JavaScript Example

/**
 * Concrete Iterators implement various traversal algorithms. These classes
 * store the current traversal position at all times.
 */
class AlphabeticalOrderIterator {
  constructor(collection, reverse = false) {
    this.collection = collection;
    /**
     * This variable indicates the traversal direction.
     */
    this.reverse = reverse;
    /**
     * Stores the current traversal position. An iterator may have a lot of
     * other fields for storing iteration state, especially when it is supposed
     * to work with a particular kind of collection.
     */
    this.position = 0;

    if (reverse) {
      this.position = collection.getCount() - 1;
    }
  }

  rewind() {
    this.position = this.reverse ? this.collection.getCount() - 1 : 0;
  }

  current() {
    return this.collection.getItems()[this.position];
  }

  key() {
    return this.position;
  }

  next() {
    const item = this.collection.getItems()[this.position];
    this.position += this.reverse ? -1 : 1;
    return item;
  }

  valid() {
    if (this.reverse) {
      return this.position >= 0;
    }

    return this.position < this.collection.getCount();
  }
}

module.exports = AlphabeticalOrderIterator;

const AlphabeticalOrderIterator = require("./AlphabeticalOrderIterator");

/**
 * Concrete Collections provide one or several methods for retrieving fresh
 * iterator instances, compatible with the collection class.
 */
class WordsCollection {
  constructor() {
    this.items = [];
  }

  getItems() {
    return this.items;
  }

  getCount() {
    return this.items.length;
  }

  addItem(item) {
    this.items.push(item);
  }

  getIterator() {
    return new AlphabeticalOrderIterator(this);
  }

  getReverseIterator() {
    return new AlphabeticalOrderIterator(this, true);
  }
}

module.exports = WordsCollection;

const WordsCollection = require("./WordsCollection");

/**
 * The client code may or may not know about the Concrete Iterator or Collection
 * classes, depending on the level of indirection you want to keep in your
 * program.
 */
const collection = new WordsCollection();
collection.addItem("First");
collection.addItem("Second");
collection.addItem("Third");

const iterator = collection.getIterator();

console.log("Straight traversal:");
while (iterator.valid()) {
  console.log(iterator.next());
}

console.log("");
console.log("Reverse traversal:");
const reverseIterator = collection.getReverseIterator();
while (reverseIterator.valid()) {
  console.log(reverseIterator.next());
}