[guide] No arrow function implicit return with side effects

Author: fvgs

README.md

@@ -369,7 +369,7 @@ Other Style Guides
     ```
 
   <a name="arrays--callback-return"></a><a name="4.5"></a>
-  - [4.5](#arrays--callback-return) Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement following [8.2](#arrows--implicit-return). eslint: [`array-callback-return`](http://eslint.org/docs/rules/array-callback-return)
+  - [4.5](#arrays--callback-return) Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following [8.2](#arrows--implicit-return). eslint: [`array-callback-return`](http://eslint.org/docs/rules/array-callback-return)
 
     ```javascript
     // good
@@ -915,7 +915,7 @@ Other Style Guides
     ```
 
   <a name="arrows--implicit-return"></a><a name="8.2"></a>
-  - [8.2](#arrows--implicit-return) If the function body consists of a single expression, omit the braces and use the implicit return. Otherwise, keep the braces and use a `return` statement. eslint: [`arrow-parens`](http://eslint.org/docs/rules/arrow-parens.html), [`arrow-body-style`](http://eslint.org/docs/rules/arrow-body-style.html) jscs:  [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam), [`requireShorthandArrowFunctions`](http://jscs.info/rule/requireShorthandArrowFunctions)
+  - [8.2](#arrows--implicit-return) If the function body consists of a single statement returning an [expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Expressions) without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a `return` statement. eslint: [`arrow-parens`](http://eslint.org/docs/rules/arrow-parens.html), [`arrow-body-style`](http://eslint.org/docs/rules/arrow-body-style.html) jscs:  [`disallowParenthesesAroundArrowParam`](http://jscs.info/rule/disallowParenthesesAroundArrowParam), [`requireShorthandArrowFunctions`](http://jscs.info/rule/requireShorthandArrowFunctions)
 
     > Why? Syntactic sugar. It reads well when multiple functions are chained together.
 
@@ -939,6 +939,24 @@ Other Style Guides
     [1, 2, 3].map((number, index) => ({
       [index]: number,
     }));
+
+    // No implicit return with side effects
+    function foo(callback) {
+      const val = callback();
+      if (val === true) {
+        // Do something if callback returns true
+      }
+    }
+
+    let bool = false;
+
+    // bad
+    foo(() => bool = true);
+
+    // good
+    foo(() => {
+      bool = true;
+    }
     ```
 
   <a name="arrows--paren-wrap"></a><a name="8.3"></a>
@@ -1354,7 +1372,9 @@ Other Style Guides
 
     // good
     let sum = 0;
-    numbers.forEach(num => sum += num);
+    numbers.forEach((num) => {
+      sum += num;
+    );
     sum === 15;
 
     // best (use the functional force)
@@ -1369,7 +1389,9 @@ Other Style Guides
 
     // good
     const increasedByOne = [];
-    numbers.forEach(num => increasedByOne.push(num + 1));
+    numbers.forEach((num) => {
+      increasedByOne.push(num + 1);
+    );
 
     // best (keeping it functional)
     const increasedByOne = numbers.map(num => num + 1);