"private" and "public" in Angular component

Question

If I don’t add private before foo, loadBar, andtext, I believe they are public by default.

export class RandomComponent {
  @Input() foo: string;
  @Output() loadBar = new EventEmitter();
  text: string;
}

Is there any use case when they are public in the component?

For encapsulation/security reason, should I always add private for all of them like below?

export class RandomComponent {
  @Input() private foo: string;
  @Output() private loadBar = new EventEmitter();
  private text: string;
}

To be simple a Private function can only use in the component. It is not possible to access a private function from other component. Suppose in service, If a function declare as Private then it can't be access from any other component . — Kevin Jose, Mar 27 '20 at 05:17

drew moore · Accepted Answer · 2022-03-06T11:24:11.730

There's a lot to say in response to this question, these are the first thoughts that jumped to my mind:

First and foremost, keep in mind that private is only a compile-time construct - it cannot be enforced at runtime (see here and here for relevant discussion): Please dismiss any notions of private being useful in any way for security purposes. That's simply not what it's about.

It is about encapsulation, and when you have a field or method on your component that you want to encapsulate in it, making it clear that it shouldn't be accessed from anywhere else, then you should absolutely make it private: That's what private is for: It signals your intent that whatever you've put it on shouldn't be touched from outside the class.

The same goes for public: It too is a compile-time-only construct, so the fact that class members are public by default, while true, has exactly zero meaning at runtime. But when you have a member that you explicitly intend to expose to the outside world as a part of your class's API, you should absolutely make it public to signal this intent: That's what public is for.

This is all applicable to Typescript in general. In Angular specifically, there are definitely valid use-cases for having public members on component classes: For instance, when implementing the container/component (aka smart/dumb) pattern, with "dumb" children injecting "smart" parents via constructor injection, it's extremely important to communicate your intent about what members on the parent should and should not be touched by the children: Otherwise, don't be surprised when you catch those dumb kids fooling around in their parents' liquor cabinet.

So, my answer to your question:

should I always add private for all of them like below?

is an emphatic no. You shouldn't always add private because in doing so you defeat the purpose of the keyword, because it no longer signals any intent if you put it everywhere: You might as well not put it anywhere.

Thanks for the explanation. But maybe I get it wrong : I understand that most of the time properties and methods have to be private (= "for this component use only"). So the answer should be "YES by default, as long as the guy does not need to expose the property/method to the outside". No ? Why conclude your explanation by answering "no" (which sounds like a "never") ? — M'sieur Toph', Mar 21 '17 at 13:50
I red angular 2 tutorials, and I didn't figure out when I should use public. There are a lot of variants for component communication: https://angular.io/docs/ts/latest/cookbook/component-communication.html# Probably we should use public only for properties and methods defined via Input Output. But I'm not sure. — Ruslan Borovok, Mar 27 '17 at 11:55
I usually have to use 'public' when I declare a property in the constructor that is not used inside the class, but is used outside (i.e. inside a template or another component). — tuliomarchetto, Aug 20 '19 at 21:07
According to others answers, people generally recommend to make private methods as much as you can and then if you need make them public (check the reasons in the links). https://stackoverflow.com/questions/2676774/when-should-methods-be-made-private/2676798 https://stackoverflow.com/questions/2620699/why-private-methods-in-the-object-oriented https://www.codewithjason.com/purpose-private-methods-use/ But yeah, I have filings like Angular developing is a little bit about another stuff and no need to use private so wide. To be honest I would like to here more reasons and arguments ) — Experimenter, Oct 11 '21 at 13:16

score 23 · Answer 2 · answered Mar 19 '17 at 18:42

23

@drewmoore provides a good answer in that private/public boils down to intent. But there are a few more things to consider when using injected private values:

Both typescript (noUnusedLocals) and tslint (no-unused-variable) will report errors if you use @Import() private foo, or constructor(private foo) {}, and only use foo in your template
AOT wont work:

If we want to emit TypeScript as output of the AoT compilation process we must make sure we access only public fields in the templates of our components**

It is considered an anti-pattern

answered Mar 19 '17 at 18:42

Lucas

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re your first point: Those errors have nothing to do with a member being private or public, they simply indicate that one is not used. Your second two points would be valid if the question was about referencing private members *in templates*, but it isn't. [See here for a question that is](http://stackoverflow.com/questions/34574167/angular2-should-private-variables-be-accessible-in-the-template/34574732#34574732) – drew moore Apr 20 '17 at 04:01
@drewmoore, you are correct, and I understand that they indicate they are not used. However, public variables may be used externally so it would _never_ trigger that warning. As to the questions itself "Is there any use case when they are public in the component?", I believe my second and third points at least indicate _a use case when they are public_, specifically, when you want to use them in the template (as indicated in the quoted text). – Lucas Apr 20 '17 at 18:38
What about when you import a service into a component ? The service is basically available to any component that imports it, so its basically public by nature. should it be declared as public or private or does it not matter at all ? I understand that defining a variable in a component which shall only be used in that component should be private, it makes sense but i dont see any reason or difference to mark things as public or private when you can import them from anywhere into any component – Kravitz Oct 13 '22 at 08:38

"private" and "public" in Angular component

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