Add mostFrequent

test/mostFrequent.test.js

@@ -0,0 +1,8 @@
+const {mostFrequent} = require('./_30s.js');
+
+test('mostFrequent is a Function', () => {
+  expect(mostFrequent).toBeInstanceOf(Function);
+});
+test('returns the most frequent value', () => {
+  expect(mostFrequent(['a', 'b', 'a', 'c', 'a', 'a', 'b'])).toBe('a');
+});