Add comments to Queue class.

Author: trekhleb

src/data-structures/queue/Queue.js

@@ -2,13 +2,18 @@ import LinkedList from '../linked-list/LinkedList';
 
 export default class Queue {
   constructor() {
+    // We're going to implement Queue based on LinkedList since this
+    // structures a quite similar. Namely they both operates mostly with
+    // with theirs beginning and the end. Compare enqueue/de-queue
+    // operations of the Queue with append/prepend operations of LinkedList.
     this.linkedList = new LinkedList();
   }
 
   /**
    * @return {boolean}
    */
   isEmpty() {
+    // The queue is empty in case if its linked list don't have tail.
     return !this.linkedList.tail;
   }
 
@@ -17,23 +22,30 @@ export default class Queue {
    */
   peek() {
     if (!this.linkedList.head) {
+      // If linked list is empty then there is nothing to peek from.
       return null;
     }
 
+    // Just read the value from the end of linked list without deleting it.
     return this.linkedList.head.value;
   }
 
   /**
    * @param {*} value
    */
   enqueue(value) {
+    // Enqueueing means to stand in the line. Therefore let's just add
+    // new value at the beginning of the linked list. It will need to wait
+    // until all previous nodes will be processed.
     this.linkedList.append(value);
   }
 
   /**
    * @return {*}
    */
   dequeue() {
+    // Let's try to delete the last node from linked list (the tail).
+    // If there is no tail in linked list (it is empty) just return null.
     const removedHead = this.linkedList.deleteHead();
     return removedHead ? removedHead.value : null;
   }
@@ -43,6 +55,7 @@ export default class Queue {
    * @return {string}
    */
   toString(callback) {
+    // Return string representation of the queue's linked list.
     return this.linkedList.toString(callback);
   }
 }