linked-list.js

var LinkedList = function() {
  
}

LinkedList.prototype.addToTail = function(value) {
  var newNode = new Node(value);
  
  if (!this.tail && !this.head) {
    this.tail = newNode;
    this.head = newNode;
  } else {  
    var newTail = newNode;
    var oldTail = this.tail;

    oldTail.next = newTail;
    newTail.previous = oldTail;
    
    this.tail = newTail;    
  }
}

LinkedList.prototype.addToHead = function(value) {
  var newNode = new Node(value);
  
  if (!this.head && !this.tail) {
    this.head = newNode;
    this.tail = newNode;
  } else {
    var oldHead = this.head;
  
    oldHead.previous = newNode;
    newNode.next = oldHead;
    this.head = newNode;
    
  }
}

LinkedList.prototype.removeHead = function() {
  if (this.head) {
    var headToRemove = this.head;
    if (headToRemove.next) {
      var headToPromote = headToRemove.next;

      headToPromote.previous = null;
      this.head = headToPromote;      
    } else {
      this.head = undefined;
      this.tail = undefined;
    }

    return headToRemove.value;    
  }
}

LinkedList.prototype.removeTail = function() {
  if (this.tail) {
    var tailToRemove = this.tail;
  } if (tailToRemove.previous) {
    var tailToPromote = tailToRemove.previous;
    tailToPromote.next = null;
    this.tail = tailToPromote;
  } else {
    this.tail = undefined;
    this.head = undefined;
  }
  return tailToRemove.value;
}

LinkedList.prototype.search = function(value) {
  
  var findValue = function(value, currentNode) {
    //base case
    //if the value is found, return the value
    if (typeof value === 'function') {
      if (value(currentNode.value)) {
        return currentNode.value;
      }
    } else {
      if (currentNode.value.valueOf() === value) {
        return currentNode.value;
      }
    }

    //edge case
    //if .next = null return null
    if (!currentNode.next) {
      return null;
    }

    //recursive case
    return findValue(value, currentNode.next);
    
  }
  return findValue(value, this.head);
}

var Node = function(value) {
  this.value = value;
  this.next = null;
  this.previous = null;
}