Yup Skill

Overview

Schema-based validation library for JavaScript and TypeScript with intuitive API.

Objective

Enable developers to define and validate data schemas with proper TypeScript integration and error handling.

When to Use

Use this skill when:

• •Validating form data with Yup schemas
• •Creating validation logic for JavaScript/TypeScript objects
• •Implementing async validation
• •Integrating with Formik or other form libraries
• •Defining reusable validation schemas

Don't use this skill for:

• •Zod-specific patterns (use zod skill)
• •TypeScript compile-time typing only (use typescript skill)
• •Runtime parsing without validation (use zod for better TypeScript integration)

Critical Patterns

✅ REQUIRED: Use InferType for Type Extraction

// ✅ CORRECT: Automatic type inference
import * as yup from "yup";

const schema = yup.object({
  name: yup.string().required(),
});

type User = yup.InferType<typeof schema>; // { name: string }

// ❌ WRONG: Manual type definition (can drift)
interface User {
  name: string;
}
const schema = yup.object({
  /* ... */
});

✅ REQUIRED: Use validate() with try/catch

// ✅ CORRECT: Proper error handling
try {
  const valid = await schema.validate(data);
  console.log(valid);
} catch (error) {
  if (error instanceof yup.ValidationError) {
    console.error(error.message);
  }
}

// ❌ WRONG: No error handling
const valid = await schema.validate(data); // Throws on error

✅ REQUIRED: Chain Validations

// ✅ CORRECT: Multiple validations
const email = yup
  .string()
  .email("Invalid email")
  .required("Email is required")
  .max(100, "Too long");

// ❌ WRONG: Single validation
const email = yup.string();

Conventions

Refer to conventions for:

• •Error handling

Refer to typescript for:

• •Type inference from schemas

Yup Specific

• •Define schemas with proper types
• •Use TypeScript's InferType for type extraction
• •Implement custom validation methods
• •Handle validation errors appropriately
• •Use schema composition

Decision Tree

String validation? → Use .string() with .email(), .url(), .min(), .max(), .matches().

Number validation? → Use .number() with .positive(), .integer(), .min(), .max().

Optional field? → Use .nullable() or .notRequired(). Use .default() for default values.

Conditional validation? → Use .when('field', { is: value, then: schema }) for dependent fields.

Custom validation? → Use .test('name', 'message', (value) => boolean).

Async validation? → Use .test() with async function, handle promises.

Schema composition? → Use .concat() to merge schemas or extract common schemas.

Array validation? → Use .array(itemSchema) with .min(), .max(), .of().

Example

import * as yup from "yup";

const userSchema = yup.object({
  name: yup.string().required("Name is required"),
  email: yup.string().email("Invalid email").required("Email is required"),
  age: yup.number().positive().integer().min(18, "Must be 18 or older"),
});

type User = yup.InferType<typeof userSchema>;

try {
  const validUser: User = await userSchema.validate({
    name: "John",
    email: "john@example.com",
    age: 25,
  });
} catch (error) {
  if (error instanceof yup.ValidationError) {
    console.error(error.message);
  }
}

Edge Cases

Circular references: Use yup.lazy() for recursive schemas that reference themselves.

Transform values: Use .transform((value, original) => transformed) to modify values during validation.

Strict vs default: Yup removes unknown keys by default. Use .strict() to throw error on unknown keys.

Multiple errors: Use .abortEarly: false in validate options to collect all errors, not just first.

Nullable vs optional: .nullable() allows null, .notRequired() allows undefined. .optional() is alias for .notRequired().

References

• •https://github.com/jquense/yup