javascript - What does the @ (at sign) mean in the latest TypeScript (presumably v1.5) example?

Question

There is a very interesting picture was posted in the official TypeScript blog.

I wonder what the @ (at sign) symbol is doing there since (as far as I know) it cannot be used in JavaScript identifiers.

Answer 1

The big news this week is the merging of AtScript and TypeScript.

The following example from the AtScript documentation...

@Component()
class MyApp {
  server:Server;
  @Bind('name') name:string;
  @Event('foo') fooFn:Function;
  @Inject()
  constructor(@parent server:Server) {}
  greet():string {}
}

Compiles into the following JavaScript...

function MyApp() {}
MyApp.properties = {
  'server': { is: Server },
  'name': { is:string,
            annotate: [new Bind('name']},
  'fooFn': { is:Function,
             annotate:[new Event('foo')]}
}
MyApp.annotate = [
  new Component(),
  new Inject()
];
MyApp.parameters = [
  {is:Server, annotate:[parent]}
];
MyApp.prototype.greet = function() {}
MyApp.prototype.greet.returns = string;

AtScript was planned to be a layer on top of TypeScript (i.e. a super-set of a super-set) - but now the two projects are one.

Annotations are described thus:

• AtScript annotation syntax is just a shorthand of placing the same information in ES5. It would be reasonable for an ES5 developer to write these annotations manually. A helper library could even be provided.

• Annotations can only be placed on functions.

• An annotation placed on a class is an annotation placed on the class’ constructor function.

• An annotation placed on a field gets moved to the constructor function.

• All annotations are translated as properties on a function.