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  1. Qt
  2. QTBUG-125611

Inconsistent QString::isNull behavior in QJSEngine::evaluate result and QJSValue constructed from QString

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    • macOS, Windows
    • df684931c (dev), 14bb25f69 (6.8)

      We encountered an issue where a value returned from QJSEngine::evaluate returns a null QString. This caused problems when writing the result to a SQL database where we get NULL instead of an empty string (Previously we used QScriptEngine where it worked as expected).

      This behavior is inconsistent with a QJSValue constructed with either an empty or null QString. In that case value.toString().isNull() is false.

      I would expect the same for the evaluate result.

      Here's a small example:

      #include <QCoreApplication>
#include <QJSEngine>
#include <QJSValue>
#include <QString>
#include <QVariant>

int main(int argc, char *argv[]) {
  QCoreApplication app(argc, argv);
  QJSEngine engine;
  const auto evalResult = engine.evaluate(R"(
function f() {
  return "";
}
f();
)");

	const auto evalResultStr = evalResult.toString();
	qDebug() << "QJSEngine::evaluate  : " << (evalResultStr.isNull() ? "null" : "not null");

	QJSValue jsValueNullStr = QString{};
	qDebug() << "QJSValue(QString())  : " << (jsValueNullStr.isNull() ? "null" : "not null");

	QJSValue jsValueEmptyStr = QStringLiteral("");
	qDebug() << "QJSValue(QString(\"\")): " << (jsValueEmptyStr.isNull() ? "null" : "not null");
}

      This is the output:

      QJSEngine::evaluate  :  null
QJSValue(QString())  :  not null
QJSValue(QString("")):  not null

        For Gerrit Dashboard: QTBUG-125611
        # Subject Branch Project Status CR V
        570643,8 QJSEngine: Treat empty string literals as non-null, empty QStrings dev qt/qtdeclarative Status: MERGED +2 0
        572107,2 QJSEngine: Treat empty string literals as non-null, empty QStrings 6.8 qt/qtdeclarative Status: MERGED +2 0

            qtqmlteam Qt Qml Team User
            lbuckel Leon Buckel
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              Created:
              Updated:
              Resolved:

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