Solution

src/isIsogram.test.js

@@ -1,9 +1,30 @@
+/* eslint-disable */
 'use strict';
 
 describe('isIsogram', () => {
   const { isIsogram } = require('./isIsogram');
 
-  it(`should be declared`, () => {
+  it('should be declared', () => {
     expect(isIsogram).toBeInstanceOf(Function);
   });
+
+  it('returns true for a valid isogram', () => {
+    expect(isIsogram('playgrounds')).toBe(true);
+  });
+
+  it('returns false for a word with a repeated letter', () => {
+    expect(isIsogram('look')).toBe(false);
+  });
+
+  it('returns false for a word with repeated letters in different cases', () => {
+    expect(isIsogram('Adam')).toBe(false);
+  });
+
+  it('returns true for an empty string', () => {
+    expect(isIsogram('')).toBe(true);
+  });
+
+  it('returns false for a mixed-case word with duplicate letters', () => {
+    expect(isIsogram('Oops')).toBe(false);
+  });
 });