Why does angular's dependency injection need private or public to work?

Question

I have noticed that an angular class doesn't recognize an injected service if it hasn't been injected with explicit scope definition.

This following code doesn't work

constructor(router: Router) {}

but this one does.

constructor(private router: Router) {}

Can anyone explain why? I believe that if you don't explicitly write the property's scope definition, then it is public by default, like classes properties, but seems like is not the case in here.

Answer 1

Whatever you define in your constructor is taken as a parameter. It is a TypeScript convenience that you can attach an accessor to it. For instance:

constructor(private router: Router) {}

is a shorthand for ES6:

constructor(router) {
  this.router = router;
}

Dependency injection still works though, if you do the following:

constructor(router: Router) {
  // router only available in this scope
}

It's just that it's only available inside the constructor, and not in the class instance, because it's inside the constructor {} scope. A class field is defined in the class {} scope, and therefore accessible in the entire class