mutation-observer-guard.js

// Select the node that will be observed for mutations
const bank = document.getElementById('bank');

// Options for the observer (which mutations to observe)
const config = { childList: true };

// Callback function to execute when mutations are observed
const callback = function(mutationsList, observer) {
  // Loop through all mutations
  mutationsList.forEach(mutation => {
    // Loop through all added nodes
    mutation.addedNodes.forEach(node => {
      // If the node is a "robber"
      if (node.id === 'robber') {
        console.log('kicking robber out of bank')
        node.parentNode.removeChild(node);
      }  
    })
  })
};

// Create an observer instance linked to the callback function
const observer = new MutationObserver(callback);

// Start observing the target node for configured mutations
observer.observe(bank, config);

setTimeout(()=> {
  // Add a "robber" to the bank
  const robber = document.createElement('div');
  robber.id = 'robber';
  bank.appendChild(robber);
}, 1500);