Deep clone objects with circular references

// Deep clone plain data, preserving Date/Map/Set/RegExp and cycles.
function deepClone(value, seen = new WeakMap()) {
  if (value === null || typeof value !== 'object') return value;
  if (seen.has(value)) return seen.get(value);

  if (value instanceof Date) return new Date(value.getTime());
  if (value instanceof RegExp) return new RegExp(value.source, value.flags);

  if (value instanceof Map) {
    const copy = new Map();
    seen.set(value, copy);
    for (const [key, item] of value) copy.set(deepClone(key, seen), deepClone(item, seen));
    return copy;
  }

  if (value instanceof Set) {
    const copy = new Set();
    seen.set(value, copy);
    for (const item of value) copy.add(deepClone(item, seen));
    return copy;
  }

  const copy = Array.isArray(value) ? [] : Object.create(Object.getPrototypeOf(value));
  seen.set(value, copy);
  for (const key of Reflect.ownKeys(value)) {
    copy[key] = deepClone(value[key], seen);
  }
  return copy;
}

const source = { name: 'root', when: new Date(0) };
source.self = source;
const copy = deepClone(source);
console.log(copy !== source, copy.self === copy, copy.when instanceof Date); // true true true