From 4844882f5e5d79facbcb254e9467c712f45e793a Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Sun, 18 Jan 2015 21:37:37 +1100
Subject: [PATCH] break let scoping transformer if there are no block scoped
 references

---
 .../transformation/transformers/es6-let-scoping.js     | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/lib/6to5/transformation/transformers/es6-let-scoping.js b/lib/6to5/transformation/transformers/es6-let-scoping.js
index ebff0352b3..5a9c829b31 100644
--- a/lib/6to5/transformation/transformers/es6-let-scoping.js
+++ b/lib/6to5/transformation/transformers/es6-let-scoping.js
@@ -83,6 +83,7 @@ function LetScoping(loopParent, block, parent, scope, file) {
   this.file       = file;
 
   this.outsideLetReferences = {};
+  this.hasLetReferences     = false;
   this.letReferences        = {};
   this.body                 = [];
 }
@@ -99,9 +100,12 @@ LetScoping.prototype.run = function () {
   var needsClosure = this.getLetReferences();
   this.checkTDZ();
 
-  // this is a block within a `Function` so we can safely leave it be
+  // this is a block within a `Function/Program` so we can safely leave it be
   if (t.isFunction(this.parent) || t.isProgram(this.block)) return;
 
+  // we can skip everything
+  if (!this.hasLetReferences) return;
+
   if (needsClosure) {
     this.needsClosure();
   } else {
@@ -284,8 +288,12 @@ LetScoping.prototype.getLetReferences = function () {
     declar = declarators[i];
     var keys = t.getIds(declar, true);
     _.extend(this.letReferences, keys);
+    this.hasLetReferences = true;
   }
 
+  // no let references so we can just quit
+  if (!this.hasLetReferences) return;
+
   // set let references to plain var references
   standardiseLets(declarators);