From a55f210c7f01b2b26fe052f9e9cdc19c41f3d0be Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Tue, 10 Nov 2015 03:40:35 -0800
Subject: [PATCH] fix buildExternalHelpers whitelist not including underscored
 helpers - fixes #2940

---
 .../src/tools/build-external-helpers.js       | 20 ++++++++++++++++++-
 packages/babel-core/test/api.js               |  5 +++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/packages/babel-core/src/tools/build-external-helpers.js b/packages/babel-core/src/tools/build-external-helpers.js
index 9bc0221977..fd1e91940b 100644
--- a/packages/babel-core/src/tools/build-external-helpers.js
+++ b/packages/babel-core/src/tools/build-external-helpers.js
@@ -73,8 +73,26 @@ function buildVar(namespace, builder) {
 }
 
 function buildHelpers(body, namespace, whitelist) {
+  function shouldIgnore(name) {
+    if (!whitelist) {
+      return false;
+    }
+
+    // check for the raw name
+    if (whitelist.indexOf(name) >= 0) {
+      return false;
+    }
+
+    // typeof -> _typeof
+    if (name[0] === "_" && whitelist.indexOf(name.slice(1)) >= 0) {
+      return false;
+    }
+
+    return true;
+  }
+
   each(helpers.list, function (name) {
-    if (whitelist && whitelist.indexOf(name) === -1) return;
+    if (shouldIgnore(name)) return;
 
     let key = t.identifier(t.toIdentifier(name));
     body.push(t.expressionStatement(
diff --git a/packages/babel-core/test/api.js b/packages/babel-core/test/api.js
index e279d5f613..c9efbb2e59 100644
--- a/packages/babel-core/test/api.js
+++ b/packages/babel-core/test/api.js
@@ -387,5 +387,10 @@ suite("api", function () {
       assert.ok(script.indexOf("classCallCheck") === -1);
       assert.ok(script.indexOf("inherits") === -1);
     });
+
+    test("underscored", function () {
+      var script = buildExternalHelpers(["typeof"]);
+      assert.ok(script.indexOf("typeof") >= 0);
+    });
   });
 });