Fix for atob not loading in React Native (Azure#12412)

For some reason React native always tries to load .browser for `atob` so as workaround we can put the polyfill in the browser file.

Author: southpolesteve

sdk/cosmosdb/cosmos/src/utils/atob.browser.ts

@@ -1,8 +1,50 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT license.
 
+let safeatob: any;
+
+// base64 character set, plus padding character (=)
+const b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
+// Regular expression to check formal correctness of base64 encoded strings
+const b64re = /^(?:[A-Za-z\d+\/]{4})*?(?:[A-Za-z\d+\/]{2}(?:==)?|[A-Za-z\d+\/]{3}=?)?$/;
+
 if ("function" !== typeof atob) {
-  throw new Error("Your browser environment is missing the global `atob` function");
+  // atob implementation for React Native
+  safeatob = (str: string) => {
+    // atob can work with strings with whitespaces, even inside the encoded part,
+    // but only \t, \n, \f, \r and ' ', which can be stripped.
+    str = String(str).replace(/[\t\n\f\r ]+/g, "");
+    if (!b64re.test(str)) {
+      throw new TypeError(
+        "Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded."
+      );
+    }
+
+    // Adding the padding if missing, for simplicity
+    str += "==".slice(2 - (str.length & 3));
+    let bitmap;
+    let result = "";
+    let r1;
+    let r2;
+    let i = 0;
+    for (; i < str.length; ) {
+      bitmap =
+        (b64.indexOf(str.charAt(i++)) << 18) |
+        (b64.indexOf(str.charAt(i++)) << 12) |
+        ((r1 = b64.indexOf(str.charAt(i++))) << 6) |
+        (r2 = b64.indexOf(str.charAt(i++)));
+
+      result +=
+        r1 === 64
+          ? String.fromCharCode((bitmap >> 16) & 255)
+          : r2 === 64
+          ? String.fromCharCode((bitmap >> 16) & 255, (bitmap >> 8) & 255)
+          : String.fromCharCode((bitmap >> 16) & 255, (bitmap >> 8) & 255, bitmap & 255);
+    }
+    return result;
+  };
+} else {
+  safeatob = atob;
 }
 
-export default atob;
+export default safeatob;