From 2c1cc26d9b06cf8e724faf6849059fe78d53f127 Mon Sep 17 00:00:00 2001
From: Joe Haddad <timer150@gmail.com>
Date: Mon, 1 Oct 2018 12:01:53 -0400
Subject: [PATCH] Fix IE 11 compatibility

---
 packages/react-error-overlay/src/utils/mapper.js | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/packages/react-error-overlay/src/utils/mapper.js b/packages/react-error-overlay/src/utils/mapper.js
index 95aa006a2c0..311dc3814b1 100644
--- a/packages/react-error-overlay/src/utils/mapper.js
+++ b/packages/react-error-overlay/src/utils/mapper.js
@@ -34,9 +34,10 @@ async function map(
   });
   await settle(
     files.map(async fileName => {
-      const fetchUrl = fileName.startsWith('webpack-internal:')
-        ? `/__get-internal-source?fileName=${encodeURIComponent(fileName)}`
-        : fileName;
+      const fetchUrl =
+        fileName.indexOf('webpack-internal:') === 0
+          ? `/__get-internal-source?fileName=${encodeURIComponent(fileName)}`
+          : fileName;
 
       const fileSource = await fetch(fetchUrl).then(r => r.text());
       const map = await getSourceMap(fileName, fileSource);