Parameter Decorators

function logPosition(target: any, propertyKey: string, parameterIndex: number) {
  console.log(parameterIndex);
}

class Cow {
  say(b: string, @logPosition c: boolean) {
    console.log(b);
  }
}

new Cow().say('hello', false); // outputs 1 (newline) hello

The above demonstrates decorating method parameters. Readers familiar with Angular can now imagine how Angular implemented their @Inject() system.