Decorators

• Decorators are functions when placed in a decorator context will be called with special contextual parameters.
• If you want to create decorators that accept parameters during "decoration", use a factory.

Class decorators

Called with the constructor function.

function sealed(ctor: Function) {
  Object.seal(ctor);
  Object.seal(ctor.prototype);
}

@sealed
class Greeter {
  greeting: string;
  // ...
}

Method and accessor decorators

Parameters:

1. constructor (static member) or prototype (instance member)
2. name
3. property descriptor

Only one accessor (either the getter or the setter) may be decorated, since the very same decorator would operate on the same member.

function friendly (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
  let oldFn = descriptor.value;
  descriptor.value = function () { return oldFn.call(this) + ', my friend'; };
}

class GreeterBot {
  greeting: string;
  constructor (message: string) {
    this.greeting = message;
  }

  @friendly
  public greet () {
    return 'Hello, ' + this.greeting;
  }
}

let rosey = new GreeterBot('Judy');
rosey.greet(); // prints "Hello Judy, my friend"

Property and parameter decorators

🚀 Since es6 class properties are not enumerable, manipulating their descriptors is not exactly trivial. The official examples themselves are relying heavily on the reflect-metadata package (future metadata API polyfill) for easier reflection.