	}
}

Thanks for all your contributions!

Jan

PS: We will stepwise adapt our libraries to throw Errors instead of Strings!

medini_1024169437

Well, a rookie mistake has crept into my script and my statement. Thanks to Marian, I recognized that line 11 is nonsense, not working as expected:

e.capureStackTrace();

Well, I misspelled the function name which leads to a script error (!) itself. As I was expecting that an Error is throw, I just double checked the line number and the file but totally overlooked that the error was "Cannot find function capureStackTrace in object Error", omg. I apologize.

I then figured that even

e.captureStackTrace();

does not work. I had to really understand the Rhino better and finally understood that this is not a function of an Error instance, but of the Error "class".

So this one is working fine:

Error.captureStackTrace(e);

Happy Scripting

Jan