diff --git a/src/data-structures/linked-list/doubly-linked-list.ts b/src/data-structures/linked-list/doubly-linked-list.ts
index dcb2da5..6fc50ce 100644
--- a/src/data-structures/linked-list/doubly-linked-list.ts
+++ b/src/data-structures/linked-list/doubly-linked-list.ts
@@ -7,8 +7,8 @@
  */
 export class DoublyLinkedListNode<E = any> {
   value: E;
-  next: DoublyLinkedListNode<E> | null;
-  prev: DoublyLinkedListNode<E> | null;
+  next: DoublyLinkedListNode<E> | undefined;
+  prev: DoublyLinkedListNode<E> | undefined;
 
   /**
    * The constructor function initializes the value, next, and previous properties of an object.
@@ -17,8 +17,8 @@ export class DoublyLinkedListNode<E = any> {
    */
   constructor(value: E) {
     this.value = value;
-    this.next = null;
-    this.prev = null;
+    this.next = undefined;
+    this.prev = undefined;
   }
 }
 
@@ -27,8 +27,8 @@ export class DoublyLinkedList<E = any> {
    * The constructor initializes the linked list with an empty head, tail, and length.
    */
   constructor(elements?: Iterable<E>) {
-    this._head = null;
-    this._tail = null;
+    this._head = undefined;
+    this._tail = undefined;
     this._length = 0;
     if (elements) {
       for (const el of elements) {
@@ -37,15 +37,15 @@ export class DoublyLinkedList<E = any> {
     }
   }
 
-  protected _head: DoublyLinkedListNode<E> | null;
+  protected _head: DoublyLinkedListNode<E> | undefined;
 
-  get head(): DoublyLinkedListNode<E> | null {
+  get head(): DoublyLinkedListNode<E> | undefined {
     return this._head;
   }
 
-  protected _tail: DoublyLinkedListNode<E> | null;
+  protected _tail: DoublyLinkedListNode<E> | undefined;
 
-  get tail(): DoublyLinkedListNode<E> | null {
+  get tail(): DoublyLinkedListNode<E> | undefined {
     return this._tail;
   }
 
@@ -133,17 +133,17 @@ export class DoublyLinkedList<E = any> {
    *
    * The `pop()` function removes and returns the value of the last node in a doubly linked list.
    * @returns The method is returning the value of the removed node (removedNode.value) if the list is not empty. If the
-   * list is empty, it returns null.
+   * list is empty, it returns undefined.
    */
   pop(): E | undefined {
     if (!this.tail) return undefined;
     const removedNode = this.tail;
     if (this.head === this.tail) {
-      this._head = null;
-      this._tail = null;
+      this._head = undefined;
+      this._tail = undefined;
     } else {
       this._tail = removedNode.prev;
-      this.tail!.next = null;
+      this.tail!.next = undefined;
     }
     this._length--;
     return removedNode.value;
@@ -160,7 +160,7 @@ export class DoublyLinkedList<E = any> {
    *
    * The `popLast()` function removes and returns the value of the last node in a doubly linked list.
    * @returns The method is returning the value of the removed node (removedNode.value) if the list is not empty. If the
-   * list is empty, it returns null.
+   * list is empty, it returns undefined.
    */
   popLast(): E | undefined {
     return this.pop();
@@ -183,11 +183,11 @@ export class DoublyLinkedList<E = any> {
     if (!this.head) return undefined;
     const removedNode = this.head;
     if (this.head === this.tail) {
-      this._head = null;
-      this._tail = null;
+      this._head = undefined;
+      this._tail = undefined;
     } else {
       this._head = removedNode.next;
-      this.head!.prev = null;
+      this.head!.prev = undefined;
     }
     this._length--;
     return removedNode.value;
@@ -262,8 +262,8 @@ export class DoublyLinkedList<E = any> {
    * Time Complexity: O(n), where n is the number of elements in the linked list.
    * Space Complexity: O(1)
    *
-   * The `getFirst` function returns the first node in a doubly linked list, or null if the list is empty.
-   * @returns The method `getFirst()` returns the first node of the doubly linked list, or `null` if the list is empty.
+   * The `getFirst` function returns the first node in a doubly linked list, or undefined if the list is empty.
+   * @returns The method `getFirst()` returns the first node of the doubly linked list, or `undefined` if the list is empty.
    */
   getFirst(): E | undefined {
     return this.head?.value;
@@ -278,8 +278,8 @@ export class DoublyLinkedList<E = any> {
    * Time Complexity: O(n), where n is the number of elements in the linked list.
    * Space Complexity: O(1)
    *
-   * The `getLast` function returns the last node in a doubly linked list, or null if the list is empty.
-   * @returns The method `getLast()` returns the last node of the doubly linked list, or `null` if the list is empty.
+   * The `getLast` function returns the last node in a doubly linked list, or undefined if the list is empty.
+   * @returns The method `getLast()` returns the last node of the doubly linked list, or `undefined` if the list is empty.
    */
   getLast(): E | undefined {
     return this.tail?.value;
@@ -294,11 +294,11 @@ export class DoublyLinkedList<E = any> {
    * Time Complexity: O(n), where n is the number of elements in the linked list.
    * Space Complexity: O(1)
    *
-   * The `getAt` function returns the value at a specified index in a linked list, or null if the index is out of bounds.
+   * The `getAt` function returns the value at a specified index in a linked list, or undefined if the index is out of bounds.
    * @param {number} index - The index parameter is a number that represents the position of the element we want to
    * retrieve from the list.
    * @returns The method is returning the value at the specified index in the linked list. If the index is out of bounds
-   * or the linked list is empty, it will return null.
+   * or the linked list is empty, it will return undefined.
    */
   getAt(index: number): E | undefined {
     if (index < 0 || index >= this.length) return undefined;
@@ -318,15 +318,15 @@ export class DoublyLinkedList<E = any> {
    * Time Complexity: O(n), where n is the number of elements in the linked list.
    * Space Complexity: O(1)
    *
-   * The function `getNodeAt` returns the node at a given index in a doubly linked list, or null if the index is out of
+   * The function `getNodeAt` returns the node at a given index in a doubly linked list, or undefined if the index is out of
    * range.
    * @param {number} index - The `index` parameter is a number that represents the position of the node we want to
    * retrieve from the doubly linked list. It indicates the zero-based index of the node we want to access.
    * @returns The method `getNodeAt(index: number)` returns a `DoublyLinkedListNode<E>` object if the index is within the
-   * valid range of the linked list, otherwise it returns `null`.
+   * valid range of the linked list, otherwise it returns `undefined`.
    */
-  getNodeAt(index: number): DoublyLinkedListNode<E> | null {
-    if (index < 0 || index >= this.length) return null;
+  getNodeAt(index: number): DoublyLinkedListNode<E> | undefined {
+    if (index < 0 || index >= this.length) return undefined;
     let current = this.head;
     for (let i = 0; i < index; i++) {
       current = current!.next;
@@ -344,12 +344,12 @@ export class DoublyLinkedList<E = any> {
    * Space Complexity: O(1)
    *
    * The function `findNodeByValue` searches for a node with a specific value in a doubly linked list and returns the
-   * node if found, otherwise it returns null.
+   * node if found, otherwise it returns undefined.
    * @param {E} value - The `value` parameter is the value that we want to search for in the doubly linked list.
    * @returns The function `findNodeByValue` returns a `DoublyLinkedListNode<E>` if a node with the specified value `value`
-   * is found in the linked list. If no such node is found, it returns `null`.
+   * is found in the linked list. If no such node is found, it returns `undefined`.
    */
-  getNode(value: E | null): DoublyLinkedListNode<E> | null {
+  getNode(value: E | undefined): DoublyLinkedListNode<E> | undefined {
     let current = this.head;
 
     while (current) {
@@ -359,7 +359,7 @@ export class DoublyLinkedList<E = any> {
       current = current.next;
     }
 
-    return null;
+    return undefined;
   }
 
   /**
@@ -502,7 +502,7 @@ export class DoublyLinkedList<E = any> {
    * The `deleteAt` function removes an element at a specified index from a linked list and returns the removed element.
    * @param {number} index - The index parameter represents the position of the element that needs to be deleted in the
    * data structure. It is of type number.
-   * @returns The method `deleteAt` returns the value of the node that was deleted, or `null` if the index is out of
+   * @returns The method `deleteAt` returns the value of the node that was deleted, or `undefined` if the index is out of
    * bounds.
    */
   deleteAt(index: number): E | undefined {
@@ -534,8 +534,8 @@ export class DoublyLinkedList<E = any> {
    * @returns The `delete` method returns a boolean value. It returns `true` if the value or node was successfully
    * deleted from the doubly linked list, and `false` if the value or node was not found in the list.
    */
-  delete(valOrNode: E | DoublyLinkedListNode<E> | null): boolean {
-    let node: DoublyLinkedListNode<E> | null;
+  delete(valOrNode: E | DoublyLinkedListNode<E> | undefined): boolean {
+    let node: DoublyLinkedListNode<E> | undefined;
 
     if (valOrNode instanceof DoublyLinkedListNode) {
       node = valOrNode;
@@ -569,11 +569,11 @@ export class DoublyLinkedList<E = any> {
   }
 
   /**
-   * The `clear` function resets the linked list by setting the head, tail, and length to null and 0 respectively.
+   * The `clear` function resets the linked list by setting the head, tail, and length to undefined and 0 respectively.
    */
   clear(): void {
-    this._head = null;
-    this._tail = null;
+    this._head = undefined;
+    this._tail = undefined;
     this._length = 0;
   }
 
@@ -590,9 +590,9 @@ export class DoublyLinkedList<E = any> {
    * @param callback - A function that takes a value of type E as its parameter and returns a boolean value. This
    * function is used to determine whether a particular value in the linked list satisfies a certain condition.
    * @returns The method `find` returns the first element in the linked list that satisfies the condition specified by
-   * the callback function. If no element satisfies the condition, it returns `null`.
+   * the callback function. If no element satisfies the condition, it returns `undefined`.
    */
-  find(callback: (value: E) => boolean): E | null {
+  find(callback: (value: E) => boolean): E | undefined {
     let current = this.head;
     while (current) {
       if (callback(current.value)) {
@@ -600,7 +600,7 @@ export class DoublyLinkedList<E = any> {
       }
       current = current.next;
     }
-    return null;
+    return undefined;
   }
 
   /**
@@ -641,13 +641,13 @@ export class DoublyLinkedList<E = any> {
    * Space Complexity: O(1)
    *
    * The `findBackward` function iterates through a linked list from the last node to the first node and returns the last
-   * value that satisfies the given callback function, or null if no value satisfies the callback.
+   * value that satisfies the given callback function, or undefined if no value satisfies the callback.
    * @param callback - A function that takes a value of type E as its parameter and returns a boolean value. This
    * function is used to determine whether a given value satisfies a certain condition.
    * @returns The method `findBackward` returns the last value in the linked list that satisfies the condition specified by
-   * the callback function. If no value satisfies the condition, it returns `null`.
+   * the callback function. If no value satisfies the condition, it returns `undefined`.
    */
-  findBackward(callback: (value: E) => boolean): E | null {
+  findBackward(callback: (value: E) => boolean): E | undefined {
     let current = this.tail;
     while (current) {
       if (callback(current.value)) {
@@ -655,7 +655,7 @@ export class DoublyLinkedList<E = any> {
       }
       current = current.prev;
     }
-    return null;
+    return undefined;
   }
 
   /**
diff --git a/src/data-structures/linked-list/singly-linked-list.ts b/src/data-structures/linked-list/singly-linked-list.ts
index d1d9a1a..60bc03b 100644
--- a/src/data-structures/linked-list/singly-linked-list.ts
+++ b/src/data-structures/linked-list/singly-linked-list.ts
@@ -7,16 +7,16 @@
  */
 export class SinglyLinkedListNode<E = any> {
   value: E;
-  next: SinglyLinkedListNode<E> | null;
+  next: SinglyLinkedListNode<E> | undefined;
 
   /**
-   * The constructor function initializes an instance of a class with a given value and sets the next property to null.
+   * The constructor function initializes an instance of a class with a given value and sets the next property to undefined.
    * @param {E} value - The "value" parameter is of type E, which means it can be any data type. It represents the value that
    * will be stored in the node of a linked list.
    */
   constructor(value: E) {
     this.value = value;
-    this.next = null;
+    this.next = undefined;
   }
 }
 
@@ -25,8 +25,8 @@ export class SinglyLinkedList<E = any> {
    * The constructor initializes the linked list with an empty head, tail, and length.
    */
   constructor(elements?: Iterable<E>) {
-    this._head = null;
-    this._tail = null;
+    this._head = undefined;
+    this._tail = undefined;
     this._length = 0;
     if (elements) {
       for (const el of elements)
@@ -34,15 +34,15 @@ export class SinglyLinkedList<E = any> {
     }
   }
 
-  protected _head: SinglyLinkedListNode<E> | null;
+  protected _head: SinglyLinkedListNode<E> | undefined;
 
-  get head(): SinglyLinkedListNode<E> | null {
+  get head(): SinglyLinkedListNode<E> | undefined {
     return this._head;
   }
 
-  protected _tail: SinglyLinkedListNode<E> | null;
+  protected _tail: SinglyLinkedListNode<E> | undefined;
 
-  get tail(): SinglyLinkedListNode<E> | null {
+  get tail(): SinglyLinkedListNode<E> | undefined {
     return this._tail;
   }
 
@@ -128,14 +128,14 @@ export class SinglyLinkedList<E = any> {
    * The `pop()` function removes and returns the value of the last element in a linked list, updating the head and tail
    * pointers accordingly.
    * @returns The method `pop()` returns the value of the node that is being removed from the end of the linked list. If
-   * the linked list is empty, it returns `null`.
+   * the linked list is empty, it returns `undefined`.
    */
   pop(): E | undefined {
     if (!this.head) return undefined;
     if (this.head === this.tail) {
       const value = this.head.value;
-      this._head = null;
-      this._tail = null;
+      this._head = undefined;
+      this._tail = undefined;
       this._length--;
       return value;
     }
@@ -145,7 +145,7 @@ export class SinglyLinkedList<E = any> {
       current = current.next!;
     }
     const value = this.tail!.value;
-    current.next = null;
+    current.next = undefined;
     this._tail = current;
     this._length--;
     return value;
@@ -163,7 +163,7 @@ export class SinglyLinkedList<E = any> {
    * The `popLast()` function removes and returns the value of the last element in a linked list, updating the head and tail
    * pointers accordingly.
    * @returns The method `pop()` returns the value of the node that is being removed from the end of the linked list. If
-   * the linked list is empty, it returns `null`.
+   * the linked list is empty, it returns `undefined`.
    */
   popLast(): E | undefined {
     return this.pop();
@@ -256,11 +256,11 @@ export class SinglyLinkedList<E = any> {
    * Time Complexity: O(n) - Linear time, where n is the index, as it may need to traverse the list to find the desired node.
    * Space Complexity: O(1) - Constant space.
    *
-   * The function `getAt` returns the value at a specified index in a linked list, or null if the index is out of range.
+   * The function `getAt` returns the value at a specified index in a linked list, or undefined if the index is out of range.
    * @param {number} index - The index parameter is a number that represents the position of the element we want to
    * retrieve from the list.
-   * @returns The method `getAt(index: number): E | null` returns the value at the specified index in the linked list, or
-   * `null` if the index is out of bounds.
+   * @returns The method `getAt(index: number): E | undefined` returns the value at the specified index in the linked list, or
+   * `undefined` if the index is out of bounds.
    */
   getAt(index: number): E | undefined {
     if (index < 0 || index >= this.length) return undefined;
@@ -284,9 +284,9 @@ export class SinglyLinkedList<E = any> {
    * @param {number} index - The `index` parameter is a number that represents the position of the node we want to
    * retrieve from the linked list. It indicates the zero-based index of the node we want to access.
    * @returns The method `getNodeAt(index: number)` returns a `SinglyLinkedListNode<E>` object if the node at the
-   * specified index exists, or `null` if the index is out of bounds.
+   * specified index exists, or `undefined` if the index is out of bounds.
    */
-  getNodeAt(index: number): SinglyLinkedListNode<E> | null {
+  getNodeAt(index: number): SinglyLinkedListNode<E> | undefined {
     let current = this.head;
     for (let i = 0; i < index; i++) {
       current = current!.next;
@@ -306,7 +306,7 @@ export class SinglyLinkedList<E = any> {
    * The `deleteAt` function removes an element at a specified index from a linked list and returns the removed element.
    * @param {number} index - The index parameter represents the position of the element that needs to be deleted in the
    * data structure. It is of type number.
-   * @returns The method `deleteAt` returns the value of the node that was deleted, or `null` if the index is out of
+   * @returns The method `deleteAt` returns the value of the node that was deleted, or `undefined` if the index is out of
    * bounds.
    */
   deleteAt(index: number): E | undefined {
@@ -336,7 +336,7 @@ export class SinglyLinkedList<E = any> {
    * @returns The `delete` method returns a boolean value. It returns `true` if the value or node is found and
    * successfully deleted from the linked list, and `false` if the value or node is not found in the linked list.
    */
-  delete(valueOrNode: E | SinglyLinkedListNode<E> | null | undefined): boolean {
+  delete(valueOrNode: E | SinglyLinkedListNode<E> | undefined | undefined): boolean {
     if (!valueOrNode) return false;
     let value: E;
     if (valueOrNode instanceof SinglyLinkedListNode) {
@@ -345,14 +345,14 @@ export class SinglyLinkedList<E = any> {
       value = valueOrNode;
     }
     let current = this.head,
-      prev = null;
+      prev = undefined;
 
     while (current) {
       if (current.value === value) {
-        if (prev === null) {
+        if (prev === undefined) {
           this._head = current.next;
           if (current === this.tail) {
-            this._tail = null;
+            this._tail = undefined;
           }
         } else {
           prev.next = current.next;
@@ -416,11 +416,11 @@ export class SinglyLinkedList<E = any> {
   }
 
   /**
-   * The `clear` function resets the linked list by setting the head, tail, and length to null and 0 respectively.
+   * The `clear` function resets the linked list by setting the head, tail, and length to undefined and 0 respectively.
    */
   clear(): void {
-    this._head = null;
-    this._tail = null;
+    this._head = undefined;
+    this._tail = undefined;
     this._length = 0;
   }
 
@@ -461,9 +461,9 @@ export class SinglyLinkedList<E = any> {
   reverse(): void {
     if (!this.head || this.head === this.tail) return;
 
-    let prev: SinglyLinkedListNode<E> | null = null;
-    let current: SinglyLinkedListNode<E> | null = this.head;
-    let next: SinglyLinkedListNode<E> | null = null;
+    let prev: SinglyLinkedListNode<E> | undefined = undefined;
+    let current: SinglyLinkedListNode<E> | undefined = this.head;
+    let next: SinglyLinkedListNode<E> | undefined = undefined;
 
     while (current) {
       next = current.next;
@@ -488,9 +488,9 @@ export class SinglyLinkedList<E = any> {
    * @param callback - A function that takes a value of type E as its parameter and returns a boolean value. This
    * function is used to determine whether a particular value in the linked list satisfies a certain condition.
    * @returns The method `find` returns the first element in the linked list that satisfies the condition specified by
-   * the callback function. If no element satisfies the condition, it returns `null`.
+   * the callback function. If no element satisfies the condition, it returns `undefined`.
    */
-  find(callback: (value: E) => boolean): E | null {
+  find(callback: (value: E) => boolean): E | undefined {
     let current = this.head;
     while (current) {
       if (callback(current.value)) {
@@ -498,7 +498,7 @@ export class SinglyLinkedList<E = any> {
       }
       current = current.next;
     }
-    return null;
+    return undefined;
   }
 
   /**
@@ -540,12 +540,12 @@ export class SinglyLinkedList<E = any> {
    * Space Complexity: O(1) - Constant space.
    *
    * The function finds a node in a singly linked list by its value and returns the node if found, otherwise returns
-   * null.
+   * undefined.
    * @param {E} value - The value parameter is the value that we want to search for in the linked list.
    * @returns a `SinglyLinkedListNode<E>` if a node with the specified value is found in the linked list. If no node with
-   * the specified value is found, the function returns `null`.
+   * the specified value is found, the function returns `undefined`.
    */
-  getNode(value: E): SinglyLinkedListNode<E> | null {
+  getNode(value: E): SinglyLinkedListNode<E> | undefined {
     let current = this.head;
 
     while (current) {
@@ -555,7 +555,7 @@ export class SinglyLinkedList<E = any> {
       current = current.next;
     }
 
-    return null;
+    return undefined;
   }
 
   /**
@@ -620,7 +620,7 @@ export class SinglyLinkedList<E = any> {
    * existing value or node, and false if the existing value or node was not found in the linked list.
    */
   insertAfter(existingValueOrNode: E | SinglyLinkedListNode<E>, newValue: E): boolean {
-    let existingNode: E | SinglyLinkedListNode<E> | null;
+    let existingNode: E | SinglyLinkedListNode<E> | undefined;
 
     if (existingValueOrNode instanceof SinglyLinkedListNode) {
       existingNode = existingValueOrNode;
diff --git a/src/data-structures/linked-list/skip-linked-list.ts b/src/data-structures/linked-list/skip-linked-list.ts
index 5784712..9631f02 100644
--- a/src/data-structures/linked-list/skip-linked-list.ts
+++ b/src/data-structures/linked-list/skip-linked-list.ts
@@ -27,7 +27,7 @@ export class SkipList<K, V> {
    * level in the skip list. It is used to determine the height of each node in the skip list.
    */
   constructor(maxLevel = 16, probability = 0.5) {
-    this._head = new SkipListNode<K, V>(null as any, null as any, maxLevel);
+    this._head = new SkipListNode<K, V>(undefined as any, undefined as any, maxLevel);
     this._level = 0;
     this._maxLevel = maxLevel;
     this._probability = probability;
@@ -88,7 +88,7 @@ export class SkipList<K, V> {
       update[i].forward[i] = newNode;
     }
 
-    if (newNode.forward[0] !== null) {
+    if (!newNode.forward[0]) {
       this._level = Math.max(this.level, newNode.forward.length);
     }
   }
@@ -172,7 +172,7 @@ export class SkipList<K, V> {
         }
         update[i].forward[i] = current.forward[i];
       }
-      while (this.level > 0 && this.head.forward[this.level - 1] === null) {
+      while (this.level > 0 && !this.head.forward[this.level - 1]) {
         this._level--;
       }
       return true;
@@ -259,7 +259,7 @@ export class SkipList<K, V> {
    */
   lower(key: K): V | undefined {
     let current = this.head;
-    let lastLess = null;
+    let lastLess = undefined;
 
     for (let i = this.level - 1; i >= 0; i--) {
       while (current.forward[i] && current.forward[i].key < key) {
diff --git a/test/unit/data-structures/linked-list/doubly-linked-list.test.ts b/test/unit/data-structures/linked-list/doubly-linked-list.test.ts
index 21c3877..89405f2 100644
--- a/test/unit/data-structures/linked-list/doubly-linked-list.test.ts
+++ b/test/unit/data-structures/linked-list/doubly-linked-list.test.ts
@@ -17,8 +17,8 @@ describe('DoublyLinkedList Operation Test', () => {
   });
 
   it('should out of bound index', () => {
-    expect(list.getNodeAt(-1)).toBe(null);
-    expect(list.getNodeAt(5)).toBe(null);
+    expect(list.getNodeAt(-1)).toBe(undefined);
+    expect(list.getNodeAt(5)).toBe(undefined);
     expect(list.insertAt(5, 6)).toBe(true);
   });
 
@@ -37,16 +37,16 @@ describe('DoublyLinkedList Operation Test', () => {
     expect(list.tail?.value).toBe(4);
   });
 
-  it('should find null', () => {
-    expect(list.find(value => value === 6)).toBe(null);
+  it('should find undefined', () => {
+    expect(list.find(value => value === 6)).toBe(undefined);
   });
 
   it('should indexOf -1', () => {
     expect(list.indexOf(6)).toBe(-1);
   });
 
-  it('should findBackward null', () => {
-    expect(list.findBackward(value => value === 0)).toBe(null);
+  it('should findBackward undefined', () => {
+    expect(list.findBackward(value => value === 0)).toBe(undefined);
   });
 
   it('should insertAfter tail', () => {
@@ -69,8 +69,8 @@ describe('DoublyLinkedList Operation Test', () => {
 
   it('should initialize an empty list', () => {
     expect(list.length).toBe(0);
-    expect(list.head).toBeNull();
-    expect(list.tail).toBeNull();
+    expect(list.head).toBe(undefined);
+    expect(list.tail).toBe(undefined);
   });
 
   it('should push elements to the list', () => {
@@ -134,8 +134,8 @@ describe('DoublyLinkedList Operation Test', () => {
     // Deleting from the end
     list.deleteAt(0);
     expect(list.length).toBe(0);
-    expect(list.head).toBeNull();
-    expect(list.tail).toBeNull();
+    expect(list.head).toBe(undefined);
+    expect(list.tail).toBe(undefined);
   });
 
   it('should delete elements by value', () => {
@@ -154,8 +154,8 @@ describe('DoublyLinkedList Operation Test', () => {
 
     list.delete(3);
     expect(list.length).toBe(0);
-    expect(list.head).toBeNull();
-    expect(list.tail).toBeNull();
+    expect(list.head).toBe(undefined);
+    expect(list.tail).toBe(undefined);
   });
 
   it('should reverse the linked list', () => {
@@ -259,8 +259,8 @@ describe('DoublyLinkedList Operation Test', () => {
     list.clear();
 
     expect(list.length).toBe(0);
-    expect(list.head).toBe(null);
-    expect(list.tail).toBe(null);
+    expect(list.head).toBe(undefined);
+    expect(list.tail).toBe(undefined);
   });
 
   it('should create a reversed array of values', () => {
diff --git a/test/unit/data-structures/linked-list/singly-linked-list.test.ts b/test/unit/data-structures/linked-list/singly-linked-list.test.ts
index 49cf484..66c531e 100644
--- a/test/unit/data-structures/linked-list/singly-linked-list.test.ts
+++ b/test/unit/data-structures/linked-list/singly-linked-list.test.ts
@@ -333,7 +333,7 @@ describe('SinglyLinkedList Operation Test', () => {
       list.push(1);
       list.push(3);
       const result = list.find(data => data % 2 === 0);
-      expect(result).toBeNull();
+      expect(result).toBe(undefined);
     });
   });
 
@@ -402,8 +402,8 @@ describe('SinglyLinkedList', () => {
   });
 
   it('should initialize an empty list', () => {
-    expect(list.head).toBeNull();
-    expect(list.tail).toBeNull();
+    expect(list.head).toBe(undefined);
+    expect(list.tail).toBe(undefined);
     expect(list.length).toBe(0);
   });
 
diff --git a/test/unit/data-structures/linked-list/skip-list.test.ts b/test/unit/data-structures/linked-list/skip-list.test.ts
index e8dc368..830be02 100644
--- a/test/unit/data-structures/linked-list/skip-list.test.ts
+++ b/test/unit/data-structures/linked-list/skip-list.test.ts
@@ -81,6 +81,6 @@ describe('SkipList', () => {
 
   it('lower(key) should return the getLast element less than the given key', () => {
     expect(skipList.lower(2)).toBe('One');
-    expect(skipList.lower(1)).toBe(null);
+    expect(skipList.lower(1)).toBe(undefined);
   });
 });