Backwards compatibility issue with private # sigil.

[JorisBlanken]

Consider the following class definition:

class Example {

    constructor(x){
        this['#x'] = x;
    }

}

Currently this value can be accessed from outside the instance:

let example = new Example(5);
console.log(example['#x']); // Prints 5

If the # sigil syntax follows current member referencing standards, such that object members can both be accessed directly using the dot operator (if a valid identifier) and using a string literal, then currently accessible members can become inaccessible when this proposal is implemented.

[jkrems]

Afaik the proposal doesn't affect this syntax. The code above will still work after this proposal is implemented. this.#x and this['#x'] are independent values.

[ljharb]

Indeed, this isn't an issue precisely because private fields can only use dot access.

[bakkot]

See faq.

[MichaelTheriot]

This may not be an issue but it is clear that intuition led the poster to suspect this should work due to # syntax, which is an issue?

[ljharb]

I don’t see it as an issue, i see it as something someone will have to learn once.

[MichaelTheriot]

I respect that is consensus of TC39. I think developer consensus would be valuable too.

[littledan]

I don't really see this as a compatibility issue, as it's not about how existing code works. Unfortunately, this syntax is sort of at odds with privacy itself; we have considered other syntaxes like this.#['x'], but ultimately decided it was unnecessary (you can just use an object or map in a private field for the same functionality). At this point, implementations are moving forward with the proposal in this repository; see the README for details.