Function warnOnInvalidKey

packages/react-reconciler/src/ReactChildFiber.js:1069–1122

* Warns if there is a duplicate or missing key

(
    returnFiber: Fiber,
    workInProgress: Fiber,
    child: mixed,
    knownKeys: Set<string> | null,
  )

   * Warns if there is a duplicate or missing key
   */
  function warnOnInvalidKey(
    returnFiber: Fiber,
    workInProgress: Fiber,
    child: mixed,
    knownKeys: Set<string> | null,
  ): Set<string> | null {
    if (__DEV__) {
      if (typeof child !== 'object' || child === null) {
        return knownKeys;
      }
      switch (child.$$typeof) {
        case REACT_ELEMENT_TYPE:
        case REACT_PORTAL_TYPE:
          warnForMissingKey(returnFiber, workInProgress, child);
          const key = child.key;
          if (typeof key !== 'string') {
            break;
          }
          if (knownKeys === null) {
            knownKeys = new Set();
            knownKeys.add(key);
            break;
          }
          if (!knownKeys.has(key)) {
            knownKeys.add(key);
            break;
          }
          runWithFiberInDEV(workInProgress, () => {
            console.error(
              'Encountered two children with the same key, `%s`. ' +
                'Keys should be unique so that components maintain their identity ' +
                'across updates. Non-unique keys may cause children to be ' +
                'duplicated and/or omitted — the behavior is unsupported and ' +
                'could change in a future version.',
              key,
            );
          });
          break;
        case REACT_LAZY_TYPE: {
          const resolvedChild = resolveLazy((child: any));
          warnOnInvalidKey(
            returnFiber,
            workInProgress,
            resolvedChild,
            knownKeys,
          );
          break;
        }
        default:
          break;
      }
    }
    return knownKeys;
  }

  function reconcileChildrenArray(
    returnFiber: Fiber,