Exempt variables prefixed with underscore from no-unused-vars rule

[valscion]

This is useful when e.g. using object spread operator to remove only a
certain field from the object.

For example, this can be used to ignore a property from React
component's this.props:

class OpinionatedDebugger extends React.Component {
  render() {
    const { justIgnoreMe: _unused, ...rest } = this.props;
    return <pre>{ JSON.stringify(rest) }</pre>;
  }
}

This PR was asked by Dan via Twitter when he replied to my tweet about ignoring unused variables with underscore prefix: https://twitter.com/dan_abramov/status/775764929894809602

Test plan:

Write this to any JS file in the project and observe no warnings are shown:

var obj = { a: 1, b: 2 };
var { a: _unused, ...rest } = obj;
console.log(rest);

Then try to change it to this and observe warnings are shown:

var obj = { a: 1, b: 2 };
var { a, ...rest } = obj;
console.log(rest);

## Same approach elsewhere

This approach is similar in nature to what rubocop, the de-facto linter in Ruby, specifies in its default config as well: https://github.com/bbatsov/ruby-style-guide#underscore-unused-vars

Prefix with _ unused block parameters and local variables. It's also acceptable to use just _(although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings.

[valscion]

Let me know if you'd like this change to also be documented somewhere :)

[gaearon]

Let's mention this somewhere in the usage guide.

[valscion]

I'm looking at the template/README.md file and I feel a bit unsure where this would fit in. This change is quite small so I had hoped there would've been an existing section for linting or object spreads but there isn't one.

That file itself is already quite large so that makes me even more wary about tucking in this little detail as I fear I'd too much irrelevant details.

What would a section covering this lint rule have? Should it contain more details about the linting in general, or just a one sentence note about this feature?

[vjeux]

(FWIW, we have the same lint rule on our PHP codebase at FB)

[valscion]

I'm quite stuck here, I don't know how to document this. Any help would be greatly appreciated 💕

[gaearon]

Ah, okay, let’s just sneak it in and maybe document it in some way when we release a package of our lint rules separately.

[gaearon]

Thank you!

[valscion]

Ah, okay, let’s just sneak it in and maybe document it in some way when we release a package of our lint rules separately.

That sounds cool! 👍