annotations.js

// Annotations can be put on:
// - functions / classes
// - function parameters

@Foo('bar')
function foo(@Baz() param1, @Baz() param2: Foo) {}

// GETS TRANSPILED INTO:
function foo() {}
// Using property getters, so that it works with circular references.
Object.defineProperty(foo, 'annotations', {get: function() {
  return [new Foo('bar')];
}});
Object.defineProperty(foo, "parameters", {get: function() {
  return [
    [new Baz],     // annotations on param1
    [Foo, new Baz] // annotations on param2
  ];
}});

di.js

// An example of Dependency Injection using the annotations in runtime.
class Engine {}

class Car {
  constructor(engine: Engine) {}
}

var injector = new Injector();
var car = injector.get(Car);

// This is roughly what happens inside the `injector.get(Car)` call.
// - do I have an instance of Car (check a cache/map)?
// - if yes, return
// - if not, instantiate:
//   - check Car.parameters annotations and instantiate the dependencies first:
//     - var dep1 = injector.get(Car.parameters[0][0]);
//   - return new Car(... already instantiated deps ...)