Why is this not a syntax / compile time error?

I accidentally typed : instead of = at line 2 of this piece of code.

Why is this valid Typescript? Shouldn't colon expect a known Type declaration on a property definition?

I am certain that there is somewhere in the TS spec that allows it, but what is the purpose?

export class CategoriesService {
  private _url : "http://localhost:3000/someurl"; // ???????????

  constructor(private http: Http) {}

  getData(): Observable<Category[]> {
    return this.http.get(this._url).map(res => res.json()); //fails here, as _url is undefined
  }
}