[eslint config][major][guide] Always require parens for arrow functio…

…n args

README.md

@@ -962,9 +962,12 @@ Other Style Guides
       `A string containing the ${nextNumber}.`;
     });
 
-    // good
+    // bad
     [1, 2, 3].map(number => `A string containing the ${number}.`);
 
+    // good
+    [1, 2, 3].map((number) => `A string containing the ${number}.`);
+
     // good
     [1, 2, 3].map((number) => {
       const nextNumber = number + 1;
@@ -1018,22 +1021,27 @@ Other Style Guides
     ```
 
   <a name="arrows--one-arg-parens"></a><a name="8.4"></a>
-  - [8.4](#arrows--one-arg-parens) If your function takes a single argument and doesn’t use braces, omit the parentheses. Otherwise, always include parentheses around arguments for clarity and consistency. Note: it is also acceptable to always use parentheses, in which case use the [“always” option](https://eslint.org/docs/rules/arrow-parens#always) for eslint. eslint: [`arrow-parens`](https://eslint.org/docs/rules/arrow-parens.html)
+  - [8.4](#arrows--one-arg-parens) Always include parentheses around arguments for clarity and consistency. eslint: [`arrow-parens`](https://eslint.org/docs/rules/arrow-parens.html)
 
-    > Why? Less visual clutter.
+    > Why? Minimizes diff churn when adding or removing arguments.
 
     ```javascript
     // bad
-    [1, 2, 3].map((x) => x * x);
-
-    // good
     [1, 2, 3].map(x => x * x);
 
     // good
+    [1, 2, 3].map((x) => x * x);
+
+    // bad
     [1, 2, 3].map(number => (
       `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
     ));
 
+    // good
+    [1, 2, 3].map((number) => (
+      `A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
+    ));
+
     // bad
     [1, 2, 3].map(x => {
       const y = x + 1;

packages/eslint-config-airbnb-base/rules/es6.js

@@ -21,9 +21,7 @@ module.exports = {
 
     // require parens in arrow function arguments
     // https://eslint.org/docs/rules/arrow-parens
-    'arrow-parens': ['error', 'as-needed', {
-      requireForBlockBody: true,
-    }],
+    'arrow-parens': ['error', 'always'],
 
     // require space before/after arrow function's arrow
     // https://eslint.org/docs/rules/arrow-spacing

packages/eslint-config-airbnb-base/test/test-base.js

@@ -26,7 +26,7 @@ Object.keys(files).forEach((
 
     // scan rules for react/ and fail if any exist
     const reactRuleIds = Object.keys(config.rules)
-      .filter(ruleId => ruleId.indexOf('react/') === 0);
+      .filter((ruleId) => ruleId.indexOf('react/') === 0);
     t.deepEquals(reactRuleIds, [], 'there are no react/ rules');
   });
 });

packages/eslint-config-airbnb/test/test-base.js

@@ -28,7 +28,7 @@ Object.keys(files).forEach((name) => {
 
     // scan rules for react/ and fail if any exist
     const reactRuleIds = Object.keys(config.rules)
-      .filter(ruleId => ruleId.indexOf('react/') === 0);
+      .filter((ruleId) => ruleId.indexOf('react/') === 0);
     t.deepEquals(reactRuleIds, [], 'there are no react/ rules');
   });
 });