fix(isBase64): reject invalid Base64 length

- Reject strings where length % 4 === 1 (impossible in valid Base64)
- Simplify return statement by removing redundant condition
- Add tests for invalid length cases

Author: coti-z

src/lib/isBase64.js

@@ -12,6 +12,7 @@ export default function isBase64(str, options) {
 
   if (str === '') return true;
 
+  if (str.length % 4 === 1) return false;
   if (options.padding && str.length % 4 !== 0) return false;
 
   let regex;
@@ -21,5 +22,5 @@ export default function isBase64(str, options) {
     regex = options.padding ? base64WithPadding : base64WithoutPadding;
   }
 
-  return (!options.padding || str.length % 4 === 0) && regex.test(str);
+  return regex.test(str);
 }

test/validators/isBase64.test.js

@@ -26,6 +26,9 @@ describe('isBase64', () => {
         'HQIDAQAB',
       ],
       invalid: [
+        'A',
+        'AAAAA',
+        'AAAAAAAAA',
         '12345',
         'Vml2YW11cyBmZXJtZtesting123',
         'Zg=',