Add tests for table of contents helper

src/features/sicp/__tests__/TableOfContentsHelper.ts

@@ -0,0 +1,31 @@
+import { getNext, getPrev } from '../TableOfContentsHelper';
+
+const data = {
+  '1': { next: '2' },
+  '2': { next: '3', prev: '1' },
+  '3': { prev: '2' }
+};
+
+jest.mock('../data/toc-navigation.json', () => data);
+
+describe('Table of contents helper', () => {
+  test('generate next correctly', () => {
+    expect(getNext('1')).toBe('2');
+    expect(getNext('2')).toBe('3');
+    expect(getNext('3')).toBeUndefined();
+  });
+
+  test('generate prev correctly', () => {
+    expect(getPrev('1')).toBeUndefined();
+    expect(getPrev('2')).toBe('1');
+    expect(getPrev('3')).toBe('2');
+  });
+
+  test('handle invalid values correctly', () => {
+    expect(getNext('invalid')).toBeUndefined();
+    expect(getNext('')).toBeUndefined();
+
+    expect(getPrev('invalid')).toBeUndefined();
+    expect(getPrev('')).toBeUndefined();
+  });
+});