test: add four new test cases

String/test/Lower.test.js

@@ -1,9 +1,19 @@
 import { lower } from '../Lower'
 
-describe('Lower', () => {
-  it('return uppercase strings', () => {
-    expect(lower('hello')).toBe('hello')
-    expect(lower('WORLD')).toBe('world')
-    expect(lower('hello_WORLD')).toBe('hello_world')
+describe('Testing the Lower function', () => {
+  it('Test 1: Check by invalid type', () => {
+    expect(() => lower(345)).toThrowError();
+    expect(() => lower(true)).toThrowError();
+    expect(() => lower(null)).toThrowError();
+  })
+
+  it('Test 2: Check by uppercase string', () => {
+    expect(lower('WORLD')).toBe('world');
+    expect(lower('Hello_WORLD')).toBe('hello_world');
+  })
+
+  it('Test 3: Check by lowercase string', () => {
+    expect(lower('hello')).toBe('hello');
+    expect(lower('hello_world')).toBe('hello_world');
   })
 })