Automattic/mongoose · error · TypeError

A setter must be a function.

Error message

A setter must be a function.

What it means

SchemaType#set registers a setter function on a path and throws a TypeError for any non-function argument (lib/schemaType.js:829). Common triggers are passing a method name as a string, an undefined variable (import typo), or the already-invoked result of a function.

Source

Thrown at lib/schemaType.js:829

 *     const nameSchema = new Schema({ name: String, keywords: [String] });
 *     nameSchema.path('name').set(function(v) {
 *       // Need to check if `this` is a document, because in mongoose 5
 *       // setters will also run on queries, in which case `this` will be a
 *       // mongoose query object.
 *       if (this instanceof Document && v != null) {
 *         this.keywords = v.split(' ');
 *       }
 *       return v;
 *     });
 *
 * @param {Function} fn
 * @return {SchemaType} this
 * @api public
 */

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

/**
 * Adds a getter to this schematype.
 *
 * #### Example:
 *
 *     function dob (val) {
 *       if (!val) return val;
 *       return (val.getMonth() + 1) + "/" + val.getDate() + "/" + val.getFullYear();
 *     }
 *
 *     // defining within the schema
 *     const s = new Schema({ born: { type: Date, get: dob })
 *

View on GitHub (pinned to 49cdab0136)

Solutions

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

Example fix

// before
schema.path('name').set('trim'); // throws TypeError

// after
schema.path('name').set(v => v.trim());

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

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

When it happens

Trigger: `schema.path('name').set('uppercase')`; `.set(opts.setter)` where `opts.setter` is undefined; `.set(transform.v)` where `v` is a bound-method lookup that failed silently.

Common situations: Config-driven setter wiring; optional chaining producing undefined (`config?.post`); renaming imports and missing one usage; copying setter names from docs as strings.

Related errors

• A getter 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/c5dd201d564d5b35. Report an issue: GitHub.