Visit https://fluentvalidation-ts.alexpotter.dev to get started.

Front-end validation is a must-have for any project that involves forms, but the requirements vary hugely. You might have a simple sign-up form with a few text fields, or a complex configuration page with collections and deeply nested fields.

There are plenty of libraries out there which help you to solve the problem of front-end validation, but all the ones I've tried have felt lacking in one aspect or another - whether that's TypeScript support, their capacity to handle complex requirements, or the ability to define your own reusable validation logic.

So I wrote fluentvalidation-ts, a tiny library that is:

• Designed for TypeScript
• Simple yet powerful
• Fully extensible

Whatever your validation needs, fluentvalidation-ts can handle them.

Full documentation, including a tutorial and a number of useful guides, is available at the official fluentvalidation-ts website.

• Overview
• Tutorial
• Guides
• Core API Reference
• Validation Rules API Reference
• Releases

This library has been written in, and for, TypeScript. You can still use fluentvalidation-ts without TypeScript, but the primary benefit of having strongly-typed validation rules is lost.

If using TypeScript (strongly recommended), you must be on TypeScript version 2.9 or later.

Using Yarn:

yarn add fluentvalidation-ts

Using NPM:

npm i --save fluentvalidation-ts

Note: fluentvalidation-ts has been written with first-class support for TypeScript, so there's no need to install types!

import { Validator } from 'fluentvalidation-ts';

type Person = {
  name: string;
  age: number;
};

class PersonValidator extends Validator<Person> {
  constructor() {
    super();

    this.ruleFor('name') // This is type-safe! (Argument is of type 'name' | 'age')
      .notEmpty()
      .withMessage('Please enter your name');

    this.ruleFor('age')
      .greaterThanOrEqualTo(0)
      .withMessage('Age cannot be negative');
  }
}

const validator = new PersonValidator();

validator.validate({ name: '', age: 25 });
// { name: 'Please enter your name' }

validator.validate({ name: 'Alex', age: -1 });
// { age: 'Age cannot be negative' }

validator.validate({ name: '', age: -1 });
// { name: 'Please enter your name', age: 'Age cannot be negative' }

fluentvalidation-ts has 100% test coverage via unit tests written with Jest.

Note: Some branches are incorrectly reported as uncovered due to the following issue: gotwarlost/istanbul#690.

Please report issues via GitHub.

fluentvalidation-ts is provided under the terms of an Apache-2.0 license.

To run the project locally simply clone the repo and run yarn install then yarn start in the root of the project. You can run the tests with yarn test.

Alex Potter is a full-stack Software Developer who is currently part of the team at Ghyston, an award-winning software development company based in Bristol.

Check out his blog at alexpotter.dev, or get in touch @ajp_dev on Twitter.