facebook/react · error · Error

572

572

Error message

Already initialized Map.

What it means

createMap sets $$consumed = true on the model array before building the Map, specifically so a cyclic reference cannot consume the same model twice. If the same serialized Map model reaches createMap again, the guard throws instead of producing wrong data or looping.

Source

Thrown at packages/react-server/src/ReactFlightReplyServer.js:1180

          reason: chunk.reason,
          deps: 0,
          errored: true,
        };
      }
      // Placeholder
      return null as any;
  }
}

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

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;

View on GitHub (pinned to eafeac097b)

Solutions

1. Avoid passing self-referential/cyclic Maps as server action arguments; reference members by id instead.
2. Catch the decode error and reject the request as malformed.
3. Align React versions across the boundary if skew is suspected.

Example fix

// before
const m = new Map();
m.set('self', m); // cyclic Map serialized into a self-referencing model
await save(m);

// after
const m = new Map();
m.set('rootId', 'map-1'); // reference by identifier, resolve on the server
await save({id: 'map-1', entries: [...m]});

function hasCycle(v: unknown, seen = new WeakSet()): boolean {
  if (v && (typeof v === 'object' || typeof v === 'function')) {
    if (seen.has(v)) return true;
    seen.add(v);
    if (v instanceof Map) {
      for (const [k, val] of v) if (hasCycle(k, seen) || hasCycle(val, seen)) return true;
    } else if (v instanceof Set) {
      for (const item of v) if (hasCycle(item, seen)) return true;
    } else if (Array.isArray(v)) {
      for (const item of v) if (hasCycle(item, seen)) return true;
    } else {
      for (const k in v) if (hasCycle((v as any)[k], seen)) return true;
    }
  }
  return false;
}
// before invoking the action
if (hasCycle(args)) throw new Error('Cyclic data cannot be serialized');

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

When it happens

Trigger: The same Map model is initialized twice - a cyclic structure the encoder failed to outline (a Map containing itself), duplicate references to one model in a crafted payload, or version-skewed encoders.

Common situations: Passing self-referential Maps through server actions; replayed or edited FormData where one Map id is referenced from two initializer positions.

Related errors

• 575
• 573
• 578
• 570
• 599

AI-assisted analysis of facebook/react@eafeac097b (2026-08-21). Data as JSON: /api/errors/d0b427a8860de664. Report an issue: GitHub.