tools: avoid let in for loops

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

Fixes: #9045
Ref: #8873
PR-URL: #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

@@ -2,3 +2,4 @@ rules:
  # Custom rules in tools/eslint-rules
  require-buffer: 2
  buffer-constructor: 2
+ no-let-in-for-declaration: 2

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
+ };
+ }
+};