freeCodeCamp/Data Structures/linkedListClassAddAtIndex.js

// https://www.freecodecamp.org/learn/coding-interview-prep/data-structures/add-elements-at-a-specific-index-in-a-linked-list

function LinkedList() {
  var length = 0;
  var head = null;

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

  this.size = function () {
    return length;
  };

  this.head = function () {
    return head;
  };

  this.add = function (element) {
    var node = new Node(element);
    if (head === null) {
      head = node;
    } else {
      var currentNode = head;

      while (currentNode.next) {
        currentNode = currentNode.next;
      }

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

  // Only change code below this line
  this.addAt = (index, element) => {
    if (index < 0 || index > length) {
      return false;
    }
    let i = 0;
    let previousNode = null;
    let nextNode = head;
    while (i < index && nextNode !== null) {
      i++;
      previousNode = nextNode;
      nextNode = nextNode.next;
    }
    if (i != index) {
      return false;
    }
    const node = new Node(element);
    if (previousNode === null) {
      head = node;
    } else {
      previousNode.next = node;
    }
    node.next = nextNode;
    length++;
    return true;
  };
  // Only change code above this line
}