JSFiddle - React, Tailwind, and code Playground

HTML

<body>
<div id="display">
</div>
</body>

JavaScript

let obj = {
    a: 10,
    b: null
  };

obj = new Proxy (obj, {get: (target, key) => {
 if(target[key] == undefined){
  //console.warn(key, "in", obj, "is undefined");
  document.getElementById('display').innerHTML = `
  ${key} in ${JSON.stringify(target)} is undefined`;
 }
return true;
}})

obj.a;
obj.d;
obj.a;