feat: prefer ES6 syntax

[curbengh]

based on eslint-config-airbnb.

[tomap]

I just ran your last change with --fix on hexojs/hexo master and got:

C:\dev\hexo\lib\extend\console.js
  55:13  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\extend\filter.js
  33:17  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\hexo\index.js
  435:16  error  Unexpected chained assignment  no-multi-assign
  437:17  error  Unexpected chained assignment  no-multi-assign
  439:16  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\hexo\locals.js
  17:13  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\hexo\post.js
  178:16  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\hexo\router.js
  17:17  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\plugins\console\config.js
  58:14  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\plugins\console\list\route.js
  28:16  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\plugins\helper\format.js
   6:22  error  Unexpected chained assignment  no-multi-assign
  12:21  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\plugins\helper\fragment_cache.js
  9:20  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\lib\theme\index.js
  36:17  error  Unexpected chained assignment  no-multi-assign
  69:17  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\test\scripts\box\box.js
  18:5  error  Combine this with the previous 'const' statement  one-var
  36:5  error  Combine this with the previous 'const' statement  one-var
  47:5  error  Combine this with the previous 'const' statement  one-var
  59:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\box\file.js
  37:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\console\config.js
  94:24  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\test\scripts\console\render.js
  16:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\extend\tag.js
   35:5  error  Combine this with the previous 'const' statement  one-var
   51:5  error  Combine this with the previous 'const' statement  one-var
   67:5  error  Combine this with the previous 'const' statement  one-var
   93:5  error  Combine this with the previous 'const' statement  one-var
  113:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\extend\tag_errors.js
   44:5  error  Combine this with the previous 'const' statement  one-var
   68:5  error  Combine this with the previous 'const' statement  one-var
  110:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\filters\i18n_locals.js
  102:5  error  Combine this with the previous 'const' statement  one-var
  121:5  error  Combine this with the previous 'const' statement  one-var
  140:5  error  Combine this with the previous 'const' statement  one-var
  161:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\filters\meta_generator.js
  21:5  error  Combine this with the previous 'const' statement  one-var
  31:5  error  Combine this with the previous 'const' statement  one-var
  42:5  error  Combine this with the previous 'const' statement  one-var
  47:5  error  Combine this with the previous 'const' statement  one-var
  58:5  error  Combine this with the previous 'const' statement  one-var
  63:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\css.js
  13:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\favicon_tag.js
  13:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\feed_tag.js
  13:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\gravatar.js
  12:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\image_tag.js
  13:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\js.js
  13:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\link_to.js
  13:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\list_archives.js
  15:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\list_categories.js
  17:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\list_posts.js
  14:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\list_tags.js
  17:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\mail_to.js
  15:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\open_graph.js
  226:5  error  Combine this with the previous 'const' statement  one-var
  309:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\paginator.js
  18:3  error  Combine this with the previous 'const' statement  one-var
  31:5  error  Combine this with the previous 'let' statement    one-var

C:\dev\hexo\test\scripts\helpers\partial.js
  28:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\helpers\tagcloud.js
  17:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\hexo\load_config.js
  52:24  error  Unexpected chained assignment  no-multi-assign

C:\dev\hexo\test\scripts\hexo\multi_config_path.js
  84:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\hexo\render.js
   14:3  error  Combine this with the previous 'const' statement  one-var
  277:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\hexo\router.js
  81:5  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\tags\blockquote.js
  10:3  error  Combine this with the previous 'const' statement  one-var

C:\dev\hexo\test\scripts\theme\view.js
  80:5  error  Combine this with the previous 'const' statement  one-var

and the following changes:

97 files changes :)

That is a lot of changes
Maybe we should introduce them progressively, or in two times: first the ones that are easily manually fixable, and then the others?

[curbengh]

I propose no-multi-assign PR and one-var PR in hexo. We can fix hexo before releasing this PR.

[curbengh]

Pending #25, if that is approved, this PR needs to remove some conflicting rules.

[curbengh]

Reverted one-var rule as it's not related to es6.

[curbengh]

suggest to hold off merging for now, let's refactor hexo first to see if this PR is actually viable.

[curbengh]

Pending hexojs/hexo#3717

[curbengh]

Refactored hexo:

• style: no-multi-assign hexo#3744
• style: no-var & prefer-const hexo#3745
• style: prefer-arrow-callback hexo#3746

I'm merging this since this PR is viable.