decoders

Elegant and battle-tested validation library for type-safe input data for TypeScript.

Introduction

Data entering your application from the outside world should not be trusted without validation and often is of the any type, effectively disabling your type checker around input values. It's an industry good practice to validate your expectations right at your program's boundaries. This has two benefits: (1) your inputs are getting validated, and (2) you can now statically know for sure the shape of the incoming data. Decoders help solve both of these problems at once.

Basic example

import { array, iso8601, number, object, optional, string } from 'decoders';

// Incoming data at runtime
const externalData = {
  id: 123,
  name: 'Alison Roberts',
  createdAt: '1994-01-11T12:26:37.024Z',
  tags: ['foo', 'bar'],
};

// Write the decoder (= what you expect the data to look like)
const userDecoder = object({
  id: number,
  name: string,
  createdAt: optional(iso8601),
  tags: array(string),
});

// Call .verify() on the incoming data
const user = userDecoder.verify(externalData);
//    ^^^^
//    TypeScript automatically infers this type as:
//    {
//      id: number;
//      name: string;
//      createdAt?: Date;
//      tags: string[];
//    }

Installation

npm install decoders

Requirements

You must set strict: true in your tsconfig.json in order for type inference to work correctly!

// tsconfig.json
{
  "compilerOptions": {
    "strict": true
  }
}

Documentation

Documentation can be found on https://decoders.cc.

Building your own decoders

There is a dedicated page in the docs that explains how to build your own decoders — it’s fun!

Changelog

[2.6.1] - 2025-02-25

• Fix issue that could lead to corrupt annotations if somehow two verions of decoders (or a CJS and an ESM "copy") end up in your bundle (#1231). Thanks for reporting, @craig-feldman!
• Use bundler module resolution setting (recommended setting for libraries that use a bundler)
• Upgrade dev dependencies