singly_list.js

function Node(data) {
  this.data = data; 
  this.next = null; 
}

function SinglyList() {
  this._length = 0; 
  this.head   = null;     
}

SinglyList.prototype.add = function(value) {
  var node        = new Node(value), 
      currentNode = this.head;

  // 1st use-case: an empty list 
  if (!currentNode) {
    this.head = node;
    this._length++;
    return node;    
  }

  // 2nd use-case: a non-empty list
  while (currentNode.next) {
    currentNode = currentNode.next;  
  }

  currentNode.next = node;

  this._length++;
  return node;
};

SinglyList.prototype.searchNodeAt = function(position) {
  var currentNode = this.head,
      length      = this._length,
      count       = 1,
      message     = {failure: "Failure: non-existent node in this list."};

  // 1st use-case: an invalid position 
  if (length === 0 || position < 1 || position > length) {
    throw new Error(message.failure);  
  }
  
  // 2nd use-case: a valid position 
  while (count < position) {
    currentNode = currentNode.next;  
    count++;
  }

  return currentNode; 
};

SinglyList.prototype.remove = function(position) {
  var currentNode        = this.head, 
      length             = this._length,
      count              = 0,
      message            = {failure: "Failure: non-existent node in this list."
                           },
      beforeNodeToDelete = null,
      nodeToDelete       = null,
      deletedNode        = null;
  
  // 1st use-case: an invalid position
  if (position < 0 || position > length) {
    throw new Error(message.failure);  
  }

  // 2nd use-case: the first node is removed
  if (position === 1) {
    this.head   = currentNode.next;
    deletedNode = currentNode;
    currentNode = null; 
    this._length--;
    return deletedNode;
  }
  
  // 3rd use-case: any other node is removed
  while (count < position) {
    beforeNodeToDelete = currentNode; 
    nodeToDelete       = currentNode.next;  
    count++;
  }

  beforeNodeToDelete.next = nodeToDelete.next;
  deletedNode             = nodeToDelete; 
  nodeToDelete            = null;
  this._length--;

  return deletedNode;
};