JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<meta charset="utf-8">
<body>

<script>
var handler = {
    get: function(obj, prop) {
      if (prop in obj) {
        return obj[prop];
      } else {
        throw `Unknown property '${prop}'`; // remove this line and it works
      }
    }
  };
  var attr = new Proxy({}, handler);
  //var attr = {}; // use this line instead and it works
  attr.myprop = 'Hello World'

  console.log('attr.myprop');

</script>