privileged

Modern privileged method pattern with inheritable protected state.

Installation

pnpm add privileged

Problem

JavaScript prototype methods cannot access constructor-scoped variables:

class Counter {
  constructor(initial) {
     // initial is private, but inaccessible from methods
  }

  increment() {
    initial++  // ReferenceError: initial is not defined
  }

  get_count() {
    return initial  // ReferenceError: initial is not defined
  }
}

Solution

create_protected creates a protected bridge between constructor scope and prototype methods, enabling privileged access:

import create_protected from 'privileged'

const { protect, access: _ } = create_protected()

class Counter {
  constructor(initial) {
    
    protect(this, {

      count: { get: _=> initial, set: v=> initial = v }
    })
  }

  increment() {
    _(this).count++
  }

  get_count() {
    return _(this).count
  }
}

const counter = new Counter(0)
counter.increment()
counter.get_count()  // 1

Tip: Use access: _ for a more concise syntax, similar to class #private:

const { protect, access: _ } = create_protected()

// In prototype methods:
class Counter {
  get_count() {
    return _(this).count  // vs this.#count
  }
}

Usage

• Use this to access public members — privileged and prototype members

• Use _(this) to access protected methods, getter/setter and non-callable objects

• Use protect() to define protected methods, getter/setter and non-callable objects — privileged methods with protected visibility. No primitive values!

• Use privilege() to define privileged methods, getter/setter

Basic Class Pattern

// observable.js
import create_protected, { privilege } from 'privileged'

// export protected state for Child to inherit
export const { protect, access: _ } = create_protected()

export default class Observable {

  constructor() {

    // --- Private state: Private variables, intermediate values    
    const events = {} 
  
    // Instance API: protected members
    protect(this, { 

      event(event_name) {
        if (!events[event_name]) events[event_name] = []
        return events[event_name]
      },

      emit(event_name) {
        const lsnrs = _(this).event(event_name)
        for (let i = lsnrs.length - 1; i >= 0; i--)
          lsnrs[i]()
      }
    })

    // Instance API: privileged members
    privilege(this, {
      events_length: { get: _=> events.length }
    })

    // Initialization
  }

  // Prototype methods
  on(event_name, cb) {
    _(this).event(event_name).push(cb)
    return _=> this.off(event_name, cb)
  }

  off(event_name, cb) {
    const lsnrs = _(this).event(event_name)
    for (let i = 0, l = lsnrs.length; i < l; i++) {
      if (lsnrs[i] === cb) {
        lsnrs.splice(i, 1)
        break
      }
    }
  }
}

// todostore.js
import Observable, { protect, _ } from './observable.js'
import { privilege } from 'privileged'

export default class TodoStore extends Observable {

  constructor() {
    super()

    // Protected members
    protect(this, {
      items: []
    })

    // Privileged members
    // Privileged method can access private variables directly
    privilege(this, {

      // privileged method is bound to `this`, so you can use `onChange` 
      // as a function. 
      onChange(cb) {
        return this.on('change', cb)
      },

      todos() {
        return _(this).items
      }
    })
  }

  // prototype methods
  add(title) {
    const { items, emit } = _(this)
    items.push({ id: Date.now(), title, completed: false })
    emit('change')
  }
}

API

protect(instance, protected_state)

Defines protected members for an instance.

Parameters:

• instance - The instance to define protected members on (usually this)
• protected_state - Protected member definitions

Members allowed:

Protected methods: Method shorthand.

calc() {...} bound to this automatically

Getter/Setter in Property descriptors:

fullname: { get: _=> {...}, set: v=> {...} }

Non-callable objects: objects, arrays, etc.

let count = 0

protect(this, {
  items: [], // non-callable object
  count: {get: _=> count, set: v=> count = v}, // descriptor
  compute() {} // protected method
})

Members NOT allowed

Note: object literal getter/setter is not allowed

protect(this, {
  get count() { return 0} // throw Error
})

Primitive values

Since protected state members are NOT allowed to be re-assigned, use getter/setter to manage primitive values as follows:

let count = 3

protect(this, {
  // getter/setter for primitive values
  count: { get: _=> count, set: v=> count = v }
})

access(instance)

Returns the protected members.

_(this).count      // Read
_(this).count = 5  // Write
_(this).compute()  // Call method
_(this).items      // Access reference

const { count, compute, items, cache } = _(this) // use destructuring to get all

privilege(instance, privileged_state)

Defines privileged members for an instance. Functions are auto-bound to the instance.

Parameters:

• instance - The instance to define privileged members on (usually this)
• privileged_state - Privileged member definitions

Works like Object.assign(instance, privileged_state) but auto-binds this.

import { privilege } from 'privileged'

class Counter {
  constructor() {
    
    privilege(this, {

      count: 0, // primitive values are allowed in privilege, because `this` is `public` visibility

      increment() { this.count++ },
      
      get_count() { return this.count }
    })
  }
}

const counter = new Counter()
const { increment, get_count } = counter  // this stays bound!
increment()
increment()
get_count()  // 2

Members allowed

Same as protect(), but also allows primitive values

Primitive values allowed in privilege(), but also allowed to be re-assigned

privilege(this, {
  count: 0 
})

Create only readable property in privileged getter descriptor

let isReady = false
privilege(this, {
  isReady: { get: _=> isReady }
})

Member NOT allowed

Note: object literal getter/setter is not allowed

privilege(this, {
  get count() { return 0} // throw Error
})

Reflect change

Define privileged methods in constructor, prototype methods on prototype:

import create_protected, { privilege } from 'privileged'

const { protect, access: _ } = create_protected()

class BankAccount {
  constructor(initial = 100) {
    let balance = initial

    protect(this, {

      balance: { get: _=> balance }
    })

    // Privileged method - unique per instance
    privilege(this, {

      deposit(amount) {
        if (amount > 0) balance += amount // update the private variable directly
      }
    })
  }

  // Class method - on prototype
  show() {
    return _(this).balance
  }
}

const acct = new BankAccount()

console.log(acct.show()) // 100
acct.deposit(100)
console.log(acct.show()) // 200: reflect the change

Sharing Reference Types

Reference types are stored by reference, not copied:

const { protect, access: _ } = create_protected()

class Cache {
  constructor() {
    const store = new Map()

    protect(this, {
      store  // Stored by reference, not copied
    })
  }

  set(key, value) {
    _(this).store.set(key, value)
  }

  get(key) {
    return _(this).store.get(key)
  }
}

Module Isolation

Each create_protected() call creates isolated access:

// module-a.js
import create_protected from 'privileged'
export const { protect, access } = create_protected()

// module-b.js
import create_protected from 'privileged'
export const { protect, access } = create_protected()

// These cannot access each other's protected state

Errors

Primitive Values in protect()

Primitives must use property descriptors in protect() but allowed in privilege:

protect(this, {
  count: 0  // Error: "count" is primitive, use { get, set } descriptor instead
})

// Correct:
protect(this, {
  count: { get: _=> count, set: v=> count = v }
})

Object Literal Getter/Setter is NOT allowed

Object literal getter/setter requires return and multiple lines. Descriptor pattern is more concise:

protect(this, {

  get config() { 
    return config 
  },   // Error: getter syntax not supported
  set config(v) { 
    config = v 
  }    // Error: setter syntax not supported
})

// Correct:
let config = {}
protect(this, {
  config: { get: _=> config, set: v=> config = v }
})

Accessing Protected State from Other Modules

Using mismatched protect/access pairs throws:

// module-a.js
import create_protected from 'privileged'
const { protect } = create_protected()

export default class Foo {
  constructor() {

    let value = 1
    
    protect(this, { 

      value: { get: _=> value } 
    })
  }
}

// module-b.js
import create_protected from 'privileged'
import Foo from './module-a.js'

const { access: _ } = create_protected()  // different pair!

class Bar extends Foo {
  get() { return _(this).value }   // Error: Illegal Access Exception
}

Inheritance

privileged supports inheritance. The behavior depends on whether the Child uses the same protect/access as the Parent.

Controlling Visibility (Java Analogy)

Visibility is controlled by export and import decisions:

Parent exports protect/access?	Child imports them?	Result
No	-	Private - Child cannot access Parent's protected state
Yes	No (creates own)	Isolated - Parent and Child have separate protected states
Yes	Yes	Shared - Parent and Child share the same protected state

This differs from Java where protected is always visible to subclasses. Here, both sides must agree to share.

1. Shared Protected State

If the Parent exports its protect and access functions, and the Child imports/uses them, they share the same protected state.

• Behavior: Child can read/write Parent's protected state, and can also add its own. Child can override Parent's protected members if using the same name.
• Access: _(this) returns all members (Parent's + Child's).

// Observable.js
import create_protected from 'privileged'

export const { protect, access: _ } = create_protected()

export default class Observable {
  constructor() {

    const listeners = {}

    protect(this, {

      emit(event_name) {
        for (const listener of (listeners[event_name] || [])) {
          listener()
        }
      }
    })

  }
}

// Admin.js
import Observable, { protect, _ } from './Observable.js'

export class Admin extends Observable {
  constructor() {
    super()

    protect(this, {
      log(msg) { console.log(`[Admin]: ${msg}`) }
    })
  }

  triggerCustomEvent() {

    const { emit, log } = _(this)

    emit('custom_event')    // access protected member in Observable
    log('Broadcasting...')  // access protected member in Admin
  }
}

2. Override Parent's Protected Members

When sharing protected state, the Child can override Parent's protected members by using the same name.

// animal.js
import create_protected from 'privileged'

export const { protect, access: _ } = create_protected()

export default class Animal {
  constructor(name) {

    protect(this, {
      speak() {
        return `${name} makes a sound`
      }
    })
  }

  talk() {
    return _(this).speak()
  }
}

// dog.js
import Animal, { protect } from './animal.js'

export class Dog extends Animal {
  constructor(name) {
    super(name)

    // Override parent's speak()
    protect(this, {

      speak() {
        return `${name} barks: Woof!`
      }
    })
  }
}

const dog = new Dog('Buddy')
console.log(dog.talk())  // "Buddy barks: Woof!"

3. Isolated Protected State

If the Child creates its own create_protected(), the protected states remain strictly isolated in layers.

• Behavior: Parent and Child maintain separate protected state.
• Access:
  • Parent's _(this) only sees Parent's protected members.
  • Child's _(this) only sees Child's protected members.
  • Cross-access throws Property Access Exception.

// Component.js (Parent)
import create_protected from 'privileged'

const { protect, access: _ } = create_protected()

export class Component {
  constructor(id) {

    protect(this, {
      id: { get: _=> id }
    })
  }

  get_id() { return _(this).id }
}

// Widget.js (Child)
import create_protected from 'privileged'
import { Component } from './Component.js'

// Child creates its OWN protect/access
const { protect, access: _ } = create_protected()

export class Widget extends Component {
  constructor(id, width) {
    super(id)
    protect(this, {

      width: { get: _=> width }
    })
  }

  render() {
    // _(this).id  <-- Error: Property Access Exception (wrong pair)

    // Correct: use public API for parent's protected state
    const id = this.get_id()
    const { width } = _(this)
    return `<Widget id="${id}" width="${width}" />`
  }
}

License

MIT