Fix: no-useless-computed-key false positive with __proto__

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Bug fix

Tell us about your environment

• ESLint Version: master
• Node Version: 7.3.0
• npm Version: 3.10.10

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

(none)

What did you do? Please include the actual source code causing the issue.

/* eslint no-useless-computed-key: error */

({ ['__proto__']: [] }) instanceof Array;

What did you expect to happen?

I expected no error to be reported, because using '__proto__' directly as the object key would set the value of Object.getPrototypeOf(theObject), and the expression's value would change from false to true.

What actually happened? Please include the actual, raw output from ESLint.

3:4  error  Unnecessarily computed property ['__proto__'] found  no-useless-computed-key

What changes did you make? (Give an overview)

This updates no-useless-computed-key to avoid reporting computed keys that have the literal value __proto__. __proto__ is a special key that sets the internal [[Prototype]] property of an object when used in an object literal.

Is there anything you'd like reviewers to focus on?

Nothing in particular

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @BYK, @mysticatea and @vitorbal to be potential reviewers.

[eslintbot]

LGTM

[ilyavolodin]

Are there any other cases we need to handle? Like constructor or prototype?

[not-an-aardvark]

I think __proto__ is the only case like this. (spec text)

[platinumazure]

This is in Appendix B of the spec. Do we have a position on supporting Appendix B features? Should we consider having a core or parser option for deciding whether Appendix B should be in effect?

[not-an-aardvark]

I don't think it's an official policy, but with no-useless-escape we take the conservative approach of only reporting an error if a problem exists regardless of whether the engine implements Annex B.