[reative-element] improve support for private reactive properties

[sorvell]

Should this be an RFC?

• This is not a substantial change

Which package is this a feature request for?

Lit Core (lit / lit-html / lit-element / reactive-element)

Description

This doesn't work today:

@state()
accessor #foo = 5;

This probably would only work with standard decorators, but there are a couple changes we'll need to make it possible.

Currently, requestUpdate depends on being able to lookup the current property value via this[propertyKey] and that isn't possible with a private field.

Alternatives and Workarounds

You can write a manual getter/setter today.

#_foo = 5;
get #foo() {
  return this.#_foo;
}
set #foo(value) {
  const old = this.#foo;
  if (value !== old) {
    this._#foo = value;
    this.requestUpdate('#foo', old);
  }
}

[sorvell]

It's probably not worth trying to take any action on this until Lit no longer needs to support TypeScript experimental decorators or the legacy JS static properties object. Neither of these can use private identifiers.