Js error message became [object Object]

[sixinli]

js script exception not being extracted properly after 19.x.x

we noticed that exceptions changes from having actual message (in this case we have some errors from handlebars.js), to having [object Object] when upgrading from 19.3.1 to 21.0.0, looking at https://github.com/oracle/graaljs/blob/master/graal-js/src/com.oracle.truffle.js/src/com/oracle/truffle/js/runtime/UserScriptException.java it seems like getMessage is changed between the versions...

specifically https://github.com/oracle/graaljs/blob/master/graal-js/src/com.oracle.truffle.js/src/com/oracle/truffle/js/runtime/UserScriptException.java#L125 returns null, and that's from com.oracle.truffle.object.DynamicObjectLibraryImpl

@TruffleBoundary
@Override
public Object getOrDefault(DynamicObject object, Shape cachedShape, Object key, Object defaultValue) {
    Property existing = ACCESS.getShape(object).getProperty(key);
    if (existing != null) {
        return getLocation(existing).get(object, false);
    } else {
        return defaultValue;
    }
}

thinks DynamicObject<JSError> has no constructor....

[woess]

Do you have some example code that you can share that triggers this?

[sixinli]

sorry for the late reply, we have the handlebars.js 3.x running, and it's just an error thrown in javascript like https://github.com/handlebars-lang/handlebars.js/blob/v3.0.8/lib/handlebars/runtime.js#L198, the message is lot in between

[rdosanjh]

Hey it seems that Handlebars JS has created their own custom exception which seems to be the problem.

Here is a minimal example that outlines the problem

import com.google.common.io.Resources;
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.Engine;
import org.graalvm.polyglot.Source;


public class Program {
    public static void main(String[] args) {
        Engine engine = Engine.create();
        Context context = Context.newBuilder()
                .engine(engine)
                .option("js.nashorn-compat", "true")
                .allowAllAccess(true)
                .build();


        String customErrorFunc = "function customError(errorMessage) { this.message = Error.prototype.constructor.call(this, errorMessage).message; }\n" +
                "customError.prototype = new Error();";
        try {
            context.eval(Source.create("js", customErrorFunc));
            context.eval(Source.create("js", "throw new customError('This is a basic error')"));

        } catch (Exception e) {
            System.out.println(e.getMessage());
            // 19.3.1 returns {message: "This is a basic error"}
            // 21.1.0 returns [object Object]
        }
    }
}

[rdosanjh]

I've done some digging and this was introduced in this commit. We were using the "js.nashorn-compat" mode but when they became combined it broke our functionality.

Here is the line in question before - after. (JSUserObject has since been renamed to JSOrdinary).

Would it be possible to add an option to go back to the previous way of displaying errors?

[woess]

A simple workaround for this would be to set the constructor property on the prototype of the custom error:
customError.prototype.constructor = customError;
With this simple change I see:
customError: This is a basic error

Alternatively, you could use just subclass Error, e.g. class CustomError extends Error {}

It would probably make sense to show the message also without the constructor property. We'll look into that.