QJSValue to json string (stringify) [Solved]

bundickt

Is there a way to convert a QJSValue object to a json string?

What I'm trying to do:
Send a QVariant object formatted as a QJSValue object over a TCP connection from a TCP Service written using Qt 5.4 to a TCP client written using Qt 4.8.

Problems:
Qt 4.8 does not have QJSValue objects. There is no easy was to create a json string from a QJSValue object.

Plan:
Convert the QJSValue to a json string then send the message to the client. The client can then convert the json string back to a QVariant.

SGaist

Hi,

IIRC, there was an external module call QJson for Qt 4 that could be of interest.

Hope it helps

JKSH

@bundickt said:

Is there a way to convert a QJSValue object to a json string?

Yes. QJsonDocument::toJson() produces a JSON string.

bundickt

@JKSH

I don't have a QJsonDocument object. I have a QJSValue object. The QJSValue object is being created by the Qt Quick engine.

I create a Qt Quick plugin that creates a TCP server that allows the QML developers to send object to the connect clients:

c++

Q_INVOKABLE void sendMessage(QString clientId, QVariant data);

qml

sendMessage("client1", {key1: "value1", key2: "value2"});

data will be of type QJSValue.

JKSH

@bundickt said:

I don't have a QJsonDocument object. I have a QJSValue object.

That's not a problem. We can convert it.

c++
Q_INVOKABLE void sendMessage(QString clientId, QVariant data);

qml
sendMessage("client1", {key1: "value1", key2: "value2"});

data will be of type QJSValue.

Notice that your data is inside a QVariant. So, use QJsonDocument::fromVariant() to create your JSON document, and then turn it into a string using QJsonDocument::toJson(). This works if data is a JavaScript object or JavaScript array.

bundickt

@JKSH

Thanks for your help. The documentation for QJsonDocument says the QVariant must be one of the following types: QVariantMap, QVariantList or QStringList. However, the QVariant object I have is a QJSValue. I tested it and I get an invalid QJsonDocument. After doing a little experimentation I was able to get a JSON string.

Code:

void MyTcpServer::sendMessage(QString clientId, QVariant data) {
    QJsonDocument jsonDoc = QJsonDocument::fromVariant(data.value<QJSValue>().toVariant());
    qDebug() << "json[" <<  jsonDoc.toJson() << "]";
    ....

It looks like data.value<QJSValue>().toVariant() convert the QJSValue variant to a QVariantMap variant.

JKSH

@bundickt

You're welcome, and I'm glad you've found a solution.

The documentation for QJsonDocument says the QVariant must be one of the following types: QVariantMap, QVariantList or QStringList. However, the QVariant object I have is a QJSValue. I tested it and I get an invalid QJsonDocument.
...
It looks like data.value<QJSValue>().toVariant() convert the QJSValue variant to a QVariantMap variant.

That's interesting. A QJSValue is simply a wrapper. If your QJSValue contains a JavaScript object, then it should already be stored as a QVariantMap.

Out of curiosity, could you run the following code and share the output?

void MyTcpServer::sendMessage(QString clientId, QVariant data) {
    qDebug() << data;
    ...

QJSValue stores the following datatypes like this:

JavaScript boolean -> bool
JavaScript number -> int/double
JavaScript string -> QString
JavaScript array -> QVariantList
JavaScript object -> QVariantMap
QObject -> QObject*

So, I expected your output to be: QVariant(QVariantMap, QMap(("key1", QVariant(QString, "value1") ) ( "key2" , QVariant(QString, "value2") ) ) )

bundickt

@JKSH

The output of qDebug() << data; is:

QVariant(QJSValue, )

The output of data.value<QJSValue>().toVariant() is:

QVariant(QVariantMap, QMap((key1, QVariant(QString, value1) ) ( key2 ,  QVariant(QString, value2) ) ) )

JKSH

@bundickt said:

@JKSH

The output of qDebug() << data; is:
QVariant(QJSValue, )
The output of data.value<QJSValue>().toVariant() is:
QVariant(QVariantMap, QMap((key1, QVariant(QString, value1) ) ( key2 ,  QVariant(QString, value2) ) ) )

Ah, right. I thought the QML engine would automatically convert the QJSValue to a QVariantMap, but you had to do the conversion yourself.

Thank you for sharing your findings!