Simple implementation on linked list in JS

linkedList.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);
	var 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.searchNodeByValue = function(data) {
    var currentNode = this.head;
    var count = 0;
 
    while (currentNode.data) {
    	if(currentNode.data === data){
    		return currentNode;
    	}
        currentNode = currentNode.next;
        count++;
    }
};

var linkedList = new SinglyList();

linkedList.add(10);
linkedList.add(20);
console.log(linkedList)

var searchByIndex = linkedList.searchNodeAt(1);
var searchByValue = linkedList.searchNodeByValue(20);

console.log(searchByIndex);
console.log(searchByValue);