Example of a lazy collection which only creates one new array even with multiple combinator calls

lazy-iterator.js

const assert = require('assert');

class BaseIter {
  map(fn) {
    return new MapIter(this, fn);
  }
  filter(fn) {
    return new FilterIter(this, fn);
  }
  collect() {
    const result = [];
    let {done, value} = this._iter.next();
    while (!done) {
      result.push(value);
      ({done, value} = this._iter.next());
    }
    return result;
  }
}

class MapIter extends BaseIter {
  constructor(iter, fn) {
    super();
    this._iter = iter;
    this._fn = fn;
  }
  next() {
    const {done, value} = this._iter.next();
    return {
      done,
      value: done ? value : this._fn(value)
    };
  }
}

class FilterIter extends BaseIter {
  constructor(iter, fn) {
    super();
    this._iter = iter;
    this._fn = fn;
  }
  next() {
    let {done, value} = this._iter.next();
    while (!done && !this._fn(value)) {
      ({done, value} = this._iter.next());
    }
    return {done, value};
  }
}

class LazyIter extends BaseIter {
  constructor(values) {
    super();
    assert(values[Symbol.iterator], 'Input must have a iterator');
    this._iter = values[Symbol.iterator]();
  }
  next() {
    return this._iter.next();
  }
}

const result = new LazyIter([1, 2, 3, 4])
  .map(value => {
    console.log('map1 going through value %d', value);
    return value * 3;
  })
  .filter(value => {
    console.log('filter1 going through value %d', value);
    return value % 6 === 0;
  })
  .map(value => {
    console.log('map2 going through value %d', value);
    return value * 10;
  })
  .collect();

console.log('result', result);