let currentListeners = [];
let nextListeners = currentListeners;

function ensureCanMutateNextListeners() {
  if (nextListeners === currentListeners) {
    nextListeners = currentListeners.slice();
  }
}

console.log(nextListeners === currentListeners); // true
ensureCanMutateNextListeners();
console.log(nextListeners === currentListeners); // false