diff --git a/node-atob.js b/node-atob.js
index 9e2c2b1..88bf3a3 100644
--- a/node-atob.js
+++ b/node-atob.js
@@ -1,11 +1,24 @@
 "use strict";
 
-const b64Alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
+// var b64Alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
+
+function toCharCode(ch) {
+  return ch.charCodeAt(0);
+}
+
+function isValidB64Char(charCode) {
+  return charCode >= toCharCode('A') && charCode <= toCharCode('Z')
+      || charCode >= toCharCode('a') && charCode <= toCharCode('z')
+      || charCode >= toCharCode('0') && charCode <= toCharCode('9')
+      || charCode === toCharCode('+')
+      || charCode === toCharCode('/')
+      || charCode === toCharCode('=');
+}
 
 function atob(str) {
-  for (const ch of str)
-    if (!b64Alphabet.includes(ch))
-      throw new Error(`Invalid character '${ch}' in base64 String`);
+  for (var idx in str)
+    if (!isValidB64Char(str.charCodeAt(idx)))
+      throw new Error('Invalid character ' + str.charAt(idx) + ' in base64 String');
   return Buffer.from(str, 'base64').toString('binary');
 }