Rewrite base64/ArrayBuffer conversion helpers

This rewrites 2 helpers as part of the GitLab Cleanroom project.

app/assets/javascripts/authentication/webauthn/util.js

@@ -14,31 +14,36 @@ export function isHTTPS() {
 export const FLOW_AUTHENTICATE = 'authenticate';
 export const FLOW_REGISTER = 'register';
 
-// adapted from https://stackoverflow.com/a/21797381/8204697
-function base64ToBuffer(base64) {
-  const binaryString = window.atob(base64);
-  const len = binaryString.length;
-  const bytes = new Uint8Array(len);
-  for (let i = 0; i < len; i += 1) {
-    bytes[i] = binaryString.charCodeAt(i);
-  }
-  return bytes.buffer;
-}
-
-// adapted from https://stackoverflow.com/a/9458996/8204697
-function bufferToBase64(buffer) {
-  if (typeof buffer === 'string') {
-    return buffer;
+/**
+ * Converts a base64 string to an ArrayBuffer
+ *
+ * @param {String} str - A base64 encoded string
+ * @returns {ArrayBuffer}
+ */
+export const base64ToBuffer = (str) => {
+  const rawStr = atob(str);
+  const buffer = new ArrayBuffer(rawStr.length);
+  const arr = new Uint8Array(buffer);
+  for (let i = 0; i < rawStr.length; i += 1) {
+    arr[i] = rawStr.charCodeAt(i);
   }
+  return arr.buffer;
+};
 
-  let binary = '';
-  const bytes = new Uint8Array(buffer);
-  const len = bytes.byteLength;
-  for (let i = 0; i < len; i += 1) {
-    binary += String.fromCharCode(bytes[i]);
+/**
+ * Converts ArrayBuffer to a base64-encoded string
+ *
+ * @param {ArrayBuffer, String} str -
+ * @returns {String} - ArrayBuffer to a base64-encoded string.
+ * When input is a string, returns the input as-is.
+ */
+export const bufferToBase64 = (input) => {
+  if (typeof input === 'string') {
+    return input;
   }
-  return window.btoa(binary);
-}
+  const arr = new Uint8Array(input);
+  return btoa(String.fromCharCode(...arr));
+};
 
 /**
  * Returns a copy of the given object with the id property converted to buffer

spec/frontend/authentication/webauthn/util_spec.js

+import { base64ToBuffer, bufferToBase64 } from '~/authentication/webauthn/util';
+
+const encodedString = 'SGVsbG8gd29ybGQh';
+const stringBytes = [72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 33];
+
+describe('Webauthn utils', () => {
+  it('base64ToBuffer', () => {
+    const toArray = (val) => new Uint8Array(val);
+
+    expect(base64ToBuffer(encodedString)).toBeInstanceOf(ArrayBuffer);
+
+    expect(toArray(base64ToBuffer(encodedString))).toEqual(toArray(stringBytes));
+  });
+
+  it('bufferToBase64', () => {
+    const buffer = base64ToBuffer(encodedString);
+    expect(bufferToBase64(buffer)).toBe(encodedString);
+  });
+});