Updated the test system

test/reduceWhich.test.js

@@ -0,0 +1,20 @@
+const expect = require('expect');
+const {reduceWhich} = require('./_30s.js');
+
+test('reduceWhich is a Function', () => {
+  expect(reduceWhich).toBeInstanceOf(Function);
+});
+test('Returns the minimum of an array', () => {
+  expect(reduceWhich([1, 3, 2])).toBe(1);
+});
+test('Returns the maximum of an array', () => {
+  expect(reduceWhich([1, 3, 2], (a, b) => b - a)).toBe(3);
+});
+test('Returns the object with the minimum specified value in an array', () => {
+  expect(
+    reduceWhich(
+      [{ name: 'Tom', age: 12 }, { name: 'Jack', age: 18 }, { name: 'Lucy', age: 9 }],
+      (a, b) => a.age - b.age
+    )
+  ).toEqual({ name: 'Lucy', age: 9 });
+});