Review changes and added test case for overlapping substring

Author: ajinkyac03

String/CountSubstrings.js

@@ -13,14 +13,14 @@ const countSubstrings = (str, substring) => {
     throw new TypeError('Argument should be string')
   }
 
-  if (substring.length <= 0) return (str.length + 1)
+  if (substring.length === 0) return str.length + 1
 
   let count = 0
   let position = str.indexOf(substring)
 
   while (position > -1) {
-    ++count
-    position = str.indexOf(substring, ++position)
+    count++
+    position = str.indexOf(substring, position + 1)
   }
 
   return count

String/test/CountSubstrings.test.js

@@ -42,4 +42,11 @@ describe('CountSubstrings', () => {
     const count = countSubstrings(str, substring)
     expect(count).toBe(6)
   })
+
+  it('should return correct result when input is overlapping substring', () => {
+    const str = 'aaa'
+    const substring = 'aa'
+    const count = countSubstrings(str, substring)
+    expect(count).toBe(2)
+  })
 })