zloirock/core-js · error · TypeError

Object already initialized

Error message

Object already initialized

What it means

internal-state.set attaches metadata to an object via a WeakMap-backed store; if the object already has metadata (store.has(it)), it throws TypeError 'Object already initialized' (OBJECT_ALREADY_INITIALIZED). This protects the invariant that internal state is set exactly once per instance, e.g. by a polyfilled constructor.

Source

Thrown at packages/core-js/internals/internal-state.js:37

var getterFor = function (TYPE) {
  return function (it) {
    var state;
    if (!isObject(it) || (state = get(it)).type !== TYPE) {
      throw new TypeError('Incompatible receiver, ' + TYPE + ' required');
    } return state;
  };
};

if (NATIVE_WEAK_MAP || shared.state) {
  var store = shared.state || (shared.state = new WeakMap());
  /* eslint-disable no-self-assign -- prototype methods protection */
  store.get = store.get;
  store.has = store.has;
  store.set = store.set;
  /* eslint-enable no-self-assign -- prototype methods protection */
  set = function (it, metadata) {
    if (store.has(it)) throw new TypeError(OBJECT_ALREADY_INITIALIZED);
    metadata.facade = it;
    store.set(it, metadata);
    return metadata;
  };
  get = function (it) {
    return store.get(it) || {};
  };
  has = function (it) {
    return store.has(it);
  };
} else {
  var STATE = sharedKey('state');
  hiddenKeys[STATE] = true;
  set = function (it, metadata) {
    if (hasOwn(it, STATE)) throw new TypeError(OBJECT_ALREADY_INITIALIZED);
    metadata.facade = it;
    createNonEnumerableProperty(it, STATE, metadata);
    return metadata;

View on GitHub (pinned to 84e45fba09)

Solutions

1. Construct fresh instances instead of re-invoking constructors on existing objects.
2. Guard: call set/initialization only once per object (check get(it) first).
3. Deduplicate core-js versions so only one internal-state registry initializes objects.
4. If you must reinitialize, create a new instance and copy data rather than re-running the constructor.

Example fix

// before
Map.call(existingMap); // TypeError: Object already initialized
// after
var fresh = new Map();

// assume an internal `get` accessor is available for the same TYPE
if (Object.keys(internalGet(obj)).length > 0) { /* already initialized */ return; }

try { internalSet(obj, metadata); } catch (e) { if (/Object already initialized/.test(e.message)) { metadata = internalGet(obj); } else throw e; }

When it happens

Trigger: Calling the internal set() twice on the same object — re-running a polyfilled constructor against an existing instance (e.g. Reflect.construct or .call-ing the constructor on an instance), double-initialization during subclassing tricks, or two core-js copies sharing one store but with differing type expectations.

Common situations: Constructor-invocation hacks: Map.call(existingMap); patching/re-initializing polyfilled collections; test code resetting instances by re-invoking constructors; module duplication causing two initializers for one object.

Related errors

• ArrayBuffer expected
• Target is not a typed array
• Incompatible receiver, ${TYPE} required
• Cannot convert a Symbol value to a number
• Promise can't be resolved itself

AI-assisted analysis of zloirock/core-js@84e45fba09 (2026-08-30). Data as JSON: /api/errors/c1daf2eaefb06613. Report an issue: GitHub.