Typescript: Suggestion: a streamlined way to "expose" private variables publically

[JoshuaKGoldberg]

Right now, if you want to expose a class property with a public getter and a private setter, the standard a possibility is to use methods (since gets and sets can't have different visibility):

class Container<T> {
    private value: T;

    constructor(value: T) {
        this.value = value;
    }

    public getValue(): T {
        return this.value;
    }
}

// To access the value
new Container("foo").getValue(); // "foo"

The code works fine, but with a caveat: there's an extra function defined for every exposure. That really bloats the generated JavaScript.

_Edit: the same holds true for creating a getter. See the second and third comments._

I propose some sort of exposure functionality be added for these simple private values. They should still be publicly gettable but only privately settable.

A few options would be to mix public and private, create a new exposed keyword, or use a variation C#'s { get; private set; }

class Container {
    public private value: T;
    private exposed value: T;
    public value: T { get; private set; }
    // ...
}

new Container("foo").value; // "foo"

I slightly prefer the keyword approaches, since in C# there's a difference between properties and fields, and using the property syntax on TypeScript fields could be viewed as a syntactic conflation with the existing get/set properties.

Note that there are prior, mostly-declined issues vaguely related to this. https://github.com/Microsoft/TypeScript/issues/2845 is the closest by asking for different access modifiers for getter and setter. I think this is a better approach for simulating C#'s { get; private set; }

This is also different from the readonly and imm proposals. The goal here is to allow changing the property with less code. Arguably, allowing the language to prefer directly accessing properties actually _embraces_ the third and fourth design goals by removing the need to rely on methods to guarantee property privacy and creating more streamlined output JavaScript.

[aluanhaddad]

Right now, if you want to expose a class property with a public getter and a private setter, the standard is to use methods (since gets and sets can't have different visibility):

  class Container<T> {
      private value: T;

      constructor(value: T) {
          this.value = value;
      }

      public getValue(): T {
          return this.value;
      }
  }

The standard approach is to use a getter.

  class Container<T> {
      private _value: T;

      constructor(value: T) {
          this._value = value;
      }

      get value() {
          return this._value;
      }
  }

new Container("foo").value; // "foo"

[JoshuaKGoldberg]

@aluanhaddad yes, and using a getter results in significantly more generated JavaScript on lower ES targets. The point of this proposal is to _not_ have to do that.

Without a getter:

var Container = (function () {
    function Container(value) {
        this._value = value;
    }
    return Container;
}());

With a getter:

var Container = (function () {
    function Container(value) {
        this._value = value;
    }
    Object.defineProperty(Container.prototype, "value", {
        get: function () {
            return this._value;
        },
        enumerable: true,
        configurable: true
    });
    return Container;
}());

Creating a getter is a bit of extra work that I don't think should be necessary for this type of behavior.

Edit: But yes, it would have been better for me to write "a possibility" instead of "a standard".

[aluanhaddad]

@JoshuaKGoldberg Indeed there is more emitted code but performance in es5.1 environments should be quite good.

[mhegazy]

What you are missing are two modifiers, private_set and protected_set that can adorn a property. you can combine them with public and protected to get public|protected read, private|protected write properties. (assuming public private_get is not really a scenario that is worth supporting). in this context, readonly is just an alias for constructor_set.

[JoshuaKGoldberg]

Looks like the team is collectively not in favor of adding new keywords for this. That makes sense. How about combining existing privacy keywords?

• If there is one keyword, both gets and sets use it
• If there are two keywords, gets use the first and sets use the second
• The first keyword must be strictly more "open" than the second

class AllowedMemberPrivacySettings<T> {
    public fullyPublic: T;
    public protected publicGetsProtectedSets: T;
    public private publicGetsPrivateSets: T;
    protected fullyProtected: T;
    protected private protectedGetsPrivateSets: T;
    private fullyPrivate: T;
}

The public protected/public private combo could be particularly useful for MobX stores in MobX's strict mode:

import { action, observable } from "mobx";

class Store<T> {
    @observable
    public private data: T;

    @action
    public setData(data: T): void {
        this.data = data;
    }
}

[RyanCavanaugh]

This falls victim to the same problem as others - we don't distinguish the type of a property for reading vs the type/accessibility of it for writing, so the variance problems become rather hairy and you'd quickly find yourself doing unsound writes or reads.