- Qt 6.12
- Qt Qml
- C++ Classes
- QJSValue
- Qt 6.12.0 Reference Documentation

# QJSValue Class

The QJSValue class acts as a container for Qt/JavaScript data types. More...

Header: | `#include <QJSValue>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Qml)` `target_link_libraries(mytarget PRIVATE Qt6::Qml)`
qmake: | `QT += qml`

- List of all members, including inherited members
- Deprecated members

## Public Types

enum | ErrorType { GenericError, RangeError, ReferenceError, SyntaxError, TypeError, URIError }
enum | ObjectConversionBehavior { ConvertJSObjects, RetainJSObjects }
enum | SpecialValue { UndefinedValue, NullValue }

## Public Functions

| QJSValue(QJSValue::SpecialValue value = UndefinedValue)
| QJSValue(bool value)
| QJSValue(const QLatin1String &value)
| QJSValue(const QString &value)
| QJSValue(const char *value)
| QJSValue(double value)
| QJSValue(int value)
| QJSValue(uint value)
| QJSValue(const QJSValue &other)
| QJSValue(QJSValue &&other)
| ~QJSValue()
QJSValue | call(const QJSValueList &args = QJSValueList()) const
QJSValue | callAsConstructor(const QJSValueList &args = QJSValueList()) const
QJSValue | callWithInstance(const QJSValue &instance, const QJSValueList &args = QJSValueList()) const
bool | deleteProperty(const QString &name)
bool | equals(const QJSValue &other) const
QJSValue::ErrorType | errorType() const
bool | hasOwnProperty(const QString &name) const
bool | hasProperty(const QString &name) const
bool | isArray() const
bool | isBool() const
bool | isCallable() const
bool | isDate() const
bool | isError() const
bool | isNull() const
bool | isNumber() const
bool | isObject() const
bool | isQMetaObject() const
bool | isQObject() const
bool | isRegExp() const
bool | isString() const
bool | isUndefined() const
bool | isUrl() const
QJSValue | property(const QString &name) const
QJSValue | property(quint32 arrayIndex) const
QJSValue | prototype() const
void | setProperty(const QString &name, const QJSValue &value)
void | setProperty(quint32 arrayIndex, const QJSValue &value)
void | setPrototype(const QJSValue &prototype)
bool | strictlyEquals(const QJSValue &other) const
bool | toBool() const
QDateTime | toDateTime() const
qint32 | toInt() const
double | toNumber() const
QJSPrimitiveValue | toPrimitive() const
const QMetaObject * | toQMetaObject() const
QObject * | toQObject() const
QString | toString() const
quint32 | toUInt() const
QVariant | toVariant(QJSValue::ObjectConversionBehavior behavior) const
QVariant | toVariant() const
QJSValue & | operator=(QJSValue &&other)
QJSValue & | operator=(const QJSValue &other)

## Related Non-Members

| QJSValueList

## Detailed Description

QJSValue supports the types defined in the ECMA-262 standard: The primitive types, which are Undefined, Null, Boolean, Number, and String; and the Object and Array types. Additionally, built-in support is provided for Qt/C++ types such as QVariant and QObject.

For the object-based types (including Date and RegExp), use the newT() functions in QJSEngine (e.g. QJSEngine::newObject()) to create a QJSValue of the desired type. For the primitive types, use one of the QJSValue constructor overloads. For other types, e.g. registered gadget types such as QPoint, you can use QJSEngine::toScriptValue.

The methods named isT() (e.g. isBool(), isUndefined()) can be used to test if a value is of a certain type. The methods named toT() (e.g. toBool(), toString()) can be used to convert a QJSValue to another type. You can also use the generic qjsvalue_cast() function.

Object values have zero or more properties which are themselves QJSValues. Use setProperty() to set a property of an object, and call property() to retrieve the value of a property.

     QJSEngine myEngine;
     QJSValue myObject = myEngine.newObject();
     QJSValue myOtherObject = myEngine.newObject();
     myObject.setProperty("myChild", myOtherObject);
     myObject.setProperty("name", "John Doe");

If you want to iterate over the properties of a script object, use the QJSValueIterator class.

Object values have an internal `prototype` property, which can be accessed with prototype() and setPrototype().

Function objects (objects for which isCallable()) returns true) can be invoked by calling call(). Constructor functions can be used to construct new objects by calling callAsConstructor().

Use equals() or strictlyEquals() to compare a QJSValue to another.

Note that a QJSValue for which isObject() is true only carries a reference to an actual object; copying the QJSValue will only copy the object reference, not the object itself. If you want to clone an object (i.e. copy an object's properties to another object), you can do so with the help of a `for-in` statement in script code, or QJSValueIterator in C++.

### Working With Arrays

To create an array using QJSValue, use QJSEngine::newArray():

     // Assumes that this class was declared in QML.
     QJSValue jsArray = engine->newArray(3);

To set individual elements in the array, use the setProperty(quint32 arrayIndex, const QJSValue &value) overload. For example, to fill the array above with integers:

     for (int i = 0; i < 3; ++i) {
         jsArray.setProperty(i, QRandomGenerator::global().generate());
     }

To determine the length of the array, access the `"length"` property. To access array elements, use the property(quint32 arrayIndex) overload. The following code reads the array we created above back into a list:

     QList<int> integers;
     const int length = jsArray.property("length").toInt();
     for (int i = 0; i < length; ++i) {
         integers.append(jsArray.property(i).toInt());
     }

#### Converting to JSON

It's possible to convert a QJSValue to a JSON type. For example, to convert to an array, use QJSEngine::fromScriptValue():

     const QJsonValue jsonValue = engine.fromScriptValue<QJsonValue>(jsValue);
     const QJsonArray jsonArray = jsonValue.toArray();

See also QJSEngine and QJSValueIterator.

## Member Type Documentation

### enum QJSValue::ErrorType

Use this enum for JavaScript language-specific types of Error objects.

They may be useful when emulating language features in C++ requires the use of specialized exception types. In addition, they may help to more clearly communicate certain typical conditions, instead of throwing a generic JavaScript exception. For example, code that deals with networking and resource locators may find it useful to propagate errors related to malformed locators using the URIError type.

Constant | Value | Description
--- | --- | ---
`QJSValue::GenericError` | `1` | A generic Error object, but not of a specific sub-type.
`QJSValue::RangeError` | `3` | A value did not match the expected set or range.
`QJSValue::ReferenceError` | `4` | A non-existing variable referenced.
`QJSValue::SyntaxError` | `5` | An invalid token or sequence of tokens was encountered that does not conform with the syntax of the language.
`QJSValue::TypeError` | `6` | An operand or argument is incompatible with the type expected.
`QJSValue::URIError` | `7` | A URI handling function was used incorrectly or the URI provided is malformed.

### enum QJSValue::ObjectConversionBehavior

This enum is used to specify how JavaScript objects and symbols without an equivalent native Qt type should be treated when converting to QVariant.

Constant | Value | Description
--- | --- | ---
`QJSValue::ConvertJSObjects` | `0` | A best-effort, possibly lossy, conversion is attempted. Symbols are converted to QString.
`QJSValue::RetainJSObjects` | `1` | The value is retained as QJSValue wrapped in QVariant.

### enum QJSValue::SpecialValue

This enum is used to specify a single-valued type.

Constant | Value | Description
--- | --- | ---
`QJSValue::UndefinedValue` | `1` | An undefined value.
`QJSValue::NullValue` | `0` | A null value.

## Member Function Documentation

### QJSValue::QJSValue(QJSValue::SpecialValue value = UndefinedValue)

Constructs a new QJSValue with a special value.

### QJSValue::QJSValue(bool value)

Constructs a new QJSValue with a boolean value.

### QJSValue::QJSValue(const QLatin1String &value)

Constructs a new QJSValue with a string value.

### QJSValue::QJSValue(const QString &value)

Constructs a new QJSValue with a string value.

### QJSValue::QJSValue(const char *value)

Constructs a new QJSValue with a string value.

### QJSValue::QJSValue(double value)

Constructs a new QJSValue with a number value.

### QJSValue::QJSValue(int value)

Constructs a new QJSValue with a number value.

### QJSValue::QJSValue(uint value)

Constructs a new QJSValue with a number value.

### QJSValue::QJSValue(const QJSValue &other)

Constructs a new QJSValue that is a copy of other.

Note that if other is an object (i.e., isObject() would return true), then only a reference to the underlying object is copied into the new script value (i.e., the object itself is not copied).

### QJSValue::QJSValue(QJSValue &&other)

Move constructor. Moves from other into this QJSValue object.

### `[noexcept]` QJSValue::~QJSValue()

Destroys this QJSValue.

### QJSValue QJSValue::call(const QJSValueList &args = QJSValueList()) const

Calls this QJSValue as a function, passing args as arguments to the function, and using the globalObject() as the "this"-object. Returns the value returned from the function.

If this QJSValue is not callable, call() does nothing and returns an undefined QJSValue.

Calling call() can cause an exception to occur in the script engine; in that case, call() returns the value that was thrown (typically an `Error` object). You can call isError() on the return value to determine whether an exception occurred.

See also isCallable(), callWithInstance(), and callAsConstructor().

### QJSValue QJSValue::callAsConstructor(const QJSValueList &args = QJSValueList()) const

Creates a new `Object` and calls this QJSValue as a constructor, using the created object as the `this' object and passing args as arguments. If the return value from the constructor call is an object, then that object is returned; otherwise the default constructed object is returned.

If this QJSValue is not a function, callAsConstructor() does nothing and returns an undefined QJSValue.

Calling this function can cause an exception to occur in the script engine; in that case, the value that was thrown (typically an `Error` object) is returned. You can call isError() on the return value to determine whether an exception occurred.

See also call() and QJSEngine::newObject().

### QJSValue QJSValue::callWithInstance(const QJSValue &instance, const QJSValueList &args = QJSValueList()) const

Calls this QJSValue as a function, using instance as the `this' object in the function call, and passing args as arguments to the function. Returns the value returned from the function.

If this QJSValue is not a function, call() does nothing and returns an undefined QJSValue.

Note that if instance is not an object, the global object (see QJSEngine::globalObject()) will be used as the `this' object.

Calling call() can cause an exception to occur in the script engine; in that case, call() returns the value that was thrown (typically an `Error` object). You can call isError() on the return value to determine whether an exception occurred.

See also call().

### bool QJSValue::deleteProperty(const QString &name)

Attempts to delete this object's property of the given name. Returns true if the property was deleted, otherwise returns false.

The behavior of this function is consistent with the JavaScript delete operator. In particular:

- Non-configurable properties cannot be deleted.
- This function will return true even if this object doesn't have a property of the given name (i.e., non-existent properties are "trivially deletable").
- If this object doesn't have an own property of the given name, but an object in the prototype() chain does, the prototype object's property is not deleted, and this function returns true.

See also setProperty() and hasOwnProperty().

### bool QJSValue::equals(const QJSValue &other) const

Returns true if this QJSValue is equal to other, otherwise returns false. The comparison follows the behavior described in ECMA-262 section 11.9.3, "The Abstract Equality Comparison Algorithm".

This function can return true even if the type of this QJSValue is different from the type of the other value; i.e. the comparison is not strict. For example, comparing the number 9 to the string "9" returns true; comparing an undefined value to a null value returns true; comparing a `Number` object whose primitive value is 6 to a `String` object whose primitive value is "6" returns true; and comparing the number 1 to the boolean value `true` returns true. If you want to perform a comparison without such implicit value conversion, use strictlyEquals().

Note that if this QJSValue or the other value are objects, calling this function has side effects on the script engine, since the engine will call the object's valueOf() function (and possibly toString()) in an attempt to convert the object to a primitive value (possibly resulting in an uncaught script exception).

See also strictlyEquals().

### QJSValue::ErrorType QJSValue::errorType() const

Returns the error type this QJSValue represents if it is an Error object. Otherwise, returns `NoError."`

See also isError() and QJSEngine - Script Exceptions.

### bool QJSValue::hasOwnProperty(const QString &name) const

Returns true if this object has an own (not prototype-inherited) property of the given name, otherwise returns false.

See also property() and hasProperty().

### bool QJSValue::hasProperty(const QString &name) const

Returns true if this object has a property of the given name, otherwise returns false.

See also property() and hasOwnProperty().

### bool QJSValue::isArray() const

Returns true if this QJSValue is an object of the Array class; otherwise returns false.

Note: This method is the equivalent of Array.isArray() in JavaScript. You can use it to identify JavaScript arrays, but it will return `false` for any array-like objects that are not JavaScript arrays. This includes QML list objects for either value types or object types, JavaScript typed arrays, JavaScript ArrayBuffer objects, and any custom array-like objects you may create yourself. All of these behave like JavaScript arrays, though: They generally expose the same methods and the subscript operator can be used on them. Therefore, using this method to determine whether an object could be used like an array is not advisable.

See also QJSEngine::newArray().

### bool QJSValue::isBool() const

Returns true if this QJSValue is of the primitive type Boolean; otherwise returns false.

See also toBool().

### bool QJSValue::isCallable() const

Returns true if this QJSValue is a function, otherwise returns false.

See also call().

### bool QJSValue::isDate() const

Returns true if this QJSValue is an object of the Date class; otherwise returns false.

### bool QJSValue::isError() const

Returns true if this QJSValue is an object of the Error class; otherwise returns false.

See also errorType() and QJSEngine - Script Exceptions.

### bool QJSValue::isNull() const

Returns true if this QJSValue is of the primitive type Null; otherwise returns false.

### bool QJSValue::isNumber() const

Returns true if this QJSValue is of the primitive type Number; otherwise returns false.

See also toNumber().

### bool QJSValue::isObject() const

Returns true if this QJSValue is of the Object type; otherwise returns false.

Note that function values, variant values, and QObject values are objects, so this function returns true for such values.

See also QJSEngine::newObject().

### bool QJSValue::isQMetaObject() const

Returns true if this QJSValue is a QMetaObject; otherwise returns false.

See also toQMetaObject() and QJSEngine::newQMetaObject().

### bool QJSValue::isQObject() const

Returns true if this QJSValue is a QObject; otherwise returns false.

Note: This function returns true even if the QObject that this QJSValue wraps has been deleted.

See also toQObject() and QJSEngine::newQObject().

### bool QJSValue::isRegExp() const

Returns true if this QJSValue is an object of the RegExp class; otherwise returns false.

### bool QJSValue::isString() const

Returns true if this QJSValue is of the primitive type String; otherwise returns false.

See also toString().

### bool QJSValue::isUndefined() const

Returns true if this QJSValue is of the primitive type Undefined or if the managed value has been cleared (by deleting the engine). Otherwise returns false.

### bool QJSValue::isUrl() const

Returns true if this QJSValue is an object of the URL JavaScript class; otherwise returns false.

Note: For a QJSValue that contains a QUrl, this function returns false. However, `toVariant().value<QUrl>()` works in both cases.

### QJSValue QJSValue::property(const QString &name) const

Returns the value of this QJSValue's property with the given name. If no such property exists, an undefined QJSValue is returned.

If the property is implemented using a getter function (i.e. has the PropertyGetter flag set), calling property() has side-effects on the script engine, since the getter function will be called (possibly resulting in an uncaught script exception). If an exception occurred, property() returns the value that was thrown (typically an `Error` object).

To access array elements, use the setProperty(quint32 arrayIndex, const QJSValue &value) overload instead.

See also setProperty(), hasProperty(), and QJSValueIterator.

### QJSValue QJSValue::property(quint32 arrayIndex) const

Returns the property at the given arrayIndex.

It is possible to access elements in an array in two ways. The first is to use the array index as the property name:

     qDebug() << jsValueArray.property(QLatin1String("4")).toString();

The second is to use the overload that takes an index:

     qDebug() << jsValueArray.property(4).toString();

Both of these approaches achieve the same result, except that the latter:

- Is easier to use (can use an integer directly)
- Is faster (no conversion to integer)

If this QJSValue is not an Array object, this function behaves as if property() was called with the string representation of arrayIndex.

This is an overloaded function.

### QJSValue QJSValue::prototype() const

If this QJSValue is an object, returns the internal prototype (`__proto__` property) of this object; otherwise returns an undefined QJSValue.

See also setPrototype() and isObject().

### void QJSValue::setProperty(const QString &name, const QJSValue &value)

Sets the value of this QJSValue's property with the given name to the given value.

If this QJSValue is not an object, this function does nothing.

If this QJSValue does not already have a property with name name, a new property is created.

To modify array elements, use the setProperty(quint32 arrayIndex, const QJSValue &value) overload instead.

See also property() and deleteProperty().

### void QJSValue::setProperty(quint32 arrayIndex, const QJSValue &value)

Sets the property at the given arrayIndex to the given value.

It is possible to modify elements in an array in two ways. The first is to use the array index as the property name:

     jsValueArray.setProperty(QLatin1String("4"), value);

The second is to use the overload that takes an index:

     jsValueArray.setProperty(4, value);

Both of these approaches achieve the same result, except that the latter:

- Is easier to use (can use an integer directly)
- Is faster (no conversion to integer)

If this QJSValue is not an Array object, this function behaves as if setProperty() was called with the string representation of arrayIndex.

This is an overloaded function.

See also property(quint32 arrayIndex) and Working With Arrays.

### void QJSValue::setPrototype(const QJSValue &prototype)

If this QJSValue is an object, sets the internal prototype (`__proto__` property) of this object to be prototype; if the QJSValue is null, it sets the prototype to null; otherwise does nothing.

The internal prototype should not be confused with the public property with name "prototype"; the public prototype is usually only set on functions that act as constructors.

See also prototype() and isObject().

### bool QJSValue::strictlyEquals(const QJSValue &other) const

Returns true if this QJSValue is equal to other using strict comparison (no conversion), otherwise returns false. The comparison follows the behavior described in ECMA-262 section 11.9.6, "The Strict Equality Comparison Algorithm".

If the type of this QJSValue is different from the type of the other value, this function returns false. If the types are equal, the result depends on the type, as shown in the following table:

Type | Result
--- | ---
Undefined | true
Null | true
Boolean | true if values are both true or both false, false otherwise
Number | false if either value is NaN (Not-a-Number); true if values are equal, false otherwise
String | true if both values are exactly the same sequence of characters, false otherwise
Object | true if both values refer to the same object, false otherwise

See also equals().

### bool QJSValue::toBool() const

Returns the boolean value of this QJSValue, using the conversion rules described in ECMA-262 section 9.2, "ToBoolean".

Note that if this QJSValue is an object, calling this function has side effects on the script engine, since the engine will call the object's valueOf() function (and possibly toString()) in an attempt to convert the object to a primitive value (possibly resulting in an uncaught script exception).

See also isBool().

### QDateTime QJSValue::toDateTime() const

Returns a QDateTime representation of this value, in local time. If this QJSValue is not a date, or the value of the date is NaN (Not-a-Number), an invalid QDateTime is returned.

See also isDate().

### qint32 QJSValue::toInt() const

Returns the signed 32-bit integer value of this QJSValue, using the conversion rules described in ECMA-262 section 9.5, "ToInt32".

Note that if this QJSValue is an object, calling this function has side effects on the script engine, since the engine will call the object's valueOf() function (and possibly toString()) in an attempt to convert the object to a primitive value (possibly resulting in an uncaught script exception).

See also toNumber() and toUInt().

### double QJSValue::toNumber() const

Returns the number value of this QJSValue, as defined in ECMA-262 section 9.3, "ToNumber".

Note that if this QJSValue is an object, calling this function has side effects on the script engine, since the engine will call the object's valueOf() function (and possibly toString()) in an attempt to convert the object to a primitive value (possibly resulting in an uncaught script exception).

See also isNumber(), toInt(), and toUInt().

### QJSPrimitiveValue QJSValue::toPrimitive() const

Converts the value to a QJSPrimitiveValue. If the value holds a type supported by QJSPrimitiveValue, the value is copied. Otherwise the value is converted to a string, and the string is stored in QJSPrimitiveValue.

Note: Conversion of a managed value to a string can throw an exception. In particular, symbols cannot be coerced into strings, or a custom toString() method may throw. In this case the result is the undefined value and the engine carries an error after the conversion.

### const QMetaObject *QJSValue::toQMetaObject() const

* If this QJSValue is a QMetaObject, returns the QMetaObject pointer * that the QJSValue represents; otherwise, returns `nullptr`. * *

See also isQMetaObject().

### QObject *QJSValue::toQObject() const

If this QJSValue is a QObject, returns the QObject pointer that the QJSValue represents; otherwise, returns `nullptr`.

If the QObject that this QJSValue wraps has been deleted, this function returns `nullptr` (i.e. it is possible for toQObject() to return `nullptr` even when isQObject() returns true).

See also isQObject().

### QString QJSValue::toString() const

Returns the string value of this QJSValue, as defined in ECMA-262 section 9.8, "ToString".

Note that if this QJSValue is an object, calling this function has side effects on the script engine, since the engine will call the object's toString() function (and possibly valueOf()) in an attempt to convert the object to a primitive value (possibly resulting in an uncaught script exception).

See also isString().

### quint32 QJSValue::toUInt() const

Returns the unsigned 32-bit integer value of this QJSValue, using the conversion rules described in ECMA-262 section 9.6, "ToUint32".

Note that if this QJSValue is an object, calling this function has side effects on the script engine, since the engine will call the object's valueOf() function (and possibly toString()) in an attempt to convert the object to a primitive value (possibly resulting in an uncaught script exception).

See also toNumber() and toInt().

### QVariant QJSValue::toVariant(QJSValue::ObjectConversionBehavior behavior) const

Returns the QVariant value of this QJSValue, if it can be converted to a QVariant; otherwise returns an invalid QVariant. Some JavaScript types and objects have native expressions in Qt. Those are converted to their native expressions. For example:

Input Type | Result
--- | ---
Undefined | An invalid QVariant.
Null | A QVariant containing a null pointer (QMetaType::Nullptr).
Boolean | A QVariant containing the value of the boolean.
Number | A QVariant containing the value of the number.
String | A QVariant containing the value of the string.
QVariant Object | The result is the QVariant value of the object (no conversion).
QVariantMap Object | A QVariant containing the QVariantMap stored in the object (no conversion).
QVariantHash Object | A QVariant containing the QVariantHash stored in the object (no conversion).
QObject Object | A QVariant containing a pointer to the QObject.
Date Object | A QVariant containing the date value (toDateTime()).
RegularExpression Object | A QVariant containing the regular expression value.

For other types the behavior parameter is relevant. If `ConvertJSObjects` is given, a best effort but possibly lossy conversion is attempted. Generic JavaScript objects are converted to QVariantMap. JavaScript arrays are converted to QVariantList. Each property or element is converted to a QVariant, recursively; cyclic references are not followed. JavaScript function objects are dropped. If `RetainJSObjects` is given, the QJSValue is wrapped into a QVariant via QVariant::fromValue(). The resulting conversion is lossless but the internal structure of the objects is not immediately accessible.

See also isVariant().

### QVariant QJSValue::toVariant() const

Returns toVariant(ConvertJSObjects).

This is an overloaded function.

See also isVariant().

### QJSValue &QJSValue::operator=(QJSValue &&other)

Move-assigns other to this QJSValue object.

### QJSValue &QJSValue::operator=(const QJSValue &other)

Assigns the other value to this QJSValue.

Note that if other is an object (isObject() returns true), only a reference to the underlying object will be assigned; the object itself will not be copied.

## Related Non-Members

### QJSValueList

This is a typedef for a QList<QJSValue>.