this.insert=function(head, data){
          //complete this method
          
          if(head == null){
              return {data: data, next: null}
          }
              let curr = head
              while(curr !== null){
                  if(curr.next == null){
                    let newnode = {data: data, next: null}
                    curr.next = newnode
                    break
                  }
                  curr = curr.next
              }
          return head   
    };