Update JS rules and linter

I believe that most of us agree that airbnb's eslint configuration
should be our default linter for JavaScript. I know that, for some
projects, we may feel that tweaking it would be better, but I believe
that we should agree in a set of rules and follow them. We'll never find
something that everyone likes at first. I also think that we need a
default configuration because it will remove this conversation out of
the way, and let us focus on different things. I also think that we need
consistency in our projects, and we need to remove the amount of
decisions we make whenever we start a new one. This doesn't mean that
this topic is closed forever, we should be open to new ideas, but keep
in mind that the benefits of using one styleguide or the other are
probably very small. Most times, I believe, it's a matter of taste.

Author: gabrielpoca

linters/README.md

@@ -10,4 +10,6 @@ Files to configure the linters we use for each language and technology
 * Elixir
   * [Credo](/linters/elixir/.credo.exs)
 * SVG Files
-* * [SVGO](/linters/svg/.svgo.yml)
+  * [SVGO](/linters/svg/.svgo.yml)
+* JavaScript
+  * [ESLint](/linters/js/README.md)

linters/js/.eslintrc

@@ -0,0 +1,7 @@
+{
+  "extends": "airbnb",
+  "rules": {
+    "consistent-return": 0,
+    "react/no-multi-comp": [2, { "ignoreStateless": true }]
+  }
+}

linters/js/README.md

@@ -0,0 +1,45 @@
+ESLint Setup
+============
+
+1. Copy [.eslintrc](.eslintrc) to the root of the project.
+2. Install `eslint-config-airbnb`.
+
+
+    npm i --save-dev eslint-config-airbnb
+
+
+3. Install its peer dependencies following the [instructions in the repo](https://www.npmjs.com/package/eslint-config-airbnb)
+
+
+Overrides
+---------
+
+Airbnb's configuration is great, but some of their rules are too strict for our
+team. The following are the rules that we decided not to follow:
+
+1. `consistent-return`
+
+This rule requires that return statements either always or never specify values.
+See the following invalid example:
+
+```
+const fun = (props) => {
+  if (!props.value) return;
+  return props.value * 2;
+};
+```
+
+To make it valid, we would have to write something like this:
+
+```
+const fun = (props) => {
+  if (!props.value) return undefined;
+  return props.value * 2;
+};
+```
+
+2. `react/no-multi-comp`
+
+This rule prevents multiple component definition per file. This is a good
+thing, but, for stateless components, it's a strict and painful rule to
+follow.

style/js/README.md

@@ -6,7 +6,7 @@ Guides for programing JavaScript in style
 General
 ---
 * Use [ESLint](http://eslint.org/).
-* Always use `use strict`.
+* Always use ES6.
 
 Parens and Braces
 ---
@@ -157,14 +157,3 @@ if (!foo) {
   // foo is false
 }
 ```
-
-CoffeeScript
-------------
-
-* Initialize arrays using `[]`.
-* Initialize empty objects and hashes using `{}`.
-* Use `CamelCase` for classes, `lowerCamelCase` for variables and functions,
-  `SCREAMING_SNAKE_CASE` for constants, `_single_leading_underscore` for
-  private variables and functions.
-* Prefer `== ` or `===` to `is`.
-* Prefer `||` and `&&` to `or` and `and`.