Rule proposal: `private-property-names`

[michalsnik]

According to the Style Guide we should warn about following keys:

• _foo
• $foo

and encourage to prefix private properties with $_ (e.g. $_foo).

This rule could live alongside no-reserved-keys, but we might as well keep only this rule, to not duplicate warnings.

[chrisvfritz]

I would hold off on this one for a little bit, as I'd like Evan to first approve this convention when he reviews the style guide.

[michalsnik]

@chrisvfritz do we know more now?

[chrisvfritz]

I don't think we've had a definitive ruling yet, but I'll ping @yyx990803. 🙂

[chrisvfritz]

Still no word from Evan, but let's move forward on this anyway. I think no-reserved-keys can still be useful in conjunction with this, since the $_ is only for private property names, like in plugins. There are probably two scenarios we can catch:

• Any property names starting with _. In this case, we can warn that for private property names, the $_pluginName_propertyName format should be used to avoid conflicts with Vue's private property names starting with _ and also conflicts with other plugins.
• Any property names starting with $_, but not following the $_pluginName_propertyName convention. So any property names starting with $_ should match this regex: /^\$_\w+_\w+$/. If it does not match, we can show the same warning about the format.

What do you think @michalsnik?