- Qt 6.9
- Qt Qml
- C++ Classes
- QJSManagedValue
- Qt 6.9.3 Reference Documentation

# QJSManagedValue Class

QJSManagedValue represents a value on the JavaScript heap belonging to a QJSEngine. More...

Header: | `#include <QJSManagedValue>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Qml)` `target_link_libraries(mytarget PRIVATE Qt6::Qml)`
qmake: | `QT += qml`
Since: | Qt 6.1

- List of all members, including inherited members

## Public Types

enum | Type { Undefined, Boolean, Number, String, Object, …, Function }

## Public Functions

| QJSManagedValue()
| QJSManagedValue(QJSValue value, QJSEngine *engine)
| QJSManagedValue(const QJSPrimitiveValue &value, QJSEngine *engine)
| QJSManagedValue(const QString &string, QJSEngine *engine)
| QJSManagedValue(const QVariant &variant, QJSEngine *engine)
| QJSManagedValue(QJSManagedValue &&other)
| ~QJSManagedValue()
QJSValue | call(const QJSValueList &arguments = {}) const
QJSValue | callAsConstructor(const QJSValueList &arguments = {}) const
QJSValue | callWithInstance(const QJSValue &instance, const QJSValueList &arguments = {}) const
bool | deleteProperty(const QString &name)
bool | deleteProperty(quint32 arrayIndex)
QJSEngine * | engine() const
bool | equals(const QJSManagedValue &other) const
bool | hasOwnProperty(const QString &name) const
bool | hasOwnProperty(quint32 arrayIndex) const
bool | hasProperty(const QString &name) const
bool | hasProperty(quint32 arrayIndex) const
bool | isArray() const
bool | isBoolean() const
bool | isDate() const
bool | isError() const
bool | isFunction() const
bool | isInteger() const
bool | isNull() const
bool | isNumber() const
bool | isObject() const
bool | isQMetaObject() const
bool | isQObject() const
bool | isRegularExpression() const
bool | isString() const
bool | isSymbol() const
bool | isUndefined() const
bool | isUrl() const
bool | isVariant() const
QJSValue | property(const QString &name) const
QJSValue | property(quint32 arrayIndex) const
QJSManagedValue | prototype() const
void | setProperty(const QString &name, const QJSValue &value)
void | setProperty(quint32 arrayIndex, const QJSValue &value)
void | setPrototype(const QJSManagedValue &prototype)
bool | strictlyEquals(const QJSManagedValue &other) const
bool | toBoolean() const
QDateTime | toDateTime() const
int | toInteger() const
QJSValue | toJSValue() const
double | toNumber() const
QJSPrimitiveValue | toPrimitive() const
const QMetaObject * | toQMetaObject() const
QObject * | toQObject() const
QRegularExpression | toRegularExpression() const
QString | toString() const
QUrl | toUrl() const
QVariant | toVariant() const
QJSManagedValue::Type | type() const
QJSManagedValue & | operator=(QJSManagedValue &&other)

## Detailed Description

The QJSManagedValue class allows interaction with JavaScript values in most ways you can interact with them from JavaScript itself. You can get and set properties and prototypes, and you can access arrays. Additionally, you can transform the value into the Qt counterparts of JavaScript objects. For example, a Url object may be transformed into a QUrl.

A QJSManagedValue is always bound to a particular QJSEngine. You cannot use it independently. This means that you cannot have a QJSManagedValue from one engine be a property or a proptotype of a QJSManagedValue from a different engine.

In contrast to QJSValue, almost all values held by QJSManagedValue live on the JavaScript heap. There is no inline or unmanaged storage. Therefore, you can get the prototype of a primitive value, and you can get the `length` property of a string.

Only default-constructed or moved-from QJSManagedValues do not hold a value on the JavaScript heap. They represent `undefined`, which doesn't have any properties or prototypes.

Also in contrast to QJSValue, QJSManagedValue does not catch any JavaScript exceptions. If an operation on a QJSManagedValue causes an error, it will generally return an `undefined` value and QJSEngine::hasError() will return `true` afterwards. You can then catch the exception using QJSEngine::catchError(), or pass it up the stack, at your own discretion.

Note: As the reference to the value on the JavaScript heap has to be freed on destruction, you cannot move a QJSManagedValue to a different thread. The destruction would take place in the new thread, which would create a race condition with the garbage collector on the original thread. This also means that you cannot hold a QJSManagedValue beyond the lifespan of its engine.

The recommended way of working with a QJSManagedValue is creating it on the stack, possibly by moving a QJSValue and adding an engine, then performing the necessary operations on it, and finally moving it back into a QJSValue for storage. Moving between QJSManagedValue and QJSValue is fast.

## Member Type Documentation

### enum QJSManagedValue::Type

This enum represents the JavaScript native types, as specified by ECMA-262.

Constant | Value | Description
--- | --- | ---
`QJSManagedValue::Undefined` | `0` | The `undefined` type
`QJSManagedValue::Boolean` | `1` | The `boolean` type
`QJSManagedValue::Number` | `2` | The `number` type
`QJSManagedValue::String` | `3` | The `string` type
`QJSManagedValue::Object` | `4` | The `object` type
`QJSManagedValue::Symbol` | `5` | The `symbol` type
`QJSManagedValue::Function` | `6` | The `function` type

Note that the `null` value is not a type of itself but rather a special kind of object. You can query a QJSManagedValue for this condition using the isNull() method. Furthermore, JavaScript has no integer type, but it knows a special treatment of numbers in preparation for integer only operations. You can query a QJSManagedValue to find out whether it holds the result of such a treatment by using the isInteger() method.

## Member Function Documentation

### `[constexpr noexcept]` QJSManagedValue::QJSManagedValue()

Creates a QJSManagedValue that represents the JavaScript `undefined` value. This is the only value not stored on the JavaScript heap. Calling engine() on a default-constructed QJSManagedValue will return nullptr.

### QJSManagedValue::QJSManagedValue(QJSValue value, QJSEngine *engine)

Creates a QJSManagedValue from value, using the heap of engine. If value is itself managed and the engine it belongs to is not engine, the result is an `undefined` value, and a warning is generated.

### QJSManagedValue::QJSManagedValue(const QJSPrimitiveValue &value, QJSEngine *engine)

Creates a QJSManagedValue from value using the heap of engine.

### QJSManagedValue::QJSManagedValue(const QString &string, QJSEngine *engine)

Creates a QJSManagedValue from string using the heap of engine.

### QJSManagedValue::QJSManagedValue(const QVariant &variant, QJSEngine *engine)

Creates a QJSManagedValue from variant using the heap of engine.

### QJSManagedValue::QJSManagedValue(QJSManagedValue &&other)

Move-constructs a QJSManagedValue from other. This leaves other in the default-constructed state where it represents undefined and does not belong to any engine.

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

Destroys the QJSManagedValue.

Note: This frees the memory slot it holds on the JavaScript heap. You must not destroy a QJSManagedValue from a different thread than the one where the QJSEngine it belongs to lives.

### QJSValue QJSManagedValue::call(const QJSValueList &arguments = {}) const

If this QJSManagedValue represents a JavaScript FunctionObject, calls it with the given arguments, and returns the result. Otherwise returns a JavaScript `undefined` value.

The arguments have to be either primitive values or belong to the same QJSEngine as this QJSManagedValue. Otherwise the call is not carried out and a JavaScript `undefined` value is returned.

### QJSValue QJSManagedValue::callAsConstructor(const QJSValueList &arguments = {}) const

If this QJSManagedValue represents a JavaScript FunctionObject, calls it as constructor with the given arguments, and returns the result. Otherwise returns a JavaScript `undefined` value.

The arguments have to be either primitive values or belong to the same QJSEngine as this QJSManagedValue. Otherwise the call is not carried out and a JavaScript `undefined` value is returned.

### QJSValue QJSManagedValue::callWithInstance(const QJSValue &instance, const QJSValueList &arguments = {}) const

If this QJSManagedValue represents a JavaScript FunctionObject, calls it on instance with the given arguments, and returns the result. Otherwise returns a JavaScript `undefined` value.

The arguments and the instance have to be either primitive values or belong to the same QJSEngine as this QJSManagedValue. Otherwise the call is not carried out and a JavaScript `undefined` value is returned.

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

Deletes the property name from this QJSManagedValue. Returns `true` if the deletion succeeded, or `false` otherwise.

### bool QJSManagedValue::deleteProperty(quint32 arrayIndex)

Deletes the value stored at arrayIndex from this QJSManagedValue. Returns `true` if the deletion succeeded, or `false` otherwise.

### QJSEngine *QJSManagedValue::engine() const

Returns the QJSEngine this QJSManagedValue belongs to. Mind that the engine is always valid, unless the QJSManagedValue is default-constructed or moved from. In the latter case a nullptr is returned.

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

Invokes the JavaScript '==' operator on this QJSManagedValue and other, and returns the result.

See also strictlyEquals.

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

Returns `true` if this QJSManagedValue has a property name, otherwise returns `false`. The properties of the prototype chain are not considered.

### bool QJSManagedValue::hasOwnProperty(quint32 arrayIndex) const

Returns `true` if this QJSManagedValue has an array index arrayIndex, otherwise returns `false`. The properties of the prototype chain are not considered.

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

Returns `true` if this QJSManagedValue has a property name, otherwise returns `false`. The properties of the prototype chain are considered.

### bool QJSManagedValue::hasProperty(quint32 arrayIndex) const

Returns `true` if this QJSManagedValue has an array index arrayIndex, otherwise returns `false`. The properties of the prototype chain are considered.

### bool QJSManagedValue::isArray() const

Returns `true` if this value represents a JavaScript Array object, or `false` otherwise.

### bool QJSManagedValue::isBoolean() const

Returns `true` if the type of this QJSManagedValue is `boolean`, or `false` otherwise.

### bool QJSManagedValue::isDate() const

Returns `true` if this value represents a JavaScript Date object, or `false` otherwise.

### bool QJSManagedValue::isError() const

Returns `true` if this value represents a JavaScript Error object, or `false` otherwise.

### bool QJSManagedValue::isFunction() const

Returns `true` if the type of this QJSManagedValue is `function`, `false` otherwise.

### bool QJSManagedValue::isInteger() const

Returns `true` if this QJSManagedValue holds an integer value, or `false` otherwise. The storage format of a number does not affect the result of any operations performed on it, but if an integer is stored, many operations are faster.

### bool QJSManagedValue::isNull() const

Returns `true` if this QJSManagedValue holds the JavaScript `null` value, or `false` otherwise.

### bool QJSManagedValue::isNumber() const

Returns `true` if the type of this QJSManagedValue is `number`, or `false` otherwise.

### bool QJSManagedValue::isObject() const

Returns `true` if the type of this QJSManagedValue is `object`, or `false` otherwise.

### bool QJSManagedValue::isQMetaObject() const

Returns `true` if this value represents a QMetaObject pointer managed on the JavaScript heap, or `false` otherwise.

### bool QJSManagedValue::isQObject() const

Returns `true` if this value represents a QObject pointer managed on the JavaScript heap, or `false` otherwise.

### bool QJSManagedValue::isRegularExpression() const

Returns `true` if this value represents a JavaScript regular expression object, or `false` otherwise.

### bool QJSManagedValue::isString() const

Returns `true` if the type of this QJSManagedValue is `string`, or `false` otherwise.

### bool QJSManagedValue::isSymbol() const

Returns `true` if the type of this QJSManagedValue is `symbol`, or `false` otherwise.

### bool QJSManagedValue::isUndefined() const

Returns `true` if the type of this QJSManagedValue is `undefined`, or `false` otherwise.

### bool QJSManagedValue::isUrl() const

Returns `true` if this value represents a JavaScript Url object, or `false` otherwise.

### bool QJSManagedValue::isVariant() const

Returns `true` if this value represents a QVariant managed on the JavaScript heap, or `false` otherwise.

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

Returns the property name of this QJSManagedValue. The prototype chain is searched if the property is not found on the actual object.

See also setProperty().

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

Returns the property stored at arrayIndex of this QJSManagedValue. The prototype chain is searched if the property is not found on the actual object.

### QJSManagedValue QJSManagedValue::prototype() const

Returns the prototype for this QJSManagedValue. This works on any value. You can, for example retrieve the JavaScript `boolean` prototype from a `boolean` value.

See also setPrototype().

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

Sets the property name to value on this QJSManagedValue. This can only be done on JavaScript values of type `object`. Furhermore, value has to be either a primitive or belong to the same engine as this value.

See also property().

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

Stores the value at arrayIndex in this QJSManagedValue. This can only be done on JavaScript values of type `object`, and it's not recommended if the value is not an array. Furhermore, value has to be either a primitive or belong to the same engine as this value.

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

Sets the prototype of this QJSManagedValue to prototype. A precondition is that prototype belongs to the same QJSEngine as this QJSManagedValue and is an object (including null). Furthermore, this QJSManagedValue has to be an object (excluding null), too, and you cannot create prototype cycles.

See also prototype().

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

Invokes the JavaScript '===' operator on this QJSManagedValue and other, and returns the result.

See also equals.

### bool QJSManagedValue::toBoolean() const

Converts the manged value to a boolean. If the managed value holds a boolean, that one is returned. Otherwise a boolean coercion by JavaScript rules is performed.

### QDateTime QJSManagedValue::toDateTime() const

If this QJSManagedValue holds a JavaScript Date object, returns an equivalent QDateTime. Otherwise returns an invalid one.

### int QJSManagedValue::toInteger() const

Converts the manged value to an integer. This first converts the value to a number by the rules of toNumber(), and then clamps it into the integer range by the rules given for coercing the arguments to JavaScript bit shift operators into 32bit integers.

Internally, the value may already be stored as an integer, in which case a fast path is taken.

Note: Conversion of a managed value to a number can throw an exception. In particular, symbols cannot be coerced into numbers, or a custom valueOf() method may throw. In this case the result is 0 and the engine carries an error after the conversion.

Note: The JavaScript rules for coercing numbers into 32bit integers are unintuitive.

### QJSValue QJSManagedValue::toJSValue() const

Copies this QJSManagedValue into a new QJSValue. This is less efficient than move-constructing a QJSValue from a QJSManagedValue, but retains the QJSManagedValue.

### double QJSManagedValue::toNumber() const

Converts the manged value to a number. If the managed value holds a number, that one is returned. Otherwise a number coercion by JavaScript rules is performed.

Note: Conversion of a managed value to a number can throw an exception. In particular, symbols cannot be coerced into numbers, or a custom valueOf() method may throw. In this case the result is 0 and the engine carries an error after the conversion.

### QJSPrimitiveValue QJSManagedValue::toPrimitive() const

Converts the manged value to a QJSPrimitiveValue. If the managed 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 *QJSManagedValue::toQMetaObject() const

If this QJSManagedValue holds a QMetaObject pointer, returns it. Otherwise returns nullptr.

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

If this QJSManagedValue holds a QObject pointer, returns it. Otherwise returns nullptr.

### QRegularExpression QJSManagedValue::toRegularExpression() const

If this QJSManagedValue holds a JavaScript regular expression object, returns an equivalent QRegularExpression. Otherwise returns an invalid one.

### QString QJSManagedValue::toString() const

Converts the manged value to a string. If the managed value holds a string, that one is returned. Otherwise a string coercion by JavaScript rules is performed.

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 an empty string and the engine carries an error after the conversion.

### QUrl QJSManagedValue::toUrl() const

If this QJSManagedValue holds a JavaScript Url object, returns an equivalent QUrl. Otherwise returns an invalid one.

### QVariant QJSManagedValue::toVariant() const

Copies this QJSManagedValue into a new QVariant. This also creates a useful QVariant if QJSManagedValue::isVariant() returns false. QVariant can hold all types supported by QJSManagedValue.

### QJSManagedValue::Type QJSManagedValue::type() const

Returns the JavaScript type of this QJSManagedValue.

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

Move-assigns a QJSManagedValue from other. This leaves other in the default-constructed state where it represents undefined and does not belong to any engine.

Note: This frees the memory slot this QJSManagedValue holds on the JavaScript heap. You must not move-assign a QJSManagedValue on a different thread than the one where the QJSEngine it belongs to lives.