iterators.js

Array.prototype.iter = function() {
  return new ListIter(this);
};

class Iter {
  map(fn) {
    return new Map(this, fn);
  }

  collect() {
    let accum = [];
    let current;
    while (!(current = this.next()).done) {
      accum.push(current.value);
    }
    return accum;
  }

  [Symbol.iterator]() {
    return this;
  }
}

class Map extends Iter {
  constructor(iter, fn) {
    super();
    this.iter = iter;
    this.fn = fn;
  }

  next() {
    const iter = this.iter.next();
    if (iter.done) {
      return { done: true, value: undefined };
    } else {
      return {
        value: this.fn(iter.value),
        done: iter.done
      };
    }
  }
}

class ListIter extends Iter {
  constructor(list) {
    super();
    this.list = list;
    this.position = 0;
  }

  next() {
    if (this.position > this.list.length) {
      return { done: true, value: undefined };
    }
    return {
      value: this.list[this.position++],
      done: this.list.length == this.position - 1
    };
  }
}

let list = [1, 2, 3];

let iter = list
  .iter()
  .map(a => a * 2)
  .map(a => a + 1000);

console.log([...iter]);