facebook/react · error · Error

578

578

Error message

Already initialized Iterator.

What it means

extractIterator sets $$consumed = true on the model array before creating the iterator so cyclic references cannot consume it twice. If the same model reaches extractIterator again, the guard throws.

Source

Thrown at packages/react-server/src/ReactFlightReplyServer.js:1206

function createSet(response: Response, model: Array<any>): Set<any> {
  if (!isArray(model)) {
    throw new Error('Invalid Set initializer.');
  }
  if ((model as any).$$consumed === true) {
    throw new Error('Already initialized Set.');
  }
  // This needs to come first to prevent the model from being consumed again in case of a cyclic reference.
  (model as any).$$consumed = true;
  const set = new Set(model);
  return set;
}

function extractIterator(response: Response, model: Array<any>): Iterator<any> {
  if (!isArray(model)) {
    throw new Error('Invalid Iterator initializer.');
  }
  if ((model as any).$$consumed === true) {
    throw new Error('Already initialized Iterator.');
  }
  // This needs to come first to prevent the model from being consumed again in case of a cyclic reference.
  (model as any).$$consumed = true;
  // $FlowFixMe[incompatible-use]: This uses raw Symbols because we're extracting from a native array.
  const iterator = model[Symbol.iterator]();
  return iterator;
}

function createModel(
  response: Response,
  model: any,
  parentObject: Object,
  key: string,
): any {
  if (key === 'then' && typeof model === 'function') {
    // This should never happen because we always serialize objects with then-functions
    // as "thenable" which reduces to ReactPromise with no other fields.
    return null;

View on GitHub (pinned to eafeac097b)

Solutions

1. Avoid passing iterators of cyclic structures as action arguments; send materialized arrays or ids instead.
2. Catch the error during decode and reject the request as malformed.
3. Align React versions across the boundary.

try {
  const args = await decodeReply(formData);
} catch (e) {
  return new Response('Bad request', {status: 400});
}

When it happens

Trigger: The same serialized iterator model is initialized twice - cyclic structures not outlined by the encoder, duplicate references in a crafted payload, or version-skewed wire formats.

Common situations: Iterators over self-referential data passed to server actions; replayed or hand-edited FormData.

Related errors

• 572
• 573
• 577
• 570
• 599

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/c680dec3de40e2ee. Report an issue: GitHub.