Code_Listing of the Doubly Linked List main function List function Lis

/*
	Code_Listing of the Doubly Linked List
*/

/*
	main function List
*/

function List() {
	this._length = 0;
	this._head = null;
	this._tail = null;
}

List.prototype = {
	head: function() {
		return this._head.data;
	},

	tail: function() {
		return this._tail.data;
	},

	append: function( data ) {
		var node = {
			data: data,
			next: null,
			prev: null
		};

		if ( this._length == 0 ) {
			this._head = node;
			node.prev = this._tail;
			this._tail = node;
		} else {
			this._tail.next = node;
			node.prev = this._tail;
			this._tail = node;
		}

		this._length++;
		return this;
	},

	deleteAt: function( index ) {
		if ( index > -1 && index < this._length ) {
			var current = this._head, i = 0;

			if ( index === 0 ) {
				this._head = current.next;

				if ( !this._head ) {
					this._tail = null;
				} else {
					this._head.prev = null;
				}
			} else if ( index === this._length - 1 ) {
				current = this._tail;
				this._tail = current.prev;
				this._tail.next = null;
			} else {

				while ( i != index ) {
					current = current.next;
					i++;
				}

				current.prev.next = current.next;
				current.next.prev = current.prev; 
			}

			this._length--;
			return this;
		} else {
			return null;
		}
	},

	at: function( index ) {
		if ( index > -1 && index < this._length ) {
		var current = this._head, i = 0;

			while ( i != index ) {
				current = current.next;
				i++;
			}

			return null;
		}
	},

	insertAt: function( index, data ) {
		var node = {
			data: data,
			next: null,
			prev: null
		};

		if ( index > -1 && index < this._length + 1 ) {
			var current = this._head, i = 0;

			if ( this._length == 0 ) {
				this._head = node;
				this._tail = node;
			} else if ( index === 0 ) {
				this._head = node;
				node.next = current;
				current.prev = node;
			} else if ( index === this._length ) {
				current = this._tail;
				current.next = node;
				node.prev = current;
				this._tail = node;
			} else {
				while( i != index ) {
					current = current.next;
					i++;
				}

				current.prev.next = node;
				node.prev = current.prev;
				current.prev = node;
				node.next = current;
			}

			this._length++;
			return this;
		}
	},

	reverse: function() {
		var current = this.head, tmp;
		while ( current != null ) {
			tmp = current.next;
			current.next = current.prev;
			current.prev = tmp;
			current = current.prev;
		}

		tmp = this._head;
		this._head = this._tail;
		this._tail = tmp;
		return this;
	},

	each: function( func ) {
		var current = this._head;
		
		while ( current != null ) {
			current.data = func( current.data );
			current = current.next;
		}
		return this;
	},

	indexOF: function( data ) {
		var current = this._head, i = 0;
		
		while ( current != null ) {
			if ( current.data == data ) {
				return i;
			};
			i++;
		current = current.next;
		}
		return null;
	}
};