Automattic/mongoose · error · TypeError

A getter must be a function.

Error message

A getter must be a function.

What it means

SchemaType#get registers a getter function on a path and throws a TypeError for any non-function argument (lib/schemaType.js:899). Like the setter variant, it guards against strings, undefined variables, and other non-callables before pushing to the getters array.

Source

Thrown at lib/schemaType.js:899

 *       name: { type: String, required: true, get: inspector },
 *       taxonomy: { type: String, get: inspector }
 *     })
 *
 *     const Virus = db.model('Virus', VirusSchema);
 *
 *     Virus.findById(id, function (err, virus) {
 *       console.log(virus.name);     // name is required
 *       console.log(virus.taxonomy); // taxonomy is not
 *     })
 *
 * @param {Function} fn
 * @return {SchemaType} this
 * @api public
 */

SchemaType.prototype.get = function(fn) {
  if (typeof fn !== 'function') {
    throw new TypeError('A getter must be a function.');
  }
  this.getters.push(fn);
  return this;
};

/**
 * Adds multiple validators for this document path.
 * Calls `validate()` for every element in validators.
 *
 * @param {(RegExp|Function|object)[]} validators
 * @returns {SchemaType}
 */

SchemaType.prototype.validateAll = function(validators) {
  for (let i = 0; i < validators.length; i++) {
    this.validate(validators[i]);
  }
  return this;

View on GitHub (pinned to 49cdab0136)

Solutions

1. Pass a function reference: `schema.path('name').get(v => v?.toUpperCase())`
2. Guard optional getters: `if (typeof fn === 'function') path.get(fn)`
3. Fix the import/variable name that resolved to undefined

Example fix

// before
schema.path('dob').get('toISOString'); // throws TypeError

// after
schema.path('dob').get(v => (v ? v.toISOString() : v));

function addGetter(schematype, fn) {
  if (typeof fn !== 'function') {
    throw new TypeError(`getter for ${schematype.path} must be a function, got ${typeof fn}`);
  }
  return schematype.get(fn);
}

const isFunction = v => typeof v === 'function';

When it happens

Trigger: `schema.path('name').get('toJSON')`; `.get(mask)` where mask is undefined due to a missed import; passing a config object instead of a function.

Common situations: Config-driven getter wiring; optional features where the getter key is absent (`config.formatter` undefined); copy-paste of getter names as strings from documentation.

Related errors

• A setter must be a function.
• Arguments must be aggregate pipeline operators
• Union schema type requires an array of types
• Invalid addFields() argument. Must be an object
• Invalid project() argument. Must be string or object

AI-assisted analysis of Automattic/mongoose@49cdab0136 (2026-08-21). Data as JSON: /api/errors/3c009fc19981338c. Report an issue: GitHub.