addtohead.js

   addToHead(data) {
        let newNode = new node(data);
       /*
       If the list is empty , 
       newly created `node` 
       will be the `head` of the list.
       */
        if (this.head == null) {
            this.head = newNode;
        } 
      /*
       If not,
       we will point `next` 
       of the newly created `node`
       to the current `head` of the list
       and then set `head` of the list to newly created node
       */
       else {
            newNode.next = this.head;
            this.head = newNode;
        }
      /*
      increment length
      */
      this.length++;
   }