Add frequencies

snippets/frequencies.md

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+---
+title: frequencies
+tags: array,intermediate
+---
+
+Returns an object with the unique values of an array as keys and their frequencies as the values.
+
+Use `Array.prototype.reduce()` to map unique values to an object's keys, adding to existing keys every time the same value is encountered.
+
+```js
+const frequencies = arr =>
+  arr.reduce(
+    (a, v) => {
+      a[v] = a[v] ? a[v] + 1 : 1;
+      return a;
+    }, {}
+  );
+```
+
+```js
+frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']); // { a: 4, b: 2, c: 1 }
+```

test/frequencies.test.js

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+const {frequencies} = require('./_30s.js');
+
+test('frequencies is a Function', () => {
+  expect(frequencies).toBeInstanceOf(Function);
+});
+test('returns the appropriate object', () => {
+  expect(frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b'])).toEqual({ a: 4, b: 2, c: 1 });
+});