From 38d9b34b2720a2f26e9fa33365836e26d9a7a44e Mon Sep 17 00:00:00 2001
From: Joe Lencioni <joe.lencioni@airbnb.com>
Date: Tue, 2 Feb 2016 15:30:57 -0800
Subject: [PATCH 1/2] Avoid lexical declarations in case/default clauses

This commit adds guidance warning people to avoid lexical declarations
in case/default clauses and enables the corresponding ESLint rule.

  http://eslint.org/docs/rules/no-case-declarations.html

We didn't have a section on switch statements yet, so I thought about
starting one. It seemed like it would best fit between section 15
(Comparison Operators & Equality) and section 16 (Blocks), but I didn't
want to mess up all of the following numberings, since people probably
have references to them. I considered adding this near the end to
minimize this effect, but it really seemed to belong near these other
things. I landed on appending it to Section 15 (Comparison Operators &
Equality) and I think it sorta fits there since switch statements are a
related concept.
---
 README.md                                     | 41 +++++++++++++++++++
 .../rules/best-practices.js                   |  2 +
 2 files changed, 43 insertions(+)

diff --git a/README.md b/README.md
index b758421d..381d5c6b 100644
--- a/README.md
+++ b/README.md
@@ -1245,6 +1245,47 @@ Other Style Guides
     ```
 
   - [15.4](#15.4) <a name='15.4'></a> For more information see [Truth Equality and JavaScript](http://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/#more-2108) by Angus Croll.
+  - [15.5](#15.5) <a name='15.5'></a> Use braces to create blocks in `case` and `default` clauses that contain lexical declarations (e.g. `let`, `const`, `function`, and `class`).
+
+  > Why? Lexical declarations are visible in the entire `switch` block but only get initialized when assigned, which only happens when its `case` is reached. This causes problems when multiple `case` clauses attempt to define the same thing.
+
+  eslint rules: [`no-case-declarations`](http://eslint.org/docs/rules/no-case-declarations.html).
+
+    ```javascript
+    // bad
+    switch (foo) {
+      case 1:
+        let x = 1;
+        break;
+      case 2:
+        const y = 2;
+        break;
+      case 3:
+        function f() {}
+        break;
+      default:
+        class C {}
+    }
+
+    // good
+    switch (foo) {
+      case 1: {
+        let x = 1;
+        break;
+      }
+      case 2: {
+        const y = 2;
+        break;
+      }
+      case 3: {
+        function f() {}
+        break;
+      }
+      default: {
+        class C {}
+      }
+    }
+    ```
 
   - [15.5](#15.5) <a name='15.5'></a> Ternaries should not be nested and generally be single line expressions.
 
diff --git a/packages/eslint-config-airbnb/rules/best-practices.js b/packages/eslint-config-airbnb/rules/best-practices.js
index c2af1b78..d8b94e9b 100644
--- a/packages/eslint-config-airbnb/rules/best-practices.js
+++ b/packages/eslint-config-airbnb/rules/best-practices.js
@@ -24,6 +24,8 @@ module.exports = {
     'no-alert': 1,
     // disallow use of arguments.caller or arguments.callee
     'no-caller': 2,
+    // disallow lexical declarations in case/default clauses
+    'no-case-declaration': 2,
     // disallow division operators explicitly at beginning of regular expression
     'no-div-regex': 0,
     // disallow else after a return in an if

From 4d74fe1f5ab7e2f67e4ad837387debbf276e3929 Mon Sep 17 00:00:00 2001
From: Joe Lencioni <joe.lencioni@airbnb.com>
Date: Wed, 3 Feb 2016 12:23:51 -0800
Subject: [PATCH 2/2] Add example of a case clause that does not need a block

As @kesne and @ljharb pointed out, this section was unclear about
whether or not you should always include blocks for all case clauses. To
make things clearer, I am adding a case clause that does not need a
block to the good example.

We decided that we should only require blocks for case clauses that
actually need them because it matches the as-needed spirit of section
3.8 ("Only quote properties that are invalid identifiers"). Perhaps if
there was an as-needed but consistent setting for the ESLint rule, we
would consider revising this a little, but this seems good enough for
now.
---
 README.md | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/README.md b/README.md
index 381d5c6b..9920eba7 100644
--- a/README.md
+++ b/README.md
@@ -1281,6 +1281,9 @@ Other Style Guides
         function f() {}
         break;
       }
+      case 4:
+        bar();
+        break;
       default: {
         class C {}
       }