Linked list for interview

LinkedList.js

class Entity {
  constructor(item){
    this.item = item || null;
    this.next = null;
  }
}

function LinkedList() {
  this.head = new Entity();
  //Private field
  let _size = 0;
  
  //Public Getter for private field _size
  this.size = () => _size;
  
  //Private setter
  const _increment = () => _size += 1;
  const _decrement = () => _size -= 1;

  //Add element
  this.add = (item) => {
    let current = this.head;
    while(current.next != null) {
      current = current.next;
    }
    current.next = new Entity(item);
    _increment()
  };

  //Remove element from linked list
  this.remove = (item) => {
    let current = this.head;
    let prev = current;

    while(current != null) {
      if(current.item == item){
        prev.next = current.next;
        _decrement();
        return true;
      }
      prev = current;
      current = current.next;
    }
    return false;
  }

  //Show current linked list
  this.show = () => {
    console.log(`Current LinkedList: ${JSON.stringify(this.head)}`);
  }

  //Contains element in linked list
  this.inclutes = (item) => {
    let current = this.head;
    while(current != null){
      if (current.item == item) return true;
      current = current.next
    }
    return false;
  }
};

const list = new LinkedList()
 
assert = function(predicate) { 
  if (!predicate) {
    throw new Error('assertion error');
  }
}

assertNot = function(predicate) { 
  assert(!predicate);
}

list.add(1);
list.add(3);
list.add(4);
list.show();
console.log(`Size: ${list.size()}`);

// assert(list.remove(1));
assertNot(list.remove(2));
// assert(list.remove(3));
assert(list.remove(4));
assertNot(list.remove(4));

list.show();
console.log(`Size: ${list.size()}`);