Solution

src/isIsogram.test.js

@@ -6,4 +6,24 @@ describe('isIsogram', () => {
   it(`should be declared`, () => {
     expect(isIsogram).toBeInstanceOf(Function);
   });
+
+  it(`unique chars return true`, () => {
+    expect(isIsogram('playgrounds')).toBe(true);
+  });
+
+  it(`repetitive chars return false`, () => {
+    expect(isIsogram('look')).toBe(false);
+  });
+
+  it(`returns true for empty string`, () => {
+    expect(isIsogram('')).toBe(true);
+  });
+
+  it('returns false when the same letter appears in different cases', () => {
+    expect(isIsogram('Adam')).toBe(false);
+  });
+
+  it('returns false for mixed-case repeated letters', () => {
+    expect(isIsogram('Oops')).toBe(false);
+  });
 });