feat(median): Add module median

Get the median from a list of numbers

src/__tests__/median.js

@@ -0,0 +1,26 @@
+import median from '../median';
+
+describe('Core.median', () => {
+  test('returns middle value of an odd-length list', () => {
+    expect(median([2, 9, 7])).toBe(7);
+    expect(median([3, 2, 11, 7, 9])).toBe(7);
+    expect(median([2])).toBe(2);
+  });
+
+  test('returns mean of two middle values of a nonempty even-length list', () => {
+    expect(median([7, 2])).toBe(4.5);
+    expect(median([7, 2, 10, 9])).toBe(8);
+  });
+
+  test('returns NaN for an empty list', () => {
+    expect(median([])).toBe(NaN);
+  });
+
+  test('handles array-like object', () => {
+    function getArgs() {
+      return arguments; // eslint-disable-line prefer-rest-params
+    }
+
+    expect(median(getArgs(1, 2, 3))).toBe(2);
+  });
+});

src/index.js

@@ -31,6 +31,8 @@ import keys from './keys';
 import last from './last';
 import length from './length';
 import map from './map';
+import mean from './mean';
+import median from './median';
 import modulo from './modulo';
 import multiply from './multiply';
 import negate from './negate';
@@ -84,6 +86,8 @@ export {
   last,
   length,
   map,
+  mean,
+  median,
   modulo,
   multiply,
   negate,

src/median.js

@@ -0,0 +1,24 @@
+import length from './length';
+import divide from './divide';
+import modulo from './modulo';
+import nth from './nth';
+import slice from './slice';
+import mean from './mean';
+
+const median = list => {
+  const sorted = Array.prototype.sort.call(list, (a, b) => {
+    if (a > b) return 1;
+    if (a < b) return -1;
+    return 0;
+  });
+
+  const l = length(sorted);
+  const half = divide(l, 2);
+
+  if (modulo(l, 2) > 0) return nth(Math.floor(half), sorted);
+
+  const sub = slice(half - 1, half + 1, sorted);
+  return mean(sub);
+};
+
+export { median as default };