What causes "Model.init() no longer accepts a callback" in Automattic/mongoose?

Await the promise: `await Event.init();`

Automattic/mongoose · error · MongooseError

Model.init() no longer accepts a callback

Error message

Model.init() no longer accepts a callback

What it means

Model.init() builds the collection and its indexes and returns a promise; since Mongoose 7 the callback API was removed entirely. If `arguments[0]` is a function, init() throws immediately instead of silently dropping the callback. Use `await Model.init()` and handle errors via try/catch or .catch().

Source

Thrown at lib/model.js:1120

 *
 * #### Example:
 *
 *     const eventSchema = new Schema({ thing: { type: 'string', unique: true } })
 *     // This calls `Event.init()` implicitly, so you don't need to call
 *     // `Event.init()` on your own.
 *     const Event = mongoose.model('Event', eventSchema);
 *
 *     await Event.init();
 *     console.log('Indexes are done building!');
 *
 * @api public
 * @returns {Promise}
 */

Model.init = function init() {
  _checkContext(this, 'init');
  if (typeof arguments[0] === 'function') {
    throw new MongooseError('Model.init() no longer accepts a callback');
  }

  this.schema.emit('init', this);

  if (this.$init != null) {
    return this.$init;
  }

  const conn = this.db;
  const _ensureIndexes = async() => {
    const autoIndex = utils.getOption(
      'autoIndex',
      this.schema.options,
      conn.config,
      conn.base.options
    );
    if (!autoIndex) {
      return;

View on GitHub (pinned to 49cdab0136)

Solutions

Await the promise: `await Event.init();`
Or chain: `Event.init().then(onReady).catch(onErr)`
Run repo-wide search for `\.init\(` with a following function argument and migrate all matches
Check the Mongoose 7 migration guide section 'Callback API removed' for the full list of affected functions

Example fix

// before
Event.init(function(err) { if (err) throw err; });

// after
await Event.init();

Defensive patterns

Strategy: validation

Validate before calling

// Lint-time equivalent: refuse callback args before calling
const noCallbacks = (fnName, args) => {
  if (args.some(a => typeof a === 'function')) {
    throw new TypeError(`${fnName}() accepts no callback — await the returned promise`);
  }
};
// usage: noCallbacks('init', [process.env.INIT_CB]); await Event.init();

Prevention

Migrate all mongoose calls to async/await in one mechanical pass before upgrading to Mongoose 7+
Rely on TypeScript: the modern @types signatures have no callback overloads, so old code fails to compile
Add an ESLint no-restricted-syntax rule banning function arguments to known mongoose method names

When it happens

Trigger: `Event.init(err => { ... })` in code written for Mongoose 6 or earlier; passing a success handler as the first argument; tutorials/documentation predating Mongoose 7.

Common situations: Upgrading an application from mongoose 6.x to 7+/8+ without migrating call sites; legacy codebases where only a few index-setup calls still use callbacks.

Related errors

AI-assisted analysis of Automattic/mongoose@49cdab0136 (2026-08-21). Data as JSON: /api/errors/43e48608bb77618a. Report an issue: GitHub.