Seq.js

class Seq {
  constructor(iterable, transformerGenerator) {
    this.iterable = iterable;
    this.transformerGenerator = transformerGenerator;
    Object.freeze(this);
  }

  [Symbol.iterator]() {
    const cache = this.iterable[Symbol.iterator]();
    const transformer = this.transformerGenerator();
    let last;
    return {
      next: value => {
        if (last && last.done) {
          return last;
        } else {
          last = transformer(cache.next(value));
          return last;
        }
      }
    };
  }

  map(f) {
    return new Seq(this, () => e => {
      if (e.done) {
        return { value: undefined, done: true };
      } else {
        return { value: e.value && f(e.value), done: false };
      }
    });
  }

  take(n) {
    return new Seq(this, (i = 0) => e => {
      if (e.done) {
        return e;
      } else if (i >= n) {
        return { value: undefined, done: true };
      } else {
        i++;
        return e;
      }
    });
  }

  static from(iterable) {
    return new Seq(iterable, () => _ => _);
  }
}

const a = [1, 2, 3];
const s = Seq.from(a);
const ss = s.map(n => n + 1);
const sse = ss.map(n => n * 2);
const sr = s.take(2);
const ssr = ss.take(2);

console.log(Array.from(s), Array.from(ss), Array.from(sr), Array.from(ssr));

console.log(Array.from(s), Array.from(ss), Array.from(sr), Array.from(ssr));