append-guard.js

// Store a reference to the native appendChild function
const nativeAppend = Node.prototype.appendChild;

// Override the appendChild method
Node.prototype.appendChild = function(node) {
  // If the node is a "robber" don't append it
  if (node.id === 'robber') {
    console.log('denying robber entry to the bank');
    return;
  }
  console.log('letting person into the bank');
  // use the native append function to add
  //  the node to the DOM
  return nativeAppend.call(this, node);
}

// Get the "bank" from the DOM
const bank = document.body.querySelector('#bank');

const accountOwner = document.createElement('div');
accountOwner.id = 'owner';
// Let the "account owner" into the bank (append it to the DOM)
bank.appendChild(accountOwner); // logs "letting person into the bank" to console

const robber = document.createElement('div');
robber.id = 'robber';
// Prevent the "robber" from accessing the bank (don't append to DOM) 
bank.appendChild(robber); // logs "denying robber entry to the bank" to console