feat: disallow non-instanced list concatenations

__tests__/CircularDoublyLinkedList.test.js

@@ -1,6 +1,6 @@
 import test from 'tape';
 
-import { CircularDoublyLinkedList } from '../lib';
+import { CircularDoublyLinkedList, CircularSinglyLinkedList } from '../lib';
 import { Node } from '../lib/Atomics';
 
 const subject = 'The circular doubly linked list';
@@ -313,6 +313,16 @@ test(`${subject} is not modified when invoking 'moveAfter', 'moveBefore' with a
 	t.end();
 });
 
+test(`${subject} throws an error if provided a list of a different type`, t => {
+	const l = init();
+	const l2 = new CircularSinglyLinkedList();
+
+	t.throws(() => l.pushBackList(l2));
+	t.throws(() => l.pushFrontList(l2));
+
+	t.end();
+});
+
 /* refHelpers */
 
 function checkListSize (list, expectedSize) {

__tests__/CircularSinglyLinkedList.test.js

@@ -1,6 +1,6 @@
 import test from 'tape';
 
-import { CircularSinglyLinkedList } from '../lib';
+import { CircularSinglyLinkedList, CircularDoublyLinkedList } from '../lib';
 import { ForwardNode } from '../lib/Atomics';
 
 const subject = 'The circular singly linked list';
@@ -433,6 +433,16 @@ test(`${subject} maintains integrity when operating upon a single-node list`, t
 	t.end();
 });
 
+test(`${subject} throws an error if provided a list of a different type`, t => {
+	const l = init();
+	const l2 = new CircularDoublyLinkedList();
+
+	t.throws(() => l.pushBackList(l2));
+	t.throws(() => l.pushFrontList(l2));
+
+	t.end();
+});
+
 /* Helpers */
 
 function checkListSize (list, expectedSize) {

lib/CircularDoublyLinkedList.js

@@ -299,11 +299,12 @@ export class CircularDoublyLinkedList {
 	 *
 	 * The lists may be the same, but must not be null
 	 * @param {CircularDoublyLinkedList} other
+	 * @throws {TypeError} Throws if provided a list that is not an instance of `CircularDoublyLinkedList`
 	 */
 	pushBackList (other) {
 		if (!this.sentinel.next) {
 			Object.assign(this, new CircularDoublyLinkedList());
-		}
+		} else if (!(other instanceof CircularDoublyLinkedList)) throw new TypeError('other must be an instance of `CircularDoublyLinkedList`');
 
 		for (let i = other.size(), n = other.head(); i > 0; i--, n = other.next(n)) {
 			this.insertValue(n.value, this.sentinel.prev);
@@ -315,11 +316,12 @@ export class CircularDoublyLinkedList {
 	 *
 	 * The lists may be the same, but must not be null
 	 * @param {CircularDoublyLinkedList} other
+	 * @throws {TypeError} Throws if provided a list that is not an instance of `CircularDoublyLinkedList`
 	 */
 	pushFrontList (other) {
 		if (!this.sentinel.next) {
 			Object.assign(this, new CircularDoublyLinkedList());
-		}
+		} else if (!(other instanceof CircularDoublyLinkedList)) throw new TypeError('other must be an instance of `CircularDoublyLinkedList`');
 
 		for (let i = other.size(), n = other.tail(); i > 0; i--, n = other.prev(n)) {
 			this.insertValue(n.value, this.sentinel);