gistfile1.txt

const GObject = imports.gi.GObject;
const Gtk = imports.gi.Gtk;
const Lang = imports.lang;

Gtk.init(null);

const FirstObject = new Lang.Class({
    Name: 'First',
    Extends: GObject.Object,

    Properties: {
        'foo': GObject.ParamSpec.boolean('foo', '',
            '',
            GObject.ParamFlags.READWRITE, true)
    },
});

const SecondObject = new Lang.Class({
    Name: 'Second',
    Extends: GObject.Object,

    Properties: {
        'bar': GObject.ParamSpec.boolean('bar', '',
            '',
            GObject.ParamFlags.READWRITE, true)
    },

    _init: function (props) {
    	this.parent(props);
    	this.first = new FirstObject();
    	this.bind_property_full('bar', this.first, 'foo', GObject.BindingFlags.SYNC_CREATE, (binding, from_value, to_value) => {
    		print("calling bind_property_full")
    		to_value.set_boolean(true);
    		return true;
    	}, () => {});
    }
});

let second = new SecondObject();

second.bar = true;

// I would expect this to print 'true', but it prints false.
print(second.first.foo);