Javascript Linked List algorithm

LinkedList.js

function LinkedList(){
  this.length = 0;
  this.head = null

  const Node = function(element) {
    this.element = element
    this.next = null
  }

  this.add = (element) => {
    const node = new Node(element)
    if(!this.head){
      this.head = node
    } else {
      let currentNode = this.head;
       while(currentNode.next){
         currentNode = currentNode.next
       }

       currentNode.next = node
    }
    this.length++
  }

  this.remove = element => {
    let currentNode = this.head
    let previousNode;

    if(element === currentNode.element){
      this.head = currentNode.next
    } else {
      while(currentNode.element !== element){
        previousNode = currentNode;
        currentNode = currentNode.next;
      }
      
      previousNode.next = currentNode.next
    }
    this.length--
  }

  this.indexOf = element => {
    let currentNode = this.head
    let index = -1

    while(currentNode){
      index++
      if(currentNode.element === element){
        return index
      }
      currentNode = currentNode.next
    }
    return -1
  }

  this.removeAt = index => {
    let currentNode = this.head
    let currentIndex = 0; // Start at 0
    let previousNode;

    if (index < 0 || index >= this.length){
      throw new Error("Invalid index specified")
  }

    if(index === 0){
      this.head = currentNode.next
    } else {
      while(currentIndex < index){
        currentIndex++
        previousNode = currentNode
        currentNode = currentNode.next
      }
      previousNode.next = currentNode.next
    }
    this.length--;
    return currentNode.element
  }
}

const ll = new LinkedList()

ll.add(10)
ll.add(20)
ll.add(20)
ll.add(13)
ll.removeAt(2)

console.log(JSON.stringify(ll.head, null, 2))

console.log(ll.indexOf(13))