@e22m4u/ts-reflector

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A typed wrapper for The Metadata Reflection API

Installation

npm install @e22m4u/ts-reflector

Experimental decorators

To enable experimental decorators support add the following options to your tsconfig.json file.

{
  "emitDecoratorMetadata": true,
  "experimentalDecorators": true
}

MetadataKey

Here's an example of using MetadataKey for storing metadata. Values are validated against the given key type before storage. The metadata type is also automatically inferred when retrieving values.

import {Reflector} from '@e22m4u/ts-metadata';
import {MetadataKey} from '@e22m4u/ts-metadata';

type MyData = {foo: string};
class Target {/* ... */}

// Create a MetadataKey<T> for MyData type
const key = new MetadataKey<MyData>();

// Assign metadata to the Target class
Reflector.defineMetadata(key, {foo: 'bar'}, Target);

// The defineMetadata method validates the value type
// against the MetadataKey<MyData> type constraint
Reflector.defineMetadata(key, 'string', Target);
// TypeError: Argument of type 'string' is not assignable
// to parameter of type MyData.

// Return types from getMetadata and getOwnMetadata
// are automatically inferred from the key type
const result = Reflector.getMetadata(key, Target); // MyData

getDecoratorTargetType

The getDecoratorTargetType utility provides a simple way to identify where a decorator is applied in your code. When writing custom decorators, it's often necessary to implement different behavior based on whether the decorator is used on a class, method, property, or parameter. This utility returns an enum value indicating the exact location of the decorator.

import {getDecoratorTargetType} from '@e22m4u/ts-metadata';
import {DecoratorTargetType as DTT} from '@e22m4u/ts-metadata';

// Define a decorator
function myDecorator(
  target: object,
  propertyKey?: string,
  descriptorOrIndex?: PropertyDescriptor | number,
) {
  // Pass the decorator arguments to the utility
  const type = getDecoratorTargetType(
    target,
    propertyKey,
    descriptorOrIndex,
  );
  // Check the returned type to determine the decorator's location
  if (type === DTT.CONSTRUCTOR)
    console.log('@myDecorator is applied to a class');
  if (type === DTT.STATIC_METHOD)
    console.log('@myDecorator is applied to a static method');
  if (type === DTT.INSTANCE_METHOD)
    console.log('@myDecorator is applied to an instance method');
  if (type === DTT.STATIC_PROPERTY)
    console.log('@myDecorator is applied to a static property');
  if (type === DTT.INSTANCE_PROPERTY)
    console.log('@myDecorator is applied to an instance property');
  if (type === DTT.CONSTRUCTOR_PARAMETER)
    console.log('@myDecorator is applied to a constructor parameter');
  if (type === DTT.STATIC_METHOD_PARAMETER)
    console.log('@myDecorator is applied to a static method parameter');
  if (type === DTT.INSTANCE_METHOD_PARAMETER)
    console.log('@myDecorator is applied to an instance method parameter');
};

Tests

npm run test

License

MIT