<div id="app">
    <input type="text" id="inputTxt">
    <input type="text" id="showTxt">
</div>

var obj = {};
Object.defineProperty(obj, 'txt', {
    get: function () {
        return obj;
    },
    set: function (newValue) {
        document.getElementById('inputTxt').value = newValue;
        document.getElementById('showTxt').value = newValue;
    }
})
document.addEventListener('keydown', function (e) {
    obj.txt = e.target.value;
})

const obj = {};
const aProxy = new Proxy(obj, {
    get: function(target, key, receiver){
        return Reflect.get(target, key, receiver);
    },
    set: function (target, key, value, receiver) {
        if(key === 'txt' && value !== undefined) {
            document.getElementById('inputTxt').value = value;
            document.getElementById('showTxt').value = value;
        }
        return Reflect.set(target, key, value, receiver);
    }
});
document.addEventListener('keydown', function (e) {
    aProxy.txt = e.target.value;
})