Updated the test system

test/stringPermutations.test.js

@@ -0,0 +1,15 @@
+const expect = require('expect');
+const {stringPermutations} = require('./_30s.js');
+
+test('stringPermutations is a Function', () => {
+  expect(stringPermutations).toBeInstanceOf(Function);
+});
+test('Generates all stringPermutations of a string', () => {
+  expect(stringPermutations('abc')).toEqual(['abc', 'acb', 'bac', 'bca', 'cab', 'cba']);
+});
+test('Works for single-letter strings', () => {
+  expect(stringPermutations('a')).toEqual(['a']);
+});
+test('Works for empty strings', () => {
+  expect(stringPermutations('')).toEqual(['']);
+});