SOURCE

const data = {msg:''}
const input = document.getElementById('input')
const span = document.getElementById('span')

Object.defineProperty(data, 'msg', {
    configurable:true,
    enumerable:true,
    get () {
        return input.value
    },
    set (newVal) {
        input.value = newVal;
        span.innerText = newVal;
    }
})
input.onkeyup = function (e) {
    data.msg = e.target.value;
}

<input type="" id='input'>
<span id="span"></span>