Add mostFrequent

snippets/mostFrequent.md

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+---
+title: mostFrequent
+tags: array,intermediate
+---
+
+Returns the most frequent element in an array.
+
+Use `Array.prototype.reduce()` to map unique values to an object's keys, adding to existing keys every time the same value is encountered.
+Use `Object.entries()` on the result in combination with `Array.prototype.reduce()` to get the most frequent value in the array.
+
+```js
+const mostFrequent = arr =>
+  Object.entries(arr.reduce(
+    (a, v) => {
+      a[v] = a[v] ? a[v] + 1 : 1;
+      return a;
+    }, {}
+  )).reduce((a, v) => v[1] >= a[1] ? v : a, [null, 0])[0];
+```
+
+```js
+mostFrequent(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // 'a'
+```

test/mostFrequent.test.js

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+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');
+});