code: function(data){
    this.head;
    this.size;
    if(typeof data !== 'undefined'){
      this.head = new Node(data);
      this.size = 1;
    }else{
      this.head = null;
      this.size = 0;
    }
  }
docs: Single argument constructor.
  @param {*} [data] data for head node of linked list
  @return {void}