fix error in SinglyLinkedList head method (TheAlgorithms#1322)

* fix error in SinglyLinkedList head method

* test: update check head test for SinglyLinkedList

* fix: code style error

* fix: remove extra semicolons

---------

Co-authored-by: Bekzod <bekzodisakov18@gmail.com>

Author: BekzodIsakov

Data-Structures/Linked-List/SinglyLinkedList.js

@@ -40,12 +40,12 @@ class LinkedList {
 
   // Returns the head
   head () {
-    return this.headNode?.data || null
+    return this.headNode?.data ?? null
   }
 
   // Returns the tail
   tail () {
-    return this.tailNode?.data || null
+    return this.tailNode?.data ?? null
   }
 
   // Return if the list is empty

Data-Structures/Linked-List/test/SinglyLinkedList.test.js

@@ -148,6 +148,10 @@ describe('SinglyLinkedList', () => {
 
     list.addFirst(30)
     expect(list.head()).toBe(30)
+
+    // check for a falsy head data
+    list.addFirst(false)
+    expect(list.head()).toBe(false)
   })
 
   it('Check tail', () => {
@@ -162,6 +166,10 @@ describe('SinglyLinkedList', () => {
 
     list.addFirst(30)
     expect(list.tail()).toBe(20)
+
+    // check for a falsy tail data
+    list.addLast(false)
+    expect(list.tail()).toBe(false)
   })
 
   it('Check size', () => {