GType

GType is the foundation of the GObject system. Although it is rarely necessary to interact with a GType directly in GJS, there are some situations where you may need to pass a GType as a function argument or in a class definition.

GType Object

Every GObject class has a static $gtype property that holds a GType object for the given type. This is the proper way to find the GType for an object class or instance. If an instance of the class has been constructed, you could also call GObject.type_from_name().

const icon = new Gio.ThemedIcon({name: 'dialog-information'});

if (icon.constructor.$gtype === Gio.ThemedIcon.$gtype)
    console.log('These values are exactly equivalent');

if (icon.constructor.$gtype === GObject.type_from_name('GThemedIcon'))
    console.log('These values are exactly equivalent');

To check the GType of a class instance, you can use the instanceof operator to compare it to a constructor object. This will return true for any parent type, including interfaces like Gio.Icon.

if (icon instanceof Gio.ThemedIcon)
    console.log('instance is a GThemedIcon');

if (icon instanceof GObject.Object && icon instanceof Gio.Icon)
    console.log('instance is a GObject and GIcon');

In many cases, you can pass the constructor object instead of a GType, which is often more convenient and idiomatic in JavaScript.

const listStore = Gio.ListStore.new(Gio.Icon);

const pspec = GObject.ParamSpec.object(
    'gicon',
    'GIcon',
    'A property holding a GIcon',
    GObject.ParamFlags.READWRITE,
    Gio.Icon);

GType Name

The name property of a GType object gives the GType name as a string (the same name passed to GObject.type_from_name()). This is the proper way to find the type name for an object class or instance. By default, the GType name of a subclass in GJS will be the JavaScript class name prefixed with Gjs_:

const ExampleSubclass = GObject.registerClass({
}, class ExampleSubclass extends GObject.Object {
});

const objectInstance = new GObject.Object();
const subclassInstance = new ExampleSubclass();

// expected output: 'GObject'
console.log(GObject.Object.$gtype.name);
console.log(objectInstance.constructor.$gtype.name);

// expected output: 'Gjs_MySubclass'
console.log(ExampleSubclass.$gtype.name);
console.log(subclassInstance.constructor.$gtype.name);

In most cases you will not need to specify your own name, however, this may be useful when creating a GtkBuilder template class. To set the GType name, pass it as the value for the GTypeName property to GObject.registerClass().

<interface>
  <template class="Square" parent="GtkBox">
    <!-- Template Definition -->
  </template>
</interface>

const Square = GObject.registerClass({
    GTypeName: 'Square',
    Template: 'resource:///guide/gjs/Example/ui/square.ui',
}, class Square extends Gtk.Box {
});

const widget = new Square();

// expected output: 'Square'
console.log(Square.$gtype.name);
console.log(widget.constructor.$gtype.name);

JavaScript Types

WARNING

GObject.TYPE_JSOBJECT is a boxed type, so it may not be used where a GObject is expected, such as with Gio.ListModel.

GObject.TYPE_JSOBJECT is a special GType in GJS, created so that JavaScript types that inherit from Object can be used with the GObject framework. This allows you to use them as property types and in signal parameters in your GObject subclasses.

This means that you may use Object and Array types, but also more complex types such as Date and Function. However, it will not allow you to use JavaScript primitive data types like BigInt or Symbol.

const ExampleSubclass = GObject.registerClass({
    Properties: {
        'example-property': GObject.ParamSpec.jsobject(
            'example-property',
            'Example Property',
            'A property that holds a JavaScript Object',
            GObject.ParamFlags.READWRITE
        ),
    },
    Signals: {
        'example-signal': {
            param_types: [GObject.TYPE_JSOBJECT],
        },
    },
}, class ExampleSubclass extends GObject.Object {
    get example_property() {
        if (this._example_property === undefined)
            this._example_property = {};

        return this._example_property;
    }

    set example_property(obj) {
        if (this.example_property === obj)
            return;

        this._example_property = obj;
        this.notify('example-object');
    }

    emitExampleSignal(obj) {
        this.emit('example-signal', obj);
    }
});

Type Constants

For convenience, GJS has predefined constants for most built-in types.

Constant	GLib	JavaScript
GObject.TYPE_BOOLEAN	gboolean	Boolean
GObject.TYPE_STRING	gchararray	String
GObject.TYPE_INT	gint	Number
GObject.TYPE_UINT	guint	Number
GObject.TYPE_LONG	glong	Number
GObject.TYPE_ULONG	gulong	Number
GObject.TYPE_INT64	gint64	Number
GObject.TYPE_UINT64	guint64	Number
GObject.TYPE_FLOAT	gfloat	Number
GObject.TYPE_DOUBLE	gdouble	Number
GObject.TYPE_ENUM	GEnum	Number
GObject.TYPE_FLAGS	GFlags	Number
GObject.TYPE_OBJECT	GObject	GObject.Object
GObject.TYPE_INTERFACE	GInterface	GObject.Interface
GObject.TYPE_BOXED	GBoxed
GObject.TYPE_POINTER	gpointer	nothing
GObject.TYPE_PARAM	GParam	GObject.ParamSpec
GObject.TYPE_VARIANT	GVariant	GLib.Variant
GObject.TYPE_GTYPE	GType	GObject.Type
GObject.TYPE_JSOBJECT	GBoxed	Object