rauschma/scoped-private.md

## scoped-private.md

      
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              scoped-private.md
            
          
    Sketch: scoped private fields

So far, private fields are introduced as follows:
class MyClass {
  #privateField = 123;
}
I’d like to propose a small extension: scoped private fields.
Functional style (the proposal)

The following style is becoming popular in the JavaScript world. It benefits from scoped privacy:
private #data;

function createStringBuilder() {
  return {
    #data: '',
  };
}

function add(sb, str) {
  sb.#data += str;
}

function toString(sb) {
  return sb.#data;
}
New in this code:

The keyword private in the first line.
The ability to use private fields in object literals.

OOP style

As a slight downside, you now always need to use the keyword private:
class StringBuilder {
  private #data = ''; // keyword is required
  add(str) {
    this.#data += str;
  }
  toString() {
    return this.#data;
  }
}
On the upside, this gives you the freedom to widen the scope of privacy:
private #data;

class StringBuilder {
  #data = ''; // no keyword!
  add(str) {
    this.#data += str;
  }
}
function toString(stringBuilder) {
  return stringBuilder.#data;
}
Alternative syntax

Alternative syntax has been proposed:

Keyword private not needed for “normal” private fields.
Two keywords if you want to widen the scope of privacy: private and outer.

Example:
private #data;

class StringBuilder {
  outer #data = ''; // keyword is now required
  add(str) {
    this.#data += str;
  }
}
function toString(stringBuilder) {
  return stringBuilder.#data;
}
FP example:
private #data;

function createStringBuilder() {
  return {
    outer #data: '', // keyword is now required
  };
}

function add(sb, str) {
  sb.#data += str;
}

function toString(sb) {
  return sb.#data;
}