Yup - Quick Reference

Full Reference: See advanced.md for conditional validation (.when), custom validation (.test), Formik/React Hook Form integration, transforms, and localization.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: yup for comprehensive documentation.

Setup

npm install yup

Basic Usage

import * as yup from 'yup';

const userSchema = yup.object({
  name: yup.string().required().min(2).max(50),
  email: yup.string().required().email(),
  age: yup.number().required().min(18).max(120).integer(),
  bio: yup.string().optional(),
});

// Type inference
type User = yup.InferType<typeof userSchema>;

// Validate
async function validateUser(data: unknown) {
  try {
    const user = await userSchema.validate(data, { abortEarly: false });
    return { success: true, data: user };
  } catch (error) {
    if (error instanceof yup.ValidationError) {
      return { success: false, errors: error.errors };
    }
    throw error;
  }
}

Schema Types

String

const stringSchema = yup
  .string()
  .required('Name is required')
  .min(2, 'Too short')
  .max(100, 'Too long')
  .trim()
  .email('Invalid email')
  .url('Invalid URL')
  .matches(/^[a-zA-Z]+$/, 'Only letters allowed')
  .oneOf(['option1', 'option2'], 'Must be one of the options');

Number

const numberSchema = yup
  .number()
  .required()
  .min(0)
  .max(100)
  .positive()
  .integer()
  .typeError('Must be a number');

Date

const dateSchema = yup
  .date()
  .required()
  .min(new Date('2020-01-01'), 'Too old')
  .max(new Date(), 'Cannot be in future');

Array

const arraySchema = yup
  .array()
  .of(yup.string().required())
  .required()
  .min(1, 'At least one item')
  .max(10, 'Maximum 10 items')
  .ensure();  // Transform null/undefined to []

Object

const objectSchema = yup.object({
  name: yup.string().required(),
  address: yup.object({
    street: yup.string().required(),
    city: yup.string().required(),
  }),
}).noUnknown(true);  // Strip unknown keys

// Partial object (all fields optional)
const partialSchema = objectSchema.partial();

Validation Options

const result = await schema.validate(data, {
  abortEarly: false,    // Return all errors, not just first
  stripUnknown: true,   // Remove unknown keys
  strict: false,        // Allow type coercion
});

// Sync validation
schema.validateSync(data);
schema.isValidSync(data);  // Returns boolean

Comparison: Yup vs Zod

| Feature | Yup | Zod | |---------|-----|-----| | Type inference | Good | Excellent | | Bundle size | ~12KB | ~8KB | | Async validation | Built-in | .parseAsync() | | Formik | Native | Via resolver | | Conditional | .when() | .refine() |

When NOT to Use This Skill

• New projects - Prefer Zod for better TypeScript integration
• NestJS applications - Use class-validator with decorators
• Non-form validation - Zod has better API for general validation

Anti-Patterns

| Anti-Pattern | Why It's Bad | Correct Approach | |--------------|--------------|------------------| | Schema in component | Re-created on render | Define at module level | | Forgetting abortEarly: false | Only shows first error | Set abortEarly: false | | No custom messages | Generic errors | Add custom message to each | | Not using InferType | Manual type definitions | Use yup.InferType<typeof schema> |

Quick Troubleshooting

| Issue | Cause | Solution | |-------|-------|----------| | Type errors with InferType | Missing import | Import type from 'yup' | | Async validation not working | Using validateSync | Use .validate() (async) | | "Value is required" unexpectedly | Undefined vs null | Use .nullable() or .optional() | | Error messages not showing | abortEarly: true | Set abortEarly: false |

Production Checklist

• [ ] Schema defined at module level
• [ ] Type inference with InferType
• [ ] Custom error messages
• [ ] abortEarly: false for forms
• [ ] Resolver for form library
• [ ] Localization if multilingual

Reference Documentation

• Yup GitHub