Kanun

Framework-agnostic TypeScript-first validation library. Provides a rich set of built-in rules, custom rule support, nested data validation, conditional rules, localized error messages, and a flexible API for validating complex data structures in Node.js applications.

Installation

npm install kanun

Features

• Rule-based validation — Supports common rules like required, min, max, email, url, numeric, boolean, in, regex, etc.

• Nested data validation — Dot notation for nested objects (user.email, items.*.price).

• Custom error messages — Per-rule and per-field message overrides.

• Batch validation — Validate multiple datasets or groups in one call.

• Conditional validation — Rules that only apply when other fields meet conditions (required_if, sometimes, exclude_unless).

• Implicit rules — Rules that run even when attributes are missing (e.g., accepted, required).

• Custom rules — Define user-provided validation rules as functions or classes.

• Async rules — Support for async validation (e.g., checking uniqueness in a database).

• Attribute sanitization — Optional transformation (e.g., trimming, normalizing case) before validation.

• Dynamic rule sets — Rules can be generated at runtime (e.g., based on user roles).

• Dependent rules — Rules that reference other fields dynamically.

• Localized error messages — Built-in support for localization and i18n message templates.

• Structured errors — Validation errors returned as structured objects or flat key–message pairs.

• Fail-fast mode — Option to stop at the first failure or collect all errors.

• Human-readable summaries — Helper for formatting readable validation reports.

• TypeScript-first design — Full type inference for rules, messages, and validated data.

• Chainable API — Optional fluent syntax for building validators.

Usage

Quick Start

import { Validator } from 'kanun';

const validator = new Validator(
  { email: '[email protected]', age: 20 },
  { email: 'required|email', age: 'required|numeric|min:18' },
);

const passes = await validator.passes();

if (!passes) {
  console.log(validator.errors().all());
}

Throwing Validation Errors

import { Validator, ValidationException } from 'kanun';

const validator = Validator.make({ email: '' }, { email: 'required|email' });

try {
  const data = await validator.validate();
  console.log(data);
} catch (error) {
  if (error instanceof ValidationException) {
    console.log(error.errors());
  }
}

Custom Messages

const validator = new Validator(
  { email: '' },
  { email: 'required|email' },
  {
    'email.required': 'Email is required.',
    'email.email': 'Email must be valid.',
  },
);

Nested and Array Validation

const validator = new Validator(
  {
    user: { name: { first: 'John', last: '' } },
    users: [{ email: '[email protected]' }, { email: 'bad' }],
  },
  {
    'user.name.first': 'required|min:2',
    'user.name.last': 'required|min:2',
    'users.*.email': 'required|email',
  },
);

Custom Rule Class

import { ValidationRule, Validator } from 'kanun';

class StartsWithKanun extends ValidationRule {
  validate(
    attribute: string,
    value: any,
    fail: (message: string) => void,
  ): void {
    if (typeof value !== 'string' || !value.startsWith('kanun')) {
      fail(`The ${attribute} must start with kanun.`);
    }
  }
}

const validator = new Validator(
  { slug: 'kanun' },
  { slug: ['required', new StartsWithKanun()] },
);

Database-backed exists and unique

import { IDatabaseDriver, Validator } from 'kanun';

class AppDatabaseDriver extends IDatabaseDriver {
  async exists({ table, column, value, ignore }) {
    const row = await db.table(table).where(column, value).first();
    if (!row) return false;
    if (ignore != null && String(row.id) === String(ignore)) return false;
    return true;
  }
}

const driver = new AppDatabaseDriver();

Validator.useDatabase(driver);

const validator = new Validator(
  { username: 'legacy' },
  { username: 'unique:users,username' },
);

For the complete guide and API reference, visit the docs site: https://arcstack-hq.github.io/kanun

Code of Conduct

In order to ensure that the Kanun community is welcoming to all, please review and abide by the Code of Conduct.

Security Vulnerabilities

If you discover a security vulnerability within Kanun, please send an e-mail to Legacy via [email protected]. All security vulnerabilities will be promptly addressed.

License

Kanun and all Arcstack packages are open source and licensed under the MIT license.