@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<T>

Here is an example of using the MetadataKey<T> key to store metadata. A value will be validated against the given key before being stored. Furthermore, the type of metadata will be inferred automatically when it is retrieved.

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

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

// create the MetadataKey<T> of MyData
const key = new MetadataKey<MyData>();

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

// the `defineMetadata` method checks the type
// of the given value by the MetadataKey<MyData>
// key which requires MyData
Reflector.defineMetadata(key, 'string', Target);
// TypeError: Argument of type 'string' is not assignable
// to parameter of type MyData.

// return type of `getMetadata` and `getOwnMetadata`
// methods is inferred by the given key
const result = Reflector.getMetadata(key, Target); // MyData

getDecoratorTargetType

The getDecoratorTargetType utility determines where a decorator is applied.

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

// declare a decorator
function myDecorator(
  target: object,
  propertyKey?: string,
  descriptorOrIndex?: PropertyDescriptor | number,
) {
  // pass all decorator arguments
  // to the utility function
  const type = getDecoratorTargetType(
    target,
    propertyKey,
    descriptorOrIndex,
  );
  // compare the returned value
  // to determine where it's applied
  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