jquense/yup · error · TypeError

A Method name must be provided

Error message

A Method name must be provided

What it means

addMethod()'s second argument is the method name that will be assigned onto the schema prototype, so it must be a string. If it is missing, undefined, or a non-string (number, symbol, object), addMethod throws 'A Method name must be provided'. This happens after the schemaType check, so the target constructor was valid.

Solutions

1. Pass the method name as a string: yup.addMethod(yup.string, 'myName', fn).
2. Check argument order — the signature is (schemaType, name, fn).
3. Validate the name variable is defined before invoking addMethod.

Example fix

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

const addNamedMethod = (type, name, fn) => {
  if (typeof name !== 'string' || name === '') throw new TypeError('method name must be a non-empty string');
  yup.addMethod(type, name, fn);
};

const isMethodName = (v) => typeof v === 'string' && v.length > 0;

When it happens

Trigger: yup.addMethod(yup.string, undefined, fn), addMethod(yup.date, 42, fn), or name coming from a config/destructured variable that is undefined.

Common situations: Renamed exports breaking a constant that held the method name; calling addMethod with arguments swapped (schemaType, fn, name); generated plugin code with missing metadata.

Related errors

• Method function must be provided
• ref must be a string, got:
• You must provide a yup schema constructor function
• `array.of()` sub-schema must be a valid yup schema not
• conditions must return a schema object

AI-assisted analysis of jquense/yup@ff31eee8a2 (2026-08-31). Data as JSON: /api/errors/5ee99f48d24304af. Report an issue: GitHub.

Appendix: source

Thrown at src/index.ts:59

  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,
  ISchema,
  Message,
  MessageParams,

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