private-fields-on-plain-objects.ts

// Utilities for creating private fields on plain objects. As is noted in the
// proposal (tc39/proposal-class-fields), private fields are really syntax sugar
// for WeakMaps, so this is allowed.

const ReturnInput = function <T>(object: T): T {
  return object
} as unknown as new <T>(object: T) => T

function createPrivateField<Target extends object, Field>() {
  // @ts-ignore
  class PrivateField extends ReturnInput<Target> {
    #field!: Field

    constructor(object: Target, field: Field) {
      super(object)
      this.#field = field
    }

    static get(object: Target): Field {
      return (object as any).#field
    }

    static set(object: Target, value: Field): Target {
      if (!(#field in object)) {
        new PrivateField(object, value)
      }

      ;(object as any).#field = value
      return object
    }

    static has(object: object): object is Target {
      return #field in object
    }
  }

  return {
    get(object: Target): Field {
      return PrivateField.get(object)
    },
    set(object: Target, value: Field): Target {
      return PrivateField.set(object, value)
    },
    has(object: Target) {
      return PrivateField.has(object)
    },
  }
}

const name = createPrivateField<{}, string>()

const Zachary = {}

name.set(Zachary, "Zachary")

name.get(Zachary)

export {}