babel-plugin-transform-typescript-metadata

What is babel-plugin-transform-typescript-metadata?

babel-plugin-transform-typescript-metadata is a Babel plugin that enables the transformation of TypeScript metadata for use with decorators. This is particularly useful for libraries that rely on metadata reflection, such as dependency injection frameworks.

What are babel-plugin-transform-typescript-metadata's main functionalities?

Transform TypeScript Metadata

This feature allows the transformation of TypeScript metadata, making it available for use with decorators. This is particularly useful for dependency injection frameworks that rely on metadata reflection.

/* .babelrc */
{
  "plugins": [
    "babel-plugin-transform-typescript-metadata"
  ]
}

/* Example TypeScript file */
import 'reflect-metadata';

class Example {
  constructor(@Inject('MyService') private myService: MyService) {}
}

// After transformation, metadata about the 'myService' parameter will be available

Other packages similar to babel-plugin-transform-typescript-metadata

reflect-metadata

reflect-metadata is a polyfill for the Metadata Reflection API, which is a proposed feature for ECMAScript. It allows for the addition of metadata to class declarations and members. Unlike babel-plugin-transform-typescript-metadata, it does not transform TypeScript code but provides a runtime API for metadata reflection.

tslib

tslib is a runtime library for TypeScript that provides helper functions for various TypeScript features, including decorators. While it does not specifically transform TypeScript metadata, it is often used in conjunction with other tools to support TypeScript features in a runtime environment.

inversify

inversify is a powerful and lightweight inversion of control (IoC) container for JavaScript and TypeScript apps. It uses decorators and metadata to manage dependencies. While it does not transform TypeScript metadata itself, it relies on metadata reflection to function, making it a complementary tool to babel-plugin-transform-typescript-metadata.

README

babel-plugin-transform-typescript-metadata

Babel plugin to emit decorator metadata like typescript compiler

Motivation

TypeScript Decorators allows advanced reflection patterns when combined with Reflect.metadata output.

Current @babel/preset-typescript implementation however just strips all types and does not emit the relative Metadata in the output code.

Since this kind of information is used extensively in libraries like Nest and TypeORM to implement advanced features like Dependency Injection, I've thought it would be awesome to be able to provide the same functionality that TypeScript compiler experimentalDecorators and emitDecoratorMetadata flags provide.

This means that code like:

import { Injectable, Inject } from 'some-di-library'; // Just an example
import { MyService } from './MyService';
import { Configuration } from './Configuration';

@Injectable()
class AnotherService {
  @Inject()
  config: Configuration;

  constructor(private service: MyService) {}
}

will be interpreted like:

import { MyService } from './MyService';
import { Configuration } from './Configuration';

@Injectable()
@Reflect.metadata('design:paramtypes', [MyService])
class AnotherService {
  @Inject()
  @Reflect.metadata('design:type', Configuration)
  config: Configuration;

  constructor(private service: MyService) {}
}

Parameter decorators

Since decorators in typescript supports also Parameters, this plugin also provides support for them, enabling the following syntax:

@Injectable()
class Some {
  constructor(@Inject() private: SomeService);
}

This will be roughly translated to:

// ...
Inject()(Some.prototype, undefined, 0);

Installation

With npm:

npm install --dev --save babel-plugin-transform-typescript-metadata

or with Yarn:

yarn add --dev babel-plugin-transform-typescript-metadata

Usage

With .babelrc:

Note: should be placed before @babel/plugin-proposal-decorators.

{
  "plugins": [
    "babel-plugin-transform-typescript-metadata",
    ["@babel/plugin-proposal-decorators", { "legacy": true }],
    ["@babel/plugin-proposal-class-properties", { "loose": true }],
  ],
  "presets": [
    "@babel/preset-typescript"
  ]
}

Usage with InversifyJS

If you are using normal dependency injection letting Inversify create your instances, you should be fine with all kind of decorators.

Instead, if you are using property injection, when the container does not create the instances, you would likely encounter errors since babel decorators are not exactly the same as TypeScript.

You can fix it by enhancing property decorators with the following function:

import getDecorators from 'inversify-inject-decorators';
// setup the container...
let { lazyInject: originalLazyInject } = getDecorators(container);

// Additional function to make properties decorators compatible with babel.
function fixPropertyDecorator<T extends Function>(decorator: T): T {
  return ((...args: any[]) => (
    target: any,
    propertyName: any,
    ...decoratorArgs: any[]
  ) => {
    decorator(...args)(target, propertyName, ...decoratorArgs);
    return Object.getOwnPropertyDescriptor(target, propertyName);
  }) as any;
}

export const lazyInject = fixPropertyDecorator(originalLazyInject);

Current Pitfalls

• We cannot know if type annotations are just types (i.e. IMyInterface) or concrete values (like classes, etc.). In order to resolve this, we emit the following: typeof Type === 'undefined' ? Object : Type. The code has the advantage of not throwing. If you know a better way to do this, let me know!