From 87285ed984535a152a63e4ca6c047dbf8cae25af Mon Sep 17 00:00:00 2001
From: jessicaquynh <jessica.quynh.tran@gmail.com>
Date: Wed, 12 Oct 2016 01:14:45 -0400
Subject: [PATCH] tools: avoid let in for loops

This adds a new ESLint tool to check for let
declarations within the for, forIn, forOf expressions.

Fixes: https://github.com/nodejs/node/issues/9045
Ref: https://github.com/nodejs/node/pull/8873
PR-URL: https://github.com/nodejs/node/pull/9049
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
---
 lib/.eslintrc                                 |  1 +
 .../eslint-rules/no-let-in-for-declaration.js | 46 +++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 tools/eslint-rules/no-let-in-for-declaration.js

diff --git a/lib/.eslintrc b/lib/.eslintrc
index 8fb98b7c180458..d8e34f85b5759d 100644
--- a/lib/.eslintrc
+++ b/lib/.eslintrc
@@ -2,3 +2,4 @@ rules:
   # Custom rules in tools/eslint-rules
   require-buffer: 2
   buffer-constructor: 2
+  no-let-in-for-declaration: 2
diff --git a/tools/eslint-rules/no-let-in-for-declaration.js b/tools/eslint-rules/no-let-in-for-declaration.js
new file mode 100644
index 00000000000000..8b1a6783e0773d
--- /dev/null
+++ b/tools/eslint-rules/no-let-in-for-declaration.js
@@ -0,0 +1,46 @@
+/**
+ * @fileoverview Prohibit the use of `let` as the loop variable
+ *               in the initialization of for, and the left-hand
+ *               iterator in forIn and forOf loops.
+ *
+ * @author Jessica Quynh Tran
+ */
+
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+  create(context) {
+
+    const msg = 'Use of `let` as the loop variable in a for-loop is ' +
+                'not recommended. Please use `var` instead.';
+
+    /**
+     * Report function to test if the for-loop is declared using `let`.
+     */
+    function testForLoop(node) {
+      if (node.init && node.init.kind === 'let') {
+        context.report(node.init, msg);
+      }
+    }
+
+    /**
+     * Report function to test if the for-in or for-of loop
+     * is declared using `let`.
+     */
+    function testForInOfLoop(node) {
+      if (node.left && node.left.kind === 'let') {
+        context.report(node.left, msg);
+      }
+    }
+
+    return {
+      'ForStatement': testForLoop,
+      'ForInStatement': testForInOfLoop,
+      'ForOfStatement': testForInOfLoop
+    };
+  }
+};