Function renderAsyncFragment

packages/react-server/src/ReactFlightServer.js:1918–1963

(
  request: Request,
  task: Task,
  children: $AsyncIterable<ReactClientValue, ReactClientValue, void>,
  getAsyncIterator: () => $AsyncIterator<any, any, any>,
)

}

function renderAsyncFragment(
  request: Request,
  task: Task,
  children: $AsyncIterable<ReactClientValue, ReactClientValue, void>,
  getAsyncIterator: () => $AsyncIterator<any, any, any>,
): ReactJSONValue {
  if (task.keyPath !== null) {
    // We have a Server Component that specifies a key but we're now splitting
    // the tree using a fragment.
    const fragment = __DEV__
      ? [
          REACT_ELEMENT_TYPE,
          REACT_FRAGMENT_TYPE,
          task.keyPath,
          {children},
          null,
          null,
          0,
        ]
      : [REACT_ELEMENT_TYPE, REACT_FRAGMENT_TYPE, task.keyPath, {children}];
    if (!task.implicitSlot) {
      // If this was keyed inside a set. I.e. the outer Server Component was keyed
      // then we need to handle reorders of the whole set. To do this we need to wrap
      // this array in a keyed Fragment.
      return fragment;
    }
    // If the outer Server Component was implicit but then an inner one had a key
    // we don't actually need to be able to move the whole set around. It'll always be
    // in an implicit slot. The key only exists to be able to reset the state of the
    // children. We could achieve the same effect by passing on the keyPath to the next
    // set of components inside the fragment. This would also allow a keyless fragment
    // reconcile against a single child.
    // Unfortunately because of JSON.stringify, we can't call the recursive loop for
    // each child within this context because we can't return a set with already resolved
    // values. E.g. a string would get double encoded. Returning would pop the context.
    // So instead, we wrap it with an unkeyed fragment and inner keyed fragment.
    return [fragment];
  }

  // Since we're yielding here, that implicitly resets the keyPath context on the
  // way up. Which is what we want since we've consumed it. If this changes to
  // be recursive serialization, we need to reset the keyPath and implicitSlot,
  // before recursing here.
  const asyncIterator = getAsyncIterator.call(children);
  return serializeAsyncIterable(request, task, children, asyncIterator);
}

function renderClientElement(
  request: Request,