diff --git a/lib/6to5/transformation/modules/_default.js b/lib/6to5/transformation/modules/_default.js
index 924ab1cb15..c1305df2a9 100644
--- a/lib/6to5/transformation/modules/_default.js
+++ b/lib/6to5/transformation/modules/_default.js
@@ -138,7 +138,7 @@ DefaultFormatter.prototype.exportDeclaration = function (node, nodes) {
         });
 
         var newDeclar = t.variableDeclaration(declar.kind, [decl]);
-        if (i == 0) t.inherits(newDeclar, declar);
+        if (i === 0) t.inherits(newDeclar, declar);
         nodes.push(newDeclar);
       }
     } else {
diff --git a/lib/6to5/transformation/transformers/_declarations.js b/lib/6to5/transformation/transformers/_declarations.js
index 3ceee04780..a5eb107e41 100644
--- a/lib/6to5/transformation/transformers/_declarations.js
+++ b/lib/6to5/transformation/transformers/_declarations.js
@@ -3,11 +3,12 @@ var t = require("../../types");
 exports.BlockStatement =
 exports.Program = function (node) {
   var kinds = {};
+  var kind;
 
   for (var i in node._declarations) {
     var declar = node._declarations[i];
 
-    var kind = declar.kind || "var";
+    kind = declar.kind || "var";
     var declarNode = t.variableDeclarator(declar.id, declar.init);
 
     if (!declar.init) {
@@ -18,7 +19,7 @@ exports.Program = function (node) {
     }
   }
 
-  for (var kind in kinds) {
+  for (kind in kinds) {
     node.body.unshift(t.variableDeclaration(kind, kinds[kind]));
   }
 };
diff --git a/lib/6to5/transformation/transformers/es6-default-parameters.js b/lib/6to5/transformation/transformers/es6-default-parameters.js
index 69fa62755d..93b0ef284c 100644
--- a/lib/6to5/transformation/transformers/es6-default-parameters.js
+++ b/lib/6to5/transformation/transformers/es6-default-parameters.js
@@ -1,5 +1,4 @@
 var traverse = require("../../traverse");
-var Scope    = require("../../traverse/scope");
 var util     = require("../../util");
 var t        = require("../../types");
 var _        = require("lodash");
@@ -13,9 +12,11 @@ exports.Function = function (node, parent, file, scope) {
   });
 
   var iife = false;
+  var i;
+  var def;
 
-  for (var i in node.defaults) {
-    var def = node.defaults[i];
+  for (i in node.defaults) {
+    def = node.defaults[i];
     if (!def) continue;
 
     var param = node.params[i];
@@ -48,8 +49,8 @@ exports.Function = function (node, parent, file, scope) {
 
   var body = [];
 
-  for (var i in node.defaults) {
-    var def = node.defaults[i];
+  for (i in node.defaults) {
+    def = node.defaults[i];
     if (!def) continue;
 
     body.push(util.template("if-undefined-set-to", {
diff --git a/lib/6to5/transformation/transformers/es6-destructuring.js b/lib/6to5/transformation/transformers/es6-destructuring.js
index 7353760ea0..bb563bcb34 100644
--- a/lib/6to5/transformation/transformers/es6-destructuring.js
+++ b/lib/6to5/transformation/transformers/es6-destructuring.js
@@ -220,10 +220,12 @@ exports.VariableDeclaration = function (node, parent, file, scope) {
   if (t.isForInStatement(parent) || t.isForOfStatement(parent)) return;
 
   var nodes = [];
+  var i;
+  var declar;
 
   var hasPattern = false;
-  for (var i in node.declarations) {
-    var declar = node.declarations[i];
+  for (i in node.declarations) {
+    declar = node.declarations[i];
     if (t.isPattern(declar.id)) {
       hasPattern = true;
       break;
@@ -231,8 +233,8 @@ exports.VariableDeclaration = function (node, parent, file, scope) {
   }
   if (!hasPattern) return;
 
-  for (var i in node.declarations) {
-    var declar = node.declarations[i];
+  for (i in node.declarations) {
+    declar = node.declarations[i];
 
     var patternId = declar.init;
     var pattern   = declar.id;
@@ -252,10 +254,9 @@ exports.VariableDeclaration = function (node, parent, file, scope) {
   }
 
   if (!t.isProgram(parent) && !t.isBlockStatement(parent)) {
-    var declar;
 
-    for (var i in nodes) {
-      var node = nodes[i];
+    for (i in nodes) {
+      node = nodes[i];
       declar = declar || t.variableDeclaration(node.kind, []);
 
       if (!t.isVariableDeclaration(node) && declar.kind !== node.kind) {
diff --git a/lib/6to5/transformation/transformers/es6-let-scoping.js b/lib/6to5/transformation/transformers/es6-let-scoping.js
index bfcd1e6e78..625159fce6 100644
--- a/lib/6to5/transformation/transformers/es6-let-scoping.js
+++ b/lib/6to5/transformation/transformers/es6-let-scoping.js
@@ -164,7 +164,6 @@ LetScoping.prototype.noClosure = function () {
 LetScoping.prototype.remap = function () {
   var replacements = this.info.duplicates;
   var block        = this.block;
-  var file         = this.file;
 
   if (!this.info.hasDuplicates) return;
 
@@ -232,8 +231,11 @@ LetScoping.prototype.getInfo = function () {
     }
   };
 
-  for (var i in opts.declarators) {
-    var declar = opts.declarators[i];
+  var i;
+  var declar;
+
+  for (i in opts.declarators) {
+    declar = opts.declarators[i];
     opts.declarators.push(declar);
 
     var keys = t.getIds(declar, true);
@@ -244,8 +246,8 @@ LetScoping.prototype.getInfo = function () {
     opts.keys = opts.keys.concat(keys);
   }
 
-  for (var i in block.body) {
-    var declar = block.body[i];
+  for (i in block.body) {
+    declar = block.body[i];
     if (!isLet(declar, block)) continue;
 
     _.each(t.getIds(declar, true), function (id, key) {
diff --git a/lib/6to5/transformation/transformers/es6-template-literals.js b/lib/6to5/transformation/transformers/es6-template-literals.js
index dd29d7a0f5..5a5f4ce445 100644
--- a/lib/6to5/transformation/transformers/es6-template-literals.js
+++ b/lib/6to5/transformation/transformers/es6-template-literals.js
@@ -29,8 +29,9 @@ exports.TaggedTemplateExpression = function (node, parent, file) {
 
 exports.TemplateLiteral = function (node) {
   var nodes = [];
+  var i;
 
-  for (var i in node.quasis) {
+  for (i in node.quasis) {
     var elem = node.quasis[i];
 
     nodes.push(t.literal(elem.value.cooked));
@@ -46,7 +47,7 @@ exports.TemplateLiteral = function (node) {
 
     var root = buildBinaryExpression(nodes.shift(), nodes.shift());
 
-    for (var i in nodes) {
+    for (i in nodes) {
       root = buildBinaryExpression(root, nodes[i]);
     }
 
diff --git a/lib/6to5/transformation/transformers/es7-object-spread.js b/lib/6to5/transformation/transformers/es7-object-spread.js
index 32cd606ef2..ec10d971c5 100644
--- a/lib/6to5/transformation/transformers/es7-object-spread.js
+++ b/lib/6to5/transformation/transformers/es7-object-spread.js
@@ -4,8 +4,10 @@ var t = require("../../types");
 
 exports.ObjectExpression = function (node) {
   var hasSpread = false;
-  for (var i in node.properties) {
-    var prop = node.properties[i];
+  var i;
+  var prop;
+  for (i in node.properties) {
+    prop = node.properties[i];
     if (t.isSpreadProperty(prop)) {
       hasSpread = true;
       break;
@@ -22,8 +24,8 @@ exports.ObjectExpression = function (node) {
     props = [];
   };
 
-  for (var i in node.properties) {
-    var prop = node.properties[i];
+  for (i in node.properties) {
+    prop = node.properties[i];
     if (t.isSpreadProperty(prop)) {
       push();
       args.push(prop.argument);
diff --git a/lib/6to5/transformation/transformers/react.js b/lib/6to5/transformation/transformers/react.js
index 51fb6f9b8e..3fe950a5ed 100644
--- a/lib/6to5/transformation/transformers/react.js
+++ b/lib/6to5/transformation/transformers/react.js
@@ -104,18 +104,19 @@ exports.XJSOpeningElement = {
 exports.XJSElement = {
   exit: function (node) {
     var callExpr = node.openingElement;
+    var i;
 
-    for (var i in node.children) {
+    for (i in node.children) {
       var child = node.children[i];
 
       if (t.isLiteral(child)) {
         var lines = child.value.split(/\r\n|\n|\r/);
 
-        for (var i in lines) {
+        for (i in lines) {
           var line = lines[i];
 
-          var isFirstLine = i == 0;
-          var isLastLine = i == lines.length - 1;
+          var isFirstLine = i === 0;
+          var isLastLine = i === lines.length - 1;
 
           // replace rendered whitespace tabs with spaces
           var trimmedLine = line.replace(/\t/g, ' ');
@@ -175,7 +176,7 @@ var addDisplayName = function (id, call) {
   var safe = true;
 
   for (var i in props) {
-    var prop = props[i];
+    prop = props[i];
     if (t.isIdentifier(prop.key, { name: "displayName" })) {
       safe = false;
       break;