tools: add recommended linting rules

[Trott]

This change adds ESLint rules that meet two criteria:

• recommended by ESLint
• require no code changes in the current code base

These rules are:

• no-func-assign: Disallow overwriting a function that was written
  as a function declaration.
• no-negated-in-lhs: Disallow negated left operand of in operator.
  It prevents if(!a in b) when if(!(a in b)) is intended.
• no-obj-calls: Disallow global object function calls. It prevents
  errors like JSON() and Math().
• use-isnan: Prevents errors like if (foo == NaN)
• no-octal: Disallows confusing constructs like var num = 071;
• no-delete-var: Delete works on properties, not variables. Disallows
  delete foo.

This may make configuration much more concise if we turn on recommended rules and then disable ones that we aren't using.

And it may prevent some errors from making it into the PR stage and requiring a reviewer to correct.

[Trott]

CI: https://ci.nodejs.org/job/node-test-pull-request/1632/

[thefourtheye]

LGTM but regarding isNaN I remember @bnoordhuis proposing a patch to use != as it has performance impact.

[Trott]

@thefourtheye I may be misunderstanding, but I don't think that would work:

var foo = NaN;
console.log(foo != NaN); // oops, logs "true"

Anything (including NaN itself) compared to NaN returns false, so a comparison with a hardcoded NaN value is usually (always?) a coding error.

[thefourtheye]

foo !== foo will work and even spec says it.

[silverwind]

foo !== NaN could very well be optimized to true 😉

[Trott]

@thefourtheye wrote:

foo !== foo will work and even spec says it.

Ah! I see! Well, fortunately, the linting rule won't flag that. So everybody wins!

[targos]

@thefourtheyefoo !== foo is not affected by this rule.

[targos]

LGTM. Can you also add a line about no-func-assign in the commit message?

[thefourtheye]

@thefourtheye foo !== foo is not affected by this rule.

@targos I understand that. I was actually referring to the performance drop mentioned in #1925

[Trott]

@targos Whoops, missed that one! I've updated the commit message.

[Trott]

Landed in ffbc05a