Angular2 - should private variables be accessible in the template?

Question

If a variable is declared private on a component class, should I be able to access it in the template of that component?

@Component({
  selector: 'my-app',
  template: `
    <div>
      <h2>{{title}}</h2>
      <h2>Hello {{userName}}</h2> // I am getting this name
    </div>
  `,
})
export class App {
  public title = 'Angular 2';
  private userName = "Test Name"; //declared as private
}

Answer 1

UPD

Since Angular 14, it is possible to bind protected components members in the template. This should partially address the concern of exposing internal state (which should only be accessible to the template) as the component's public API.

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No, you shouldn't be using private variables in your templates.

While I like the drewmoore's answer and see perfect conceptual logic in it, implementationwise it's wrong. Templates do not exist within component classes, but outside of them. Take a look at this repo for the proof.

The only reason why it works is because TypeScript's private keyword doesn't really make member private. Just-in-Time compilation happens in a browser at runtime and JS doesn't have any concept of private members (yet?). Credit goes to Sander Elias for putting me on the right track.

With ngc and Ahead-of-Time compilation, you'll get errors if you try accessing private members of the component from template. Clone demonstration repo, change MyComponent members' visibility to private and you will get compilation errors, when running ngc. Here is also answer specific for Ahead-of-Time compilation.

Answer 2

Edit: This answer is now incorrect. There was no official guidance on the topic when I posted it, but as explained in @Yaroslov's (excellent, and correct) answer, this is no longer the case: Codelizer now warns and AoT compilation will fail on references to private variables in component templates. That said, on a conceptual level everything here remains valid, so I'll leave this answer up as it seems to have been helpful.

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Yes, this is expected.

Keep in mind that private and other access modifiers are Typescript constructs, whereas Component/controller/template are angular constructs that Typescript knows nothing about. Access modifiers control visibility between classes: Making a field private prevents other classes from having access to it, but templates and controllers are things that exist within classes.

That's not technically true, but (in lieu of understanding how classes relate to decorators and their metadata), it might be helpful to think of it this way, because the important thing (IMHO) is to shift from thinking about template and controller as separate entities into thinking of them as unified parts of the Component construct - this is one of the major aspects of the ng2 mental model.

Thinking about it that way, obviously we expect private variables on a component class to be visible in its template, for the same reason we expect them to be visible in the private methods on that class.

Answer 3

Even though the code example indicates the question is about TypeScript it doesn't have the typescript tag. Angular2 is also available for Dart and this is a notable difference to Dart.

In Dart the template can't reference private variables of the component class, because Dart in contrast to TypeScript effectively prevents access of private members from outside.

I still back @drewmoores suggestion to think about component and it's template as one unit though.

Update (TS) It seems with offline compilation access to private properties will become more limited in Angular2 TS as well https://github.com/angular/angular/issues/11422

Answer 4

A workaround could be using private variables in ts file and using getters.

private _userName = "Test Name";
get userName() {
  return this._userName;
}

This is a good approach because the ts file and the html remains independent. Even if you change the _userName variable name in ts file, you dont have to make any change in the template file.