jquense/yup · error · TypeError

You must provide a yup schema constructor function

Error message

You must provide a yup schema constructor function

What it means

addMethod() extends a yup schema constructor's prototype with custom methods, and its first argument must be a yup schema constructor whose prototype is itself a schema (checked with isSchema(schemaType.prototype)). Passing nothing, a non-constructor, or an unrelated class throws this TypeError. It is a guard so methods are only added to real yup types.

Source

Thrown at src/index.ts:56

  Message,
  MessageParams,
  ValidateOptions,
  DefaultThunk,
} from './types';

function addMethod<T extends ISchema<any>>(
  schemaType: (...arg: any[]) => T,
  name: string,
  fn: (this: T, ...args: any[]) => T,
): void;
function addMethod<T extends abstract new (...args: any) => ISchema<any>>(
  schemaType: T,
  name: string,
  fn: (this: InstanceType<T>, ...args: any[]) => InstanceType<T>,
): void;
function addMethod(schemaType: any, name: string, fn: any) {
  if (!schemaType || !isSchema(schemaType.prototype))
    throw new TypeError('You must provide a yup schema constructor function');

  if (typeof name !== 'string')
    throw new TypeError('A Method name must be provided');
  if (typeof fn !== 'function')
    throw new TypeError('Method function must be provided');

  schemaType.prototype[name] = fn;
}

export type AnyObjectSchema = ObjectSchema<any, any, any, any>;

export type CastOptions = Omit<BaseCastOptions, 'path' | 'resolved'>;

export type {
  AnyMessageParams,
  AnyObject,
  InferType,
  InferType as Asserts,

View on GitHub (pinned to ff31eee8a2)

Solutions

1. Pass the schema class, not an instance: yup.addMethod(yup.string, 'myMethod', fn).
2. Fix the import so the schema type is defined at call time (check for circular imports).
3. If using a custom class, make sure it extends a yup schema type so its prototype passes isSchema().

Example fix

// before
yup.addMethod(yup.string(), 'phone', fn)
// after
yup.addMethod(yup.string, 'phone', fn)

import { isSchema } from 'yup';
const safeAddMethod = (type, name, fn) => {
  if (!type || !isSchema(type.prototype)) throw new TypeError('addMethod target must be a yup schema constructor');
  yup.addMethod(type, name, fn);
};

const isSchemaConstructor = (t) => typeof t === 'function' && isSchema(t.prototype);

When it happens

Trigger: yup.addMethod(undefined, 'foo', fn) (bad/missing import), yup.addMethod(String, ...) with a native constructor, or passing an instance instead of the class (yup.addMethod(yup.string(), ...)).

Common situations: Circular import producing undefined yup at module init; tree-shaking/ESM-CJS interop dropping the named export; upgrading yup where addMethod moved or types changed; custom schema classes not extending yup.Schema.

Related errors

• A Method name must be provided
• Method function must be provided
• conditions must return a schema object
• lazy() functions must return a valid schema
• ref must be a string, got:

AI-assisted analysis of jquense/yup@ff31eee8a2 (2026-08-31). Data as JSON: /api/errors/5cd57aadab8c0103. Report an issue: GitHub.