Function updateFromMap

packages/react-reconciler/src/ReactChildFiber.js:941–1064

(
    existingChildren: Map<string | number, Fiber>,
    returnFiber: Fiber,
    newIdx: number,
    newChild: any,
    lanes: Lanes,
  )

  }

  function updateFromMap(
    existingChildren: Map<string | number, Fiber>,
    returnFiber: Fiber,
    newIdx: number,
    newChild: any,
    lanes: Lanes,
  ): Fiber | null {
    if (
      (typeof newChild === 'string' && newChild !== '') ||
      typeof newChild === 'number' ||
      typeof newChild === 'bigint'
    ) {
      // Text nodes don't have keys, so we neither have to check the old nor
      // new node for the key. If both are text nodes, they match.
      const matchedFiber = existingChildren.get(newIdx) || null;
      return updateTextNode(
        returnFiber,
        matchedFiber,
        // $FlowFixMe[unsafe-addition] Flow doesn't want us to use `+` operator with string and bigint
        '' + newChild,
        lanes,
      );
    }

    if (typeof newChild === 'object' && newChild !== null) {
      switch (newChild.$$typeof) {
        case REACT_ELEMENT_TYPE: {
          const matchedFiber =
            existingChildren.get(
              newChild.key === null ? newIdx : newChild.key,
            ) || null;
          const prevDebugInfo = pushDebugInfo(newChild._debugInfo);
          const updated = updateElement(
            returnFiber,
            matchedFiber,
            newChild,
            lanes,
          );
          currentDebugInfo = prevDebugInfo;
          return updated;
        }
        case REACT_PORTAL_TYPE: {
          const matchedFiber =
            existingChildren.get(
              newChild.key === null ? newIdx : newChild.key,
            ) || null;
          return updatePortal(returnFiber, matchedFiber, newChild, lanes);
        }
        case REACT_LAZY_TYPE: {
          const prevDebugInfo = pushDebugInfo(newChild._debugInfo);
          const resolvedChild = resolveLazy((newChild: any));
          const updated = updateFromMap(
            existingChildren,
            returnFiber,
            newIdx,
            resolvedChild,
            lanes,
          );