How to create a QJSValue object of type Error

pablo_worker

I add additional info:

Here it is a code snippet of the intended functionality:

void CMainWindow::slot_ExecuteJVScript()
{
   myCppClass myObject(parameters); //It has a foo() method.
   QJSEngine myEngine;
   myEngine.globalObject().setProperty("myObj", myEngine.newQObject(&myObject)); //Now we can access the C++ object from JavaScript code.
   QString myScriptJS = "var X = myObj.foo()";  //Calling a C++ function from JavaScript
   QJSValue myValue = myEngine.evaluate(myScriptJS);
   if(myValue.isError())
   {
      //If there is an error in the JavaScript code we enter here
      //But I also wanto to enter here if there is an error in my foo() C++ function. 
   }
   else
   {
      //Do things with myValue
   }
}

Mini-Example of myCppClass::foo()

QJSValue myCppClass::foo()
{       
   if(something)
   {
       //Create a QJSValue myValue with some data
       return myValue;
   }
   else
   {   //ERROR
       //Create a QJSValue of type Error
       return myValueOfTypeError;
   }
}

p3c0

@pablo_worker It seems that ErrorObject is under private headers so I doubt it could be used somehow.
A dirty workaround would be to force an error from that function :)
For eg. inside your else part.

else
   {  
       QJSEngine engine;
       QJSValue val = engine.evaluate("f();"); //undefined function
       val.isError(); // this prints true
   }

pablo_worker

@p3c0 Thank you.
However, the problem I see is now is that I can't return QJSValue. Because if it has been created inside a function it is deleted when the function ends.

QJSValue myFunction() {
   QJSEngine engine;
   QJSValue val = engine.evaluate("asdf;"); //rubbish
   val.isError(); //this prints true
   return val;
}

QJSValue foo = myFunction();
foo.isError(); //this prints false

In addition, if it has been created with a QJSEngine, it loses its properties after the engine is deleted. Example:

else
{
     QJSValue val;
     {
        QJSEngine engine;
        val = engine.evaluate("f()");
        qDebug() << "val.isError:" << val.isError(); //this prints true
     }
     qDebug() << "val.isError:" << val.isError(); //this prints false
}

I could pass the QJSEngine instance to the function, but still, I don't know how to hard-copy the QJSValue.

p3c0

@pablo_worker You can pass reference of the same QJSEngine.

QJSValue myFunction(QJSEngine &engine) {
   QJSValue val = engine.evaluate("asdf;"); //rubbish
   val.isError(); //this prints true
   return val;
}

QJSEngine myEngine;
myFunction(myEngine).isError();

pablo_worker

@p3c0 Yes, but the QJSValue is deleted after the "return" hence:

myFunction(myEngine).isError();  //this returns false

p3c0

@pablo_worker Nope. They are copied.

pablo_worker

@p3c0
I thought so, but there is something strange.
In this case it is true:
main.cpp

#include <QCoreApplication>
#include <QDebug>
#include <QJSEngine>
#include <QJSValue>

QJSValue myFunction(QJSEngine &engine);

int main(int argc, char *argv[])
{
   QCoreApplication a(argc, argv);
   QJSEngine myEngine;
   QJSValue theVal = myFunction(myEngine);
   qDebug() << "theVal.isError():" << theVal.isError(); //this prints true
   return a.exec();
}

QJSValue myFunction(QJSEngine &engine)
{
   QJSValue val = engine.evaluate("asf");
   qDebug() << "val.isError():" << val.isError(); //this prints true
   return val;
}

However, in this case it is not true

Calling the cpp function from JavaScript.
The debug output of this code is:

In myFunction(), myVal.isError(): true
Outside, theVal.isError(): false

CMainWindow.h

#include <QMainWindow>
#include <QtWidgets>
#include "myCppClass.h"

class CMainWindow : public QMainWindow
{
   Q_OBJECT
public:
   CMainWindow(QWidget *parent = 0)
   {
      setCentralWidget(new QWidget(this));

      QJSEngine myEngine;
      myCppClass myObject(this, &myEngine); //this object has a myFunction() method.
      myEngine.globalObject().setProperty("myObj" ,myEngine.newQObject(&myObject));
      QJSValue theVal = myEngine.evaluate("var X = myObj.myFunction();"); //calling the function from Javascript
      qDebug() << "Outside, theVal.isError():" << theVal.isError(); //this prints false
   }
};

myCppClass.h

#include <QObject>
#include <QJSEngine>
#include <QJSValue>
#include <QDebug>

class myCppClass : public QObject
{
   Q_OBJECT
public:
   explicit myCppClass(QObject *parent = 0, QJSEngine *theEngine = nullptr)
      : QObject(parent), m_myEng(theEngine){}
public slots:
   QJSValue myFunction(){      
      QJSValue myVal = m_myEng->evaluate("asdf"); //myVal will be a error.
      qDebug() << "In myFunction(), myVal.isError():" << myVal.isError(); //this prints true;
      return myVal;
   }
private:
   QJSEngine *m_myEng;
};

main.cpp

#include "CMainWindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
   QApplication a(argc, argv);
   CMainWindow w;
   w.show();
   return a.exec();
}

pablo_worker

I have also noted something more, if I call the C++ function (that returns a QJSValue of type error) from JavaScript like this:

QJSValue theVal = myEngine.evaluate("var X = myObj.myFunction();"); //calling the c++ function from Javascript

I don't get the error in the variable "theVal" [theVal.isError() will say "false"]

However, if I call the function like this:

QJSValue theVal = myEngine.evaluate("var X = myObj.myFunction(); X "); // note the X

In this case the variable theVal does take the value X. And if I do theVal.isError() in this case it will show true.

Why is this?

p3c0

@pablo_worker So AFAICS something from JS should be returned so that evaluate reports proper output.

pablo_worker

@p3c0 Thank you for your help.
The problem is that what I really want in the end is a way to catch errors thrown by C++ functions transparently to the user that writes JavaScript. So, the user can just write a script like:

var X = myCppFunction(foo);
var Y = myOtherCppFunction(bar);
etc.

And when it is executed (myEngine.evaluate) the errors are catched by the C++ code of my application. Something like:

try
{
    QJSValue theVal = myEngine.evaluate(the JavaScript script);
    if(theVal.isError) {//if there are errors in the JS code enter here}
}
catch( )
{
    // If there is an error in the Cpp functions called by the JS code enter here
}

I have seen in this post that this is not currently possible with QJSEngine. In addition, I have seen that a ticket is already open. Therefore, I will have to think something to get around and wait untill the functionality is implemented.

Thank you for your help.