QJSEngine Class

Member Function Documentation

QJSEngine::QJSEngine(QObject *parent)

Constructs a QJSEngine object with the given parent.

The globalObject() is initialized to have properties as described in ECMA-262, Section 15.1.

QJSEngine::QJSEngine()

Constructs a QJSEngine object.

The globalObject() is initialized to have properties as described in ECMA-262, Section 15.1.

[override virtual] QJSEngine::~QJSEngine()

Destroys this QJSEngine.

Garbage is not collected from the persistent JS heap during QJSEngine destruction. If you need all memory freed, call collectGarbage manually right before destroying the QJSEngine.

[since Qt 6.1] QJSValue QJSEngine::catchError()

If an exception is currently pending, catches it and returns it as a QJSValue. Otherwise returns undefined as QJSValue. After calling this method hasError() returns false.

This function was introduced in Qt 6.1.

void QJSEngine::collectGarbage()

Runs the garbage collector.

The garbage collector will attempt to reclaim memory by locating and disposing of objects that are no longer reachable in the script environment.

Normally you don't need to call this function; the garbage collector will automatically be invoked when the QJSEngine decides that it's wise to do so (i.e. when a certain number of new objects have been created). However, you can call this function to explicitly request that garbage collection should be performed as soon as possible.

QJSValue QJSEngine::evaluate(const QString &program, const QString &fileName = QString(), int lineNumber = 1, QStringList *exceptionStackTrace = nullptr)

Evaluates program, using lineNumber as the base line number, and returns the result of the evaluation.

The script code will be evaluated in the context of the global object.

The evaluation of program can cause an exception in the engine; in this case the return value will be the exception that was thrown (typically an Error object; see QJSValue::isError()).

lineNumber is used to specify a starting line number for program; line number information reported by the engine that pertains to this evaluation will be based on this argument. For example, if program consists of two lines of code, and the statement on the second line causes a script exception, the exception line number would be lineNumber plus one. When no starting line number is specified, line numbers will be 1-based.

fileName is used for error reporting. For example, in error objects the file name is accessible through the "fileName" property if it is provided with this function.

exceptionStackTrace is used to report whether an uncaught exception was thrown. If you pass a non-null pointer to a QStringList to it, it will set it to list of "stackframe messages" if the script threw an unhandled exception, or an empty list otherwise. A stackframe message has the format function name:line number:column:file name

See also QQmlExpression::evaluate.

template <typename T> T QJSEngine::fromScriptValue(const QJSValue &value)

Returns the given value converted to the template type T. This works with any type T that has a QMetaType.

See also toScriptValue().

QJSValue QJSEngine::globalObject() const

Returns this engine's Global Object.

By default, the Global Object contains the built-in objects that are part of ECMA-262, such as Math, Date and String. Additionally, you can set properties of the Global Object to make your own extensions available to all script code. Non-local variables in script code will be created as properties of the Global Object, as well as local variables in global code.

[since Qt 6.1] bool QJSEngine::hasError() const

Returns true if the last JavaScript execution resulted in an exception or if throwError() was called. Otherwise returns false. Mind that evaluate() catches any exceptions thrown in the evaluated code.

This function was introduced in Qt 6.1.

[since 5.12] QJSValue QJSEngine::importModule(const QString &fileName)

Imports the module located at fileName and returns a module namespace object that contains all exported variables, constants and functions as properties.

If this is the first time the module is imported in the engine, the file is loaded from the specified location in either the local file system or the Qt resource system and evaluated as an ECMAScript module. The file is expected to be encoded in UTF-8 text.

Subsequent imports of the same module will return the previously imported instance. Modules are singletons and remain around until the engine is destroyed.

The specified fileName will internally be normalized using QFileInfo::canonicalFilePath(). That means that multiple imports of the same file on disk using different relative paths will load the file only once.

This function was introduced in Qt 5.12.

See also registerModule().

[since 5.6] void QJSEngine::installExtensions(QJSEngine::Extensions extensions, const QJSValue &object = QJSValue())

Installs JavaScript extensions to add functionality that is not available in a standard ECMAScript implementation.

The extensions are installed on the given object, or on the Global Object if no object is specified.

Several extensions can be installed at once by OR-ing the enum values:

installExtensions(QJSEngine::TranslationExtension | QJSEngine::ConsoleExtension);

This function was introduced in Qt 5.6.

See also Extension.

[since 5.14] bool QJSEngine::isInterrupted() const

Returns whether JavaScript execution is currently interrupted.

This function was introduced in Qt 5.14.

See also setInterrupted().

QJSValue QJSEngine::newArray(uint length = 0)

Creates a JavaScript object of class Array with the given length.

See also newObject().

[since 5.12] QJSValue QJSEngine::newErrorObject(QJSValue::ErrorType errorType, const QString &message = QString())

Creates a JavaScript object of class Error, with message as the error message.

The prototype of the created object will be errorType.

This function was introduced in Qt 5.12.

See also newObject(), throwError(), and QJSValue::isError().

QJSValue QJSEngine::newObject()

Creates a JavaScript object of class Object.

The prototype of the created object will be the Object prototype object.

See also newArray() and QJSValue::setProperty().

[since 5.8] QJSValue QJSEngine::newQMetaObject(const QMetaObject *metaObject)

Creates a JavaScript object that wraps the given QMetaObject The metaObject must outlive the script engine. It is recommended to only use this method with static metaobjects.

When called as a constructor, a new instance of the class will be created. Only constructors exposed by Q_INVOKABLE will be visible from the script engine.

This function was introduced in Qt 5.8.

See also newQObject() and QObject Integration.

[since 5.8] template <typename T> QJSValue QJSEngine::newQMetaObject()

Creates a JavaScript object that wraps the static QMetaObject associated with class T.

This function was introduced in Qt 5.8.

See also newQObject() and QObject Integration.

QJSValue QJSEngine::newQObject(QObject *object)

Creates a JavaScript object that wraps the given QObject object, using JavaScriptOwnership.

Signals and slots, properties and children of object are available as properties of the created QJSValue.

If object is a null pointer, this function returns a null value.

If a default prototype has been registered for the object's class (or its superclass, recursively), the prototype of the new script object will be set to be that default prototype.

If the given object is deleted outside of the engine's control, any attempt to access the deleted QObject's members through the JavaScript wrapper object (either by script code or C++) will result in a script exception.

See also QJSValue::toQObject().

[since 6.2] QJSValue QJSEngine::newSymbol(const QString &name)

Creates a JavaScript object of class Symbol, with value name.

The prototype of the created object will be the Symbol prototype object.

This function was introduced in Qt 6.2.

See also newObject().

[static] QJSEngine::ObjectOwnership QJSEngine::objectOwnership(QObject *object)

Returns the ownership of object.

See also setObjectOwnership() and QJSEngine::ObjectOwnership.

bool QJSEngine::registerModule(const QString &moduleName, const QJSValue &value)

Registers a QJSValue to serve as a module. After this function is called, all modules that import moduleName will import the value of value instead of loading moduleName from the filesystem.

Any valid QJSValue can be registered, but named exports (i.e. import { name } from "info" are treated as members of an object, so the default export must be created with one of the newXYZ methods of QJSEngine.

Because this allows modules that do not exist on the filesystem to be imported, scripting applications can use this to provide built-in modules, similar to Node.js.

Returns true on success, false otherwise.

See also importModule().

[since 5.14] void QJSEngine::setInterrupted(bool interrupted)

Interrupts or re-enables JavaScript execution.

If interrupted is true, any JavaScript executed by this engine immediately aborts and returns an error object until this function is called again with a value of false for interrupted.

This function is thread safe. You may call it from a different thread in order to interrupt, for example, an infinite loop in JavaScript.

This function was introduced in Qt 5.14.

See also isInterrupted().

[static] void QJSEngine::setObjectOwnership(QObject *object, QJSEngine::ObjectOwnership ownership)

Sets the ownership of object.

An object with JavaScriptOwnership is not garbage collected as long as it still has a parent, even if there are no references to it.

See also objectOwnership() and QJSEngine::ObjectOwnership.

[since Qt 5.12] void QJSEngine::throwError(const QString &message)

Throws a run-time error (exception) with the given message.

This method is the C++ counterpart of a throw() expression in JavaScript. It enables C++ code to report run-time errors to QJSEngine. Therefore it should only be called from C++ code that was invoked by a JavaScript function through QJSEngine.

When returning from C++, the engine will interrupt the normal flow of execution and call the the next pre-registered exception handler with an error object that contains the given message. The error object will point to the location of the top-most context on the JavaScript caller stack; specifically, it will have properties lineNumber, fileName and stack. These properties are described in Script Exceptions.

In the following example a C++ method in FileAccess.cpp throws an error in qmlFile.qml at the position where readFileAsText() is called:

// qmlFile.qml
function someFunction() {
  ...
  var text = FileAccess.readFileAsText("/path/to/file.txt");
}

// FileAccess.cpp
// Assuming that FileAccess is a QObject-derived class that has been
// registered as a singleton type and provides an invokable method
// readFileAsText()

QJSValue FileAccess::readFileAsText(const QString & filePath) {
  QFile file(filePath);

  if (!file.open(QIODevice::ReadOnly)) {
    jsEngine->throwError(file.errorString());
    return QString();
  }

  ...
  return content;
}

It is also possible to catch the thrown error in JavaScript:

// qmlFile.qml
function someFunction() {
  ...
  var text;
  try {
    text = FileAccess.readFileAsText("/path/to/file.txt");
  } catch (error) {
    console.warn("In " + error.fileName + ":" + "error.lineNumber" +
                 ": " + error.message);
  }
}

If you need a more specific run-time error to describe an exception, you can use the throwError(QJSValue::ErrorType errorType, const QString &message) overload.

This function was introduced in Qt 5.12.

See also Script Exceptions.

[since Qt 5.12] void QJSEngine::throwError(QJSValue::ErrorType errorType, const QString &message = QString())

This function overloads throwError().

Throws a run-time error (exception) with the given errorType and message.

// Assuming that DataEntry is a QObject-derived class that has been
// registered as a singleton type and provides an invokable method
// setAge().

void DataEntry::setAge(int age) {
  if (age < 0 || age > 200) {
    jsEngine->throwError(QJSValue::RangeError,
                         "Age must be between 0 and 200");
  }
  ...
}

This function was introduced in Qt 5.12.

See also Script Exceptions and newErrorObject().

[since 6.1] void QJSEngine::throwError(const QJSValue &error)

This function overloads throwError().

Throws a pre-constructed run-time error (exception). This way you can use newErrorObject() to create the error and customize it as necessary.

This function was introduced in Qt 6.1.

See also Script Exceptions and newErrorObject().

template <typename T> QJSValue QJSEngine::toScriptValue(const T &value)

Creates a QJSValue with the given value. This works with any type T that has a QMetaType.

See also fromScriptValue().