QJSEngine Class

The QJSEngine class provides an environment for evaluating JavaScript code. More...

Header: #include <QJSEngine>
qmake: QT += qml
Since: Qt 5.0
Inherits: QObject
Inherited By:


This class was introduced in Qt 5.0.

Note: All functions in this class are reentrant.

Public Types

enum Extension { TranslationExtension, ConsoleExtension, GarbageCollectionExtension, AllExtensions }
flags Extensions


Public Functions

QJSEngine(QObject *parent)
virtual ~QJSEngine() override
void collectGarbage()
QJSValue evaluate(const QString &program, const QString &fileName = QString(), int lineNumber = 1)
T fromScriptValue(const QJSValue &value)
QJSValue globalObject() const
QJSValue importModule(const QString &fileName)
void installExtensions(QJSEngine::Extensions extensions, const QJSValue &object = QJSValue())
bool isInterrupted() const
QJSValue newArray(uint length = 0)
QJSValue newErrorObject(QJSValue::ErrorType errorType, const QString &message = QString())
QJSValue newObject()
QJSValue newQMetaObject(const QMetaObject *metaObject)
QJSValue newQMetaObject()
QJSValue newQObject(QObject *object)
void setInterrupted(bool interrupted)
void setUiLanguage(const QString &language)
void throwError(const QString &message)
void throwError(QJSValue::ErrorType errorType, const QString &message = QString())
QJSValue toScriptValue(const T &value)
QString uiLanguage() const


QJSEngine *qjsEngine(const QObject *object)

Detailed Description

Evaluating Scripts

Use evaluate() to evaluate script code.

QJSEngine myEngine;
QJSValue three = myEngine.evaluate("1 + 2");

evaluate() returns a QJSValue that holds the result of the evaluation. The QJSValue class provides functions for converting the result to various C++ types (e.g. QJSValue::toString() and QJSValue::toNumber()).

The following code snippet shows how a script function can be defined and then invoked from C++ using QJSValue::call():

QJSValue fun = myEngine.evaluate("(function(a, b) { return a + b; })");
QJSValueList args;
args << 1 << 2;
QJSValue threeAgain = fun.call(args);

As can be seen from the above snippets, a script is provided to the engine in the form of a string. One common way of loading scripts is by reading the contents of a file and passing it to evaluate():

QString fileName = "helloworld.qs";
QFile scriptFile(fileName);
if (!scriptFile.open(QIODevice::ReadOnly))
    // handle error
QTextStream stream(&scriptFile);
QString contents = stream.readAll();
myEngine.evaluate(contents, fileName);

Here we pass the name of the file as the second argument to evaluate(). This does not affect evaluation in any way; the second argument is a general-purpose string that is stored in the Error object for debugging purposes.

For larger pieces of functionality, you may want to encapsulate your code and data into modules. A module is a file that contains script code, variables, etc., and uses export statements to describe its interface towards the rest of the application. With the help of import statements, a module can refer to functionality from other modules. This allows building a scripted application from smaller connected building blocks in a safe way. In contrast, the approach of using evaluate() carries the risk that internal variables or functions from one evaluate() call accidentally pollute the global object and affect subsequent evaluations.

The following example provides a module that can add numbers:

export function sum(left, right)
    return left + right

This module can be loaded with QJSEngine::import() if it is saved under the name math.mjs:

QJSvalue module = myEngine.importModule("./math.mjs");
QJSValue sumFunction = module.property("sum");
QJSValue result = sumFunction.call(args);

Modules can also use functionality from other modules using import statements:

import { sum } from "./math.mjs";
export function addTwice(left, right)
    return sum(left, right) * 2;

Engine Configuration

The globalObject() function returns the Global Object associated with the script engine. Properties of the Global Object are accessible from any script code (i.e. they are global variables). Typically, before evaluating "user" scripts, you will want to configure a script engine by adding one or more properties to the Global Object:

myEngine.globalObject().setProperty("myNumber", 123);
QJSValue myNumberPlusOne = myEngine.evaluate("myNumber + 1");

Adding custom properties to the scripting environment is one of the standard means of providing a scripting API that is specific to your application. Usually these custom properties are objects created by the newQObject() or newObject() functions.

Script Exceptions

evaluate() can throw a script exception (e.g. due to a syntax error). If it does, then evaluate() returns the value that was thrown (typically an Error object). Use QJSValue::isError() to check for exceptions.

For detailed information about the error, use QJSValue::toString() to obtain an error message, and use QJSValue::property() to query the properties of the Error object. The following properties are available:

  • name
  • message
  • fileName
  • lineNumber
  • stack
QJSValue result = myEngine.evaluate(...);
if (result.isError())
            << "Uncaught exception at line"
            << result.property("lineNumber").toInt()
            << ":" << result.toString();

Script Object Creation

Use newObject() to create a JavaScript object; this is the C++ equivalent of the script statement new Object(). You can use the object-specific functionality in QJSValue to manipulate the script object (e.g. QJSValue::setProperty()). Similarly, use newArray() to create a JavaScript array object.

QObject Integration

Use newQObject() to wrap a QObject (or subclass) pointer. newQObject() returns a proxy script object; properties, children, and signals and slots of the QObject are available as properties of the proxy object. No binding code is needed because it is done dynamically using the Qt meta object system.

QPushButton *button = new QPushButton;
QJSValue scriptButton = myEngine.newQObject(button);
myEngine.globalObject().setProperty("button", scriptButton);

myEngine.evaluate("button.checkable = true");

qDebug() << scriptButton.property("checkable").toBool();
scriptButton.property("show").call(); // call the show() slot

Use newQMetaObject() to wrap a QMetaObject; this gives you a "script representation" of a QObject-based class. newQMetaObject() returns a proxy script object; enum values of the class are available as properties of the proxy object.

Constructors exposed to the meta-object system (using Q_INVOKABLE) can be called from the script to create a new QObject instance with JavaScriptOwnership. For example, given the following class definition:

class MyObject : public QObject

    Q_INVOKABLE MyObject() {}

The staticMetaObject for the class can be exposed to JavaScript like so:

QJSValue jsMetaObject = engine.newQMetaObject(&MyObject::staticMetaObject);
engine.globalObject().setProperty("MyObject", jsMetaObject);

Instances of the class can then be created in JavaScript:

engine.evaluate("var myObject = new MyObject()");

Note: Currently only classes using the Q_OBJECT macro are supported; it is not possible to expose the staticMetaObject of a Q_GADGET class to JavaScript.

Dynamic QObject Properties

Dynamic QObject properties are not supported. For example, the following code will not work:

QJSEngine engine;

QObject *myQObject = new QObject();
myQObject->setProperty("dynamicProperty", 3);

QJSValue myScriptQObject = engine.newQObject(myQObject);
engine.globalObject().setProperty("myObject", myScriptQObject);

qDebug() << engine.evaluate("myObject.dynamicProperty").toInt();


QJSEngine provides a compliant ECMAScript implementation. By default, familiar utilities like logging are not available, but they can can be installed via the installExtensions() function.

See also QJSValue, Making Applications Scriptable, and List of JavaScript Objects and Functions.

Member Type Documentation

enum QJSEngine::Extension
flags QJSEngine::Extensions

This enum is used to specify extensions to be installed via installExtensions().

QJSEngine::TranslationExtension0x1Indicates that translation functions (qsTr(), for example) should be installed. This also installs the Qt.uiLanguage property.
QJSEngine::ConsoleExtension0x2Indicates that console functions (console.log(), for example) should be installed.
QJSEngine::GarbageCollectionExtension0x4Indicates that garbage collection functions (gc(), for example) should be installed.
QJSEngine::AllExtensions0xffffffffIndicates that all extension should be installed.


The relation between script translation functions and C++ translation functions is described in the following table:

Script FunctionCorresponding C++ Function

This flag also adds an arg() function to the string prototype.

For more information, see the Internationalization with Qt documentation.


The console object implements a subset of the Console API, which provides familiar logging functions, such as console.log().

The list of functions added is as follows:

  • console.assert()
  • console.debug()
  • console.exception()
  • console.info()
  • console.log() (equivalent to console.debug())
  • console.error()
  • console.time()
  • console.timeEnd()
  • console.trace()
  • console.count()
  • console.warn()
  • print() (equivalent to console.debug())

For more information, see the Console API documentation.


The gc() function is equivalent to calling collectGarbage().

The Extensions type is a typedef for QFlags<Extension>. It stores an OR combination of Extension values.

Property Documentation

uiLanguage : QString

This property holds the language to be used for translating user interface strings

This property holds the name of the language to be used for user interface string translations. It is exposed for reading and writing as Qt.uiLanguage when the QJSEngine::TranslationExtension is installed on the engine. It is always exposed in instances of QQmlEngine.

You can set the value freely and use it in bindings. It is recommended to set it after installing translators in your application. By convention, an empty string means no translation from the language used in the source code is intended to occur.

This property was introduced in Qt 5.15.

Access functions:

QString uiLanguage() const
void setUiLanguage(const QString &language)

Notifier signal:

void uiLanguageChanged()

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.


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.

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)

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.

Note: If an exception was thrown and the exception value is not an Error instance (i.e., QJSValue::isError() returns false), the exception value will still be returned, but there is currently no API for detecting that an exception did occur in this case.

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

Returns the given value converted to the template type T.

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.

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.

Note: If an exception is thrown during the loading of the module, the return value will be the exception (typically an Error object; see QJSValue::isError()).

This function was introduced in Qt 5.12.

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.

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().

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().

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.

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().

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().

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)) {
    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.

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) {
                         "Age must be between 0 and 200");

This function was introduced in Qt 5.12.

See also Script Exceptions and newErrorObject().

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

Creates a QJSValue with the given value.

See also fromScriptValue().

Related Non-Members

QJSEngine *qjsEngine(const QObject *object)

Returns the QJSEngine associated with object, if any.

This function is useful if you have exposed a QObject to the JavaScript environment and later in your program would like to regain access. It does not require you to keep the wrapper around that was returned from QJSEngine::newQObject().

This function was introduced in Qt 5.5.

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