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

Inheritance diagram of PySide6.QtQml.QJSEngine

Inherited by: QQmlEngine, QQmlApplicationEngine




Static functions

Detailed Description

Evaluating Scripts

Use evaluate() to evaluate script code.

myEngine = QJSEngine()
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. toString() and toNumber() ).

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

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

fileName = "helloworld.qs"
scriptFile = QFile(fileName)
if not scriptFile.open(QIODevice.ReadOnly):
    # handle error
stream = QTextStream(scriptFile)
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;

Modules don’t have to be files. They can be values registered with registerModule() :

import version from "version";

export function getVersion()
    return version;
QJSValue version(610);
myEngine.registerModule("version", version);
QJSValue module = myEngine.importModule("./myprint.mjs");
QJSValue getVersion = module.property("getVersion");
QJSValue result = getVersion.call();

Named exports are supported, but because they are treated as members of an object, the default export must be an ECMAScript object. Most of the newXYZ functions in QJSValue will return an object.

QJSValue name("Qt6");
QJSValue obj = myEngine.newObject();
obj.setProperty("name", name);
myEngine.registerModule("info", obj);
import { name } from "info";

export function getName()
    return name;

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)
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 isError() to check for exceptions.

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

  • name

  • message

  • fileName

  • lineNumber

  • stack

result = myEngine.evaluate(...)
if result.isError():
    print( cout)
            << "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. 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.

button = QPushButton()
scriptButton = myEngine.newQObject(button)
myEngine.globalObject().setProperty("button", scriptButton)
myEngine.evaluate("button.checkable = True")
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(QObject):

# public
    Q_INVOKABLE MyObject() {}

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

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

Instances of the class can then be created in JavaScript:

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


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:

engine = QJSEngine()
myQObject = QObject()
myQObject.setProperty("dynamicProperty", 3)
myScriptQObject = engine.newQObject(myQObject)
engine.globalObject().setProperty("myObject", myScriptQObject)


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.

class PySide6.QtQml.QJSEngine




Constructs a QJSEngine object.

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

Constructs a QJSEngine object with the given parent.

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


controls whether or not the JavaScript memory manager automatically destroys the QObject when the corresponding JavaScript object is garbage collected by the engine. The two ownership options are:




The object is owned by C++ code and the JavaScript memory manager will never delete it. The JavaScript destroy() method cannot be used on these objects. This option is similar to QScriptEngine::QtOwnership.


The object is owned by JavaScript. When the object is returned to the JavaScript memory manager as the return value of a method call, the JavaScript memory manager will track it and delete it if there are no remaining JavaScript references to it and it has no parent() . An object tracked by one QJSEngine will be deleted during that QJSEngine ‘s destructor. Thus, JavaScript references between objects with from two different engines will not be valid if one of these engines is deleted. This option is similar to QScriptEngine::ScriptOwnership.

Generally an application doesn’t need to set an object’s ownership explicitly. The JavaScript memory manager uses a heuristic to set the default ownership. By default, an object that is created by the JavaScript memory manager has . The exception to this are the root objects created by calling create() or beginCreate() , which have by default. The ownership of these root-level objects is considered to have been transferred to the C++ caller.

Objects not-created by the JavaScript memory manager have by default. The exception to this are objects returned from C++ method calls; their ownership will be set to . This applies only to explicit invocations of Q_INVOKABLE methods or slots, but not to property getter invocations.

Calling setObjectOwnership() overrides the default ownership.

See also

Data Ownership

New in version 6.0.


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




Indicates that translation functions (qsTr(), for example) should be installed. This also installs the Qt. uiLanguage property.


Indicates that console functions (console.log(), for example) should be installed.


Indicates that garbage collection functions (gc(), for example) should be installed.


Indicates that all extension should be installed.


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

Script Function

Corresponding 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() .

New in version 5.6.

Return type


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.


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.

PySide6.QtQml.QJSEngine.evaluate(program[, fileName=""[, lineNumber=1[, exceptionStackTrace=None]]])
  • program – str

  • fileName – str

  • lineNumber – int

  • exceptionStackTrace – list of strings

Return type


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 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


In some cases, e.g. for native functions, function name and file name can be empty and line number and column can be -1.


If an exception was thrown and the exception value is not an Error instance (i.e., isError() returns false), the exception value will still be returned. Use exceptionStackTrace->isEmpty() to distinguish whether the value was a normal or an exceptional return value.

Return type


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.

Return type


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.


fileName – str

Return type


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 canonicalFilePath() . That means that multiple imports of the same file on disk using different relative paths will load the file only once.


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

See also


PySide6.QtQml.QJSEngine.installExtensions(extensions[, 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);

See also


Return type


Returns whether JavaScript execution is currently interrupted.

See also




Return type


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

See also


PySide6.QtQml.QJSEngine.newErrorObject(errorType[, message=""])
Return type


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

The prototype of the created object will be errorType.

Return type


Creates a JavaScript object of class Object.

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



Return type


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.

See also

newQObject() QObject Integration



Return type


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



name – str

Return type


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

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

See also


static PySide6.QtQml.QJSEngine.objectOwnership(arg__1)


Return type


Returns the ownership of object.

See also

setObjectOwnership() ObjectOwnership

PySide6.QtQml.QJSEngine.registerModule(moduleName, value)
Return type


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.


The QJSValue value is not called or read until it is used by another module. This means that there is no code to evaluate, so no errors will be seen until another module throws an exception while trying to load this module.


Attempting to access a named export from a QJSValue that is not an object will trigger a exception .

See also



interrupted – bool

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.

See also


static PySide6.QtQml.QJSEngine.setObjectOwnership(arg__1, arg__2)

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() ObjectOwnership


language – str

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 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.

PySide6.QtQml.QJSEngine.throwError(errorType[, message=""])

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");

See also

Script Exceptions newErrorObject()



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.

See also

Script Exceptions newErrorObject()


message – str

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 ErrorType errorType, const QString &message) overload.

See also

Script Exceptions


arg__1 – object

Return type


Return type


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 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.