chore #7070: warn properties that starts with $ or _

[guieiras-dev]

close #7070

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
As the docs says:

Properties that start with _ or $ will not be proxied on the Vue instance because they may conflict with Vue’s internal properties and API methods. You will have to access them as vm.$data._property.`

But, this is a little bit hidden during development. It would be better if a warn could be raised to show developer that the data property will not be proxied on the Vue instance.

[posva]

warn must be shown only in development mode (process.env.NODE_ENV !== production)

[guieiras-dev]

fixed

[guieiras-dev]

@posva How about this broken spec? And, as you said on the issue, isn't a little bit risky to proceed with this change as some users/plugins uses internally this kind of data?

[posva]

The broken test is using $foo and btw, is wrong because it should ready $foo from this.$data and not from this but it doesn't change anything having undefined or {} as the injected value.

I think I went too fast saying we could safely add a warning for this, sorry about that.
If users want to add properties to data that start with $ or _ they're free to do so, they only need to know that in order to access them, they have to use this.$data
I don't have any better idea that just drop this 🙁
@chrisvfritz @sdras @LinusBorg do you have any feedback on how to improve this in case you have seen people falling under this caveat?

[guieiras-dev]

@posva I was willing to contribute and I didn’t think about the consequences or how it could impact users. I’m sorry 😕.

Well, I think we can close this. Thanks for your review and the discussion.

[posva]

No worries, as I said, I should have paid more attention 🙂

[posva]

Let's close this for the moment