Private State

Daniel Ehrenberg (littledan)

Syntax

class Foo {

#x;

#y = z;

foo() { #x++; return this.#x }

}

Semantics

• Only accessible from methods inside class body (“hard-private”)
• Internal slots mapping objects to field values
• Add field after super() returns, or at beginning in base class
• Intersperse initializer evaluation with field addition
• Throw on redundant property definition
• Scope of initializers as in property declarations

Interaction with other features

• Decorators -- reify a PrivateFieldIdentifier() object (explainer)
• A decorator can provide soft-private state
• Private methods -- a clean extension with the syntax you expect (explainer)
• Static private -- ditto (explainer)
• Friends -- may be lexically exposed via static block

let barGetter;�class Foo {� #bar;� static { barGetter = instance => instance.#bar }�}

Why inaccessible private state? (bug)

• More guarantees for high-fidelity library/polyfill authors
• Platform objects with internal slots do not have an “escape hatch” within ES
• Stronger invariants--precise control of friends
• Implementations and and security frameworks see uses (without eval)
• “Soft-private” already available through explicit symbol use

let x = Symbol("x");�class Foo {� [x] = 1;�}

• or TypeScript-style tool-based checking

​

Internal slot-based specification mechanics

• We have gone back and forth here… Now:
• Private State Identifier specification type
• [[PrivateStateValues]] is a List with pairs mapping Private State Identifiers to their values
• Difference from WeakMaps unobservable except for implied GC semantics

Open questions

• Should private state be accessible within eval?
• Is this proposal too unergonomic to be workable?
• Should static private, methods be included in this proposal?
• Should we put private before the declaration?
• General field issues: = token, scope for initializer

Current status

• New internal slot spec mechanics
• New explainers for interactions with other features
• Still missing some spec text for more early errors
• Stage 2?