nicer let-scoping switch

lib/6to5/transformation/transformers/let-scoping.js

@@ -133,7 +133,7 @@ var checkFor = function (forParent, block) {
       } else if (t.isReturnStatement(node)) {
         has.hasReturn = true;
         replace = t.returnStatement(t.objectExpression([
-          t.property("init", t.identifier("v"), node.argument)
+          t.property("init", t.identifier("v"), node.argument || t.identifier("undefined"))
         ]));
       }
 
@@ -286,16 +286,13 @@ var run = function (forParent, block, parent, file, scope) {
 
     var retCheck;
 
+    var cases = [];
+
     if (has.hasReturn) {
       // typeof ret === "object"
       retCheck = util.template("let-scoping-return", {
         RETURN: ret,
       });
-
-      // there's no `break` or `continue` so we can just push in the `if`
-      if (!has.hasBreak && !has.hasContinue) {
-        body.push(retCheck);
-      }
     }
 
     if (has.hasBreak || has.hasContinue) {
@@ -303,8 +300,6 @@ var run = function (forParent, block, parent, file, scope) {
       // need to be able to access it
       var label = forParent.label = forParent.label || t.identifier(file.generateUid("loop", scope));
 
-      var cases = [];
-
       if (has.hasBreak) {
         cases.push(t.switchCase(t.literal("break"), [t.breakStatement(label)]));
       }
@@ -317,7 +312,17 @@ var run = function (forParent, block, parent, file, scope) {
         cases.push(t.switchCase(null, [retCheck]));
       }
 
-      body.push(t.switchStatement(ret, cases));
+      if (cases.length === 1) {
+        var single = cases[0];
+        body.push(t.ifStatement(
+          t.binaryExpression("===", ret, single.test),
+          single.consequent[0]
+        ));
+      } else {
+        body.push(t.switchStatement(ret, cases));
+      }
+    } else {
+      if (has.hasReturn) body.push(retCheck);
     }
   } else {
     body.push(t.expressionStatement(call));