LinkedList Implementation in Js

linkedList.js

class Node {
  constructor(val) { 
    this.prev = null
    this.val = val 
    this.next = null
  }
}

class LinkedList {
  constructor() {
    this.clearList()
  }

  clearList() {
    this.head = null
    this.size = 0
  }
}

LinkedList.prototype.add = function (val) {
  let cur
  const newNode = new Node(val)
  if (this.head === null) 
    this.head = newNode
  else {
    cur = this.head
    while (cur.next) {
      cur = cur.next
    }
    cur.next = newNode
    newNode.prev = cur
  }
  this.size++
}

LinkedList.prototype.insert = function(val, index) {
  if (index < 0 || index > 0 && index > this.size)
    return false
  else {
    const newNode = new Node(val)
    if (index == 0) {
      newNode.next = this.head
      this.head = newNode
    } else {
      let cur = this.head,
        i = 0,
        prev
      while (i < index) {
        i++;
        prev = cur,
        cur = cur.next
      }
      newNode.next = cur
      newNode.prev = prev
      prev.next = newNode
    }
    this.size++
  }
}

LinkedList.prototype.remove = function(index) {
  if (index > 0 && index > this.size) 
    return false
  else {
    let cur = this.head,
      i = 0,
      prev = cur
    if (index == 0) {
      this.head = this.head.next
      this.head.prev = null
    }
    else {
      while(i < index) {
        i++
        prev = cur
        cur = cur.next
      }
      prev.next = cur.next
      cur = cur.next
      cur.prev = prev
    }
    this.size--
    return this.cur.val
  }
}

LinkedList.prototype.removeEl = function (val) {
  if (this.size > 0) {
    let cur = this.head,
      prev = cur
    if (cur.val === val) {
      cur = cur.next
      cur.prev = null
      this.size--
      return prev.val
    } else {
      for (let i = 0; i++; i < this.size) {
        if (cur.val === val) {
          const x = cur.val
          prev.next = cur.next
          cur = cur.next
          cur.prev = prev
          return x
        } else {
          prev = cur
          cur = cur.next
          i++
        }
      }
      this.size--
      return false
    }
  } else return false
}

LinkedList.prototype.indexOf = function(val) {
  if (this.size > 0) {
    let cur = this.head,
      i = 0
    if (cur.val === val) {
      return 0
    } else {
      while(i <= this.size) {
        if (cur.val === val) {
          return i
        } else {
          cur = cur.next
          i++
        }
      }
      return false
    }
  } else return false
}

LinkedList.prototype.isEmpty = function() {
  return this.size == 0
}

LinkedList.prototype.sizeOfList = function() {
  return this.size
}

LinkedList.prototype.printList = function() {
  let cur = this.head,
    i = 0,
    string = ""
  for (i; i <= this.size; i++) {
    string += " => " + cur.element
    cur = cur.next
  }
  console.log(string)
}