proxy

        <div id="item-1">11</div>
        <div id="item-2">22</div>
        
        <script type="text/javascript">
            let view = new Proxy({
                selected: null,
                test: null
            }, {
                set: function(obj, prop, newval) {
                    let oldval = obj[prop];
                    console.log(oldval)
                    if (prop === 'selected') {
                        if (oldval) {
                            oldval.setAttribute('aria-selected', 'false');
                        }
                        if (newval) {
                            newval.setAttribute('aria-selected', 'true');
                        }
                    }

                    // The default behavior to store the value
                    obj[prop] = newval;
                }
            });

            let i1 = view.selected = document.getElementById('item-1');
            i1.getAttribute('aria-selected')

            let i2 = view.selected = document.getElementById('item-2');
            i1.getAttribute('aria-selected')
            i2.getAttribute('aria-selected')
        </script>