SOURCE

function observer(target) {
   const div = document.getElementById("container");   
   const proxy = new Proxy(target, {
       set(target, prop, value) {
           target[prop] = value;
           render();
       },
       get(target, prop) {
           return target[prop];
       }
   })
   render();

   function render() {
       let html = "";
       for (const prop of Object.keys(target)) {
           html += `<p>
       <span>${prop}:</span>
       <span>${target[prop]}</span>
   </p>`
       }
       div.innerHTML = html;
   }
   return proxy; //target
}
const target = {
   a: 1,
   b: 2
}
const obj = observer(target);

// obj.d = 499;
obj.e = 43;

<div id='container'></div>