reflect-metadata vs class-transformer vs class-validator
TypeScript Decorators and Validation Libraries Comparison
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What's TypeScript Decorators and Validation Libraries?

These libraries are essential for enhancing TypeScript applications, particularly in the context of object-oriented programming. They provide powerful tools for transforming and validating class instances, which is crucial for ensuring data integrity and consistency in applications. By leveraging decorators and metadata reflection, developers can create robust data models that are easy to manage and validate, making them particularly useful in frameworks like NestJS and for building APIs.

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reflect-metadata15,411,1463,260241 kB25a year agoApache-2.0
class-transformer5,106,7577,097776 kB2443 years agoMIT
class-validator4,169,37211,3325.12 MB302a year agoMIT
Feature Comparison: reflect-metadata vs class-transformer vs class-validator

Data Transformation

  • reflect-metadata:

    reflect-metadata provides the necessary infrastructure to support metadata reflection in TypeScript. It allows class-transformer and class-validator to function correctly by enabling decorators to store and retrieve metadata about class properties.

  • class-transformer:

    class-transformer allows you to easily convert plain objects into class instances and vice versa. This is particularly useful when dealing with data from APIs, as it enables you to instantiate classes with methods and properties, enhancing the usability of your data models.

  • class-validator:

    class-validator does not focus on data transformation but can work alongside class-transformer to ensure that the transformed data adheres to specific validation rules. It ensures that once data is transformed, it meets the required criteria before further processing.

Validation Capabilities

  • reflect-metadata:

    reflect-metadata does not provide validation capabilities but is crucial for enabling the validation decorators in class-validator to function properly by allowing them to access metadata about class properties.

  • class-transformer:

    class-transformer does not provide validation capabilities on its own; it focuses on transformation. However, it can be combined with class-validator to ensure that transformed data is also validated, creating a robust data handling mechanism.

  • class-validator:

    class-validator is specifically designed for validation, offering a rich set of decorators for various validation rules. It allows you to enforce constraints on class properties, ensuring that the data is valid before it is used in your application.

Decorator Support

  • reflect-metadata:

    reflect-metadata is essential for enabling the use of decorators in TypeScript. It allows decorators to store metadata about classes and their properties, which is crucial for both class-transformer and class-validator to function effectively.

  • class-transformer:

    class-transformer utilizes decorators to define how properties should be transformed. This makes it easy to annotate class properties with transformation rules, enhancing code readability and maintainability.

  • class-validator:

    class-validator heavily relies on decorators to apply validation rules to class properties. This declarative approach allows developers to easily see and understand the validation logic associated with each property, improving code clarity.

Integration

  • reflect-metadata:

    reflect-metadata is a foundational library that supports the functionality of both class-transformer and class-validator. It is necessary for enabling the decorator functionality that both libraries depend on for their operations.

  • class-transformer:

    class-transformer integrates seamlessly with class-validator, allowing you to transform and validate data in a cohesive manner. This integration is particularly beneficial in frameworks like NestJS, where both transformation and validation are often required.

  • class-validator:

    class-validator works best when used in conjunction with class-transformer, as it can validate the transformed data. This integration ensures that your data models are not only correctly structured but also valid according to your business rules.

Learning Curve

  • reflect-metadata:

    reflect-metadata has a low learning curve as it primarily serves as a utility library. Understanding its role in enabling decorators is essential, but it does not require extensive knowledge to implement.

  • class-transformer:

    class-transformer has a moderate learning curve, especially for developers familiar with object-oriented programming and TypeScript. Understanding how to effectively use decorators and transformation rules is key to leveraging its full potential.

  • class-validator:

    class-validator is relatively easy to learn, especially for those who are already familiar with TypeScript and decorators. Its straightforward API and rich set of built-in validators make it accessible for developers of varying skill levels.

How to Choose: reflect-metadata vs class-transformer vs class-validator
  • reflect-metadata:

    Choose reflect-metadata if you need to utilize TypeScript's decorator metadata capabilities. This package is essential for enabling decorators to work effectively, especially in conjunction with class-transformer and class-validator, as it allows for the storage and retrieval of metadata associated with classes and their properties.

  • class-transformer:

    Choose class-transformer if you need to transform plain JavaScript objects into class instances and vice versa. It is particularly useful when working with APIs where you need to map JSON data to TypeScript classes, allowing you to leverage class methods and properties directly.

  • class-validator:

    Choose class-validator if your application requires robust validation of data models. It provides a wide range of built-in validation decorators that can be easily applied to class properties, ensuring that the data adheres to specified rules before processing or saving it.

Similar Npm Packages to reflect-metadata

reflect-metadata is a library that provides a way to add and retrieve metadata in JavaScript and TypeScript applications. It is particularly useful in scenarios where decorators are used, allowing developers to attach metadata to classes, methods, and properties. This metadata can then be accessed at runtime, enabling powerful features such as dependency injection and validation. While reflect-metadata serves as a foundational tool for many applications, there are alternatives that offer additional functionalities. Here are a couple of notable alternatives:

  • class-transformer is a library that allows you to transform plain objects into class instances and vice versa. It leverages decorators to define how properties should be transformed, making it easier to work with data coming from external sources, such as APIs. If your application requires transforming data between different formats or structures, class-transformer is an excellent choice. It works seamlessly with TypeScript and can be combined with reflect-metadata to enhance its capabilities.

  • class-validator is a library that provides a way to validate class properties using decorators. It allows you to define validation rules directly on your class properties, making it easy to enforce data integrity and ensure that incoming data meets specific criteria. When used alongside class-transformer, class-validator can help ensure that the data being transformed into class instances is valid. This combination is particularly useful in applications that require strict data validation, such as those handling user input or API responses.

To explore how reflect-metadata compares with class-transformer and class-validator, check out the comparison: Comparing class-transformer vs class-validator vs reflect-metadata.

Similar Npm Packages to class-transformer

class-transformer is a powerful library for transforming plain JavaScript objects into class instances and vice versa. It is particularly useful in TypeScript applications, where you can leverage the benefits of classes and decorators to create more structured and maintainable code. By using class-transformer, developers can easily manage data transformations, making it an essential tool for applications that require serialization and deserialization of data.

While class-transformer is a robust solution for data transformation, there are several alternatives in the ecosystem that provide similar functionalities. Here are a few notable ones:

  • class-transformer-validator is an extension of class-transformer that integrates validation capabilities. It allows you to validate class instances after transformation, ensuring that the data adheres to specified rules and constraints. If you are already using class-transformer and need validation features, this library is a natural choice, as it builds upon the existing functionality while adding validation capabilities.
  • class-validator is another popular library that focuses solely on validation. It allows you to define validation rules using decorators on class properties. While it does not handle transformation like class-transformer, it can be used in conjunction with it to validate transformed class instances. If your primary requirement is to enforce validation rules on your data models, class-validator is a great option that can work alongside class-transformer.
  • io-ts is a runtime type system for TypeScript that allows you to define and validate types using a functional programming approach. Unlike class-transformer, which is class-based, io-ts focuses on creating codecs that can decode and validate data structures. If you prefer a more functional approach to type validation and transformation, io-ts provides a powerful alternative that integrates well with TypeScript's type system.

To see how class-transformer compares with class-transformer-validator, class-validator, and io-ts, check out the comparison: Comparing class-transformer vs class-transformer-validator vs class-validator vs io-ts.

Similar Npm Packages to class-validator

class-validator is a powerful validation library for TypeScript and JavaScript applications. It allows developers to define validation rules using decorators, making it easy to validate class instances. This library is particularly useful in scenarios where you want to enforce data integrity and ensure that the data being processed meets specific criteria. While class-validator is a great choice for validation, there are several alternatives available that cater to different needs and preferences. Here are a few notable alternatives:

  • ajv (Another JSON Schema Validator) is a fast and efficient JSON schema validator. It supports the latest JSON Schema standards and provides a robust way to validate JSON data structures. If you're working with JSON data and need a schema-based validation solution, ajv is an excellent choice. Its performance and compliance with JSON Schema standards make it suitable for applications that require strict validation of complex data structures.

  • joi is a popular validation library that allows you to create schemas for JavaScript objects. It provides a rich set of validation rules and is highly customizable. joi is particularly useful in Node.js applications and is often used in conjunction with frameworks like Express. If you need a flexible and expressive way to validate data structures, joi is a strong contender.

  • yup is a JavaScript schema builder for value parsing and validation. It is similar to joi but is designed to work seamlessly with React and other frontend libraries. yup provides a simple and intuitive API for defining validation schemas, making it a great choice for form validation in client-side applications. If you're looking for a lightweight and easy-to-use validation library for React, yup is worth considering.

  • zod is a TypeScript-first schema declaration and validation library. It offers a simple and expressive API for defining schemas and validating data. zod is particularly well-suited for TypeScript projects, as it provides strong type inference and validation capabilities. If you're working in a TypeScript environment and want a validation library that integrates well with TypeScript's type system, zod is an excellent option.

To see how class-validator compares with ajv, joi, yup, and zod, check out the comparison: Comparing ajv vs class-validator vs joi vs yup vs zod.

README for reflect-metadata

Metadata Reflection API

NOTE: Now that both Decorators and Decorator Metadata have achieved Stage 3 within TC39, the API proposed below is no longer being considered for standardization. However, this package will continue to support projects that leverage TypeScript's legacy --experimentalDecorators option as some projects may not be able to migrate to use standard decorators.

Installation

npm install reflect-metadata

Usage

ES Modules in NodeJS/Browser, TypeScript/Babel, Bundlers

// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Supports ESM and CommonJS.
// - Contains internal polyfills for `Map`, `Set`, and `WeakMap` for older runtimes.
import "reflect-metadata";

// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Supports ESM and CommonJS.
// - Requires runtime support for `"exports"` in `package.json`.
// - Does not include internal polyfills.
import "reflect-metadata/lite";

CommonJS

// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Contains internal polyfills for `Map`, `Set`, and `WeakMap` for older runtimes.
require("reflect-metadata");

// - Modifies global `Reflect` object (or defines one in ES5 runtimes).
// - Requires runtime support for `"exports"` in `package.json`.
// - Does not include internal polyfills.
require("reflect-metadata/lite");

In the Browser via <script>

HTML

<!-- Modifies global `Reflect` object (or defines one in ES5 runtimes). -->
<!-- Contains internal polyfills for `Map`, `Set`, and `WeakMap` for older runtimes. -->
<script src="path/to/reflect-metadata/Reflect.js"></script>

<!-- Modifies global `Reflect` object (or defines one in ES5 runtimes). -->
<!-- Does not include internal polyfills. -->
<script src="path/to/reflect-metadata/ReflectLite.js"></script>

Script

// - Makes types available in your editor.
/// <reference path="path/to/reflect-metadata/standalone.d.ts" />

Background

  • Decorators add the ability to augment a class and its members as the class is defined, through a declarative syntax.
  • Traceur attaches annotations to a static property on the class.
  • Languages like C# (.NET), and Java support attributes or annotations that add metadata to types, along with a reflective API for reading metadata.

Goals

  • A number of use cases (Composition/Dependency Injection, Runtime Type Assertions, Reflection/Mirroring, Testing) want the ability to add additional metadata to a class in a consistent manner.
  • A consistent approach is needed for various tools and libraries to be able to reason over metadata.
  • Metadata-producing decorators (nee. "Annotations") need to be generally composable with mutating decorators.
  • Metadata should be available not only on an object but also through a Proxy, with related traps.
  • Defining new metadata-producing decorators should not be arduous or over-complex for a developer.
  • Metadata should be consistent with other language and runtime features of ECMAScript.

Syntax

  • Declarative definition of metadata:
class C {
  @Reflect.metadata(metadataKey, metadataValue)
  method() {
  }
}
  • Imperative definition of metadata:
Reflect.defineMetadata(metadataKey, metadataValue, C.prototype, "method");
  • Imperative introspection of metadata:
let obj = new C();
let metadataValue = Reflect.getMetadata(metadataKey, obj, "method");

Semantics

  • Object has a new [[Metadata]] internal property that will contain a Map whose keys are property keys (or undefined) and whose values are Maps of metadata keys to metadata values.
  • Object will have a number of new internal methods for [[DefineOwnMetadata]], [[GetOwnMetadata]], [[HasOwnMetadata]], etc.
    • These internal methods can be overridden by a Proxy to support additional traps.
    • These internal methods will by default call a set of abstract operations to define and read metadata.
  • The Reflect object will expose the MOP operations to allow imperative access to metadata.
  • Metadata defined on class declaration C is stored in C.[[Metadata]], with undefined as the key.
  • Metadata defined on static members of class declaration C are stored in C.[[Metadata]], with the property key as the key.
  • Metadata defined on instance members of class declaration C are stored in C.prototype.[[Metadata]], with the property key as the key.

API

// define metadata on an object or property
Reflect.defineMetadata(metadataKey, metadataValue, target);
Reflect.defineMetadata(metadataKey, metadataValue, target, propertyKey);

// check for presence of a metadata key on the prototype chain of an object or property
let result = Reflect.hasMetadata(metadataKey, target);
let result = Reflect.hasMetadata(metadataKey, target, propertyKey);

// check for presence of an own metadata key of an object or property
let result = Reflect.hasOwnMetadata(metadataKey, target);
let result = Reflect.hasOwnMetadata(metadataKey, target, propertyKey);

// get metadata value of a metadata key on the prototype chain of an object or property
let result = Reflect.getMetadata(metadataKey, target);
let result = Reflect.getMetadata(metadataKey, target, propertyKey);

// get metadata value of an own metadata key of an object or property
let result = Reflect.getOwnMetadata(metadataKey, target);
let result = Reflect.getOwnMetadata(metadataKey, target, propertyKey);

// get all metadata keys on the prototype chain of an object or property
let result = Reflect.getMetadataKeys(target);
let result = Reflect.getMetadataKeys(target, propertyKey);

// get all own metadata keys of an object or property
let result = Reflect.getOwnMetadataKeys(target);
let result = Reflect.getOwnMetadataKeys(target, propertyKey);

// delete metadata from an object or property
let result = Reflect.deleteMetadata(metadataKey, target);
let result = Reflect.deleteMetadata(metadataKey, target, propertyKey);

// apply metadata via a decorator to a constructor
@Reflect.metadata(metadataKey, metadataValue)
class C {
  // apply metadata via a decorator to a method (property)
  @Reflect.metadata(metadataKey, metadataValue)
  method() {
  }
}

Alternatives

  • Use properties rather than a separate API.
    • Obvious downside is that this can be a lot of code:
function ParamTypes(...types) {
  return (target, propertyKey) => {
    const symParamTypes = Symbol.for("design:paramtypes");
    if (propertyKey === undefined) {
      target[symParamTypes] = types;
    }
    else {
      const symProperties = Symbol.for("design:properties");
      let properties, property;
      if (Object.prototype.hasOwnProperty.call(target, symProperties)) {
        properties = target[symProperties];
      }
      else {
        properties = target[symProperties] = {};
      }
      if (Object.prototype.hasOwnProperty.call(properties, propertyKey)) {
        property = properties[propertyKey];
      }
      else {
        property = properties[propertyKey] = {};
      }
      property[symParamTypes] = types;
    }
  };
}

Notes

  • Though it may seem counterintuitive, the methods on Reflect place the parameters for the metadata key and metadata value before the target or property key. This is due to the fact that the property key is the only optional parameter in the argument list. This also makes the methods easier to curry with Function#bind. This also helps reduce the overall footprint and complexity of a metadata-producing decorator that could target both a class or a property:
function ParamTypes(...types) {
  // as propertyKey is effectively optional, its easier to use here
  return (target, propertyKey) => { Reflect.defineMetadata("design:paramtypes", types, target, propertyKey); }

  // vs. having multiple overloads with the target and key in the front:
  //
  // return (target, propertyKey) => {
  //    if (propertyKey === undefined) {
  //      Reflect.defineMetadata(target, "design:paramtypes", types);
  //    }
  //    else {
  //      Reflect.defineMetadata(target, propertyKey, "design:paramtypes", types);
  //    }
  // }
  //
  // vs. having a different methods for the class or a property:
  //
  // return (target, propertyKey) => {
  //    if (propertyKey === undefined) {
  //      Reflect.defineMetadata(target, "design:paramtypes", types);
  //    }
  //    else {
  //      Reflect.definePropertyMetadata(target, propertyKey, "design:paramtypes", types);
  //    }
  // }
}
  • To enable experimental support for metadata decorators in your TypeScript project, you must add "experimentalDecorators": true to your tsconfig.json file.
  • To enable experimental support for auto-generated type metadata in your TypeScript project, you must add "emitDecoratorMetadata": true to your tsconfig.json file.
    • Please note that auto-generated type metadata may have issues with circular or forward references for types.

Issues

  • A poorly written mutating decorator for a class constructor could cause metadata to become lost if the prototype chain is not maintained. Though, not maintaining the prototype chain in a mutating decorator for a class constructor would have other negative side effects as well. @rbuckton
    • This is mitigated if the mutating decorator returns a class expression that extends from the target, or returns a proxy for the decorator. @rbuckton
  • Metadata for a method is attached to the class (or prototype) via the property key. It would not then be available if trying to read metadata on the function of the method (e.g. "tearing-off" the method from the class). @rbuckton
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