.js

class Peekable {
  constructor(iter) { 
    this.iter = iter;
    this.deck = [];
  }
  peek() {
    if (!this.deck.length) {
      this.deck = [this.iter.next()];
    }
    return this.deck[0];
  }
  next() {
    if (this.deck.length) {
      return this.deck.shift();
    } else {
      return this.iter.next();
    }
  } 
  [Symbol.iterator]() {
    return this;
  }
}