Stream.js

const genIterator = (iterable) =>
  typeof iterable === 'function' ? iterable() : iterable[Symbol.iterator]()

class Stream {
  constructor(iterable = []) {
    this._iterable = iterable
    this._iterator = genIterator(iterable)
  }

  next() { return this._iterator.next() }

  [Symbol.iterator]() { return genIterator(this._iterable) }

  map(fun) {
    const iterable = this._iterable
    return new Stream(function* () {
      for (let x of genIterator(iterable)) {
        yield fun(x)
      }
    })
  }

  flatMap(fun) {
    const iterable = this._iterable
    return new Stream(function* () {
      for (let x of genIterator(iterable)) {
        yield* fun(x)
      }
    })
  }

  filter(fun) {
    const iterable = this._iterable
    return new Stream(function* () {
      for (let x of genIterator(iterable)) {
        if (fun(x)) yield x
      }
    })
  }

  take(n) {
    const iterable = this._iterable
    return new Stream(function* () {
      const iterator = genIterator(iterable)
      for (let count = 0; count < n; count++) {
        const { done, value } = iterator.next()
        if (done) return value
        yield value
      }
    })
  }

  static iterate(initialValue, fun) {
    return new Stream(function* () {
      let value = initialValue
      while (true) {
        yield value
        value = fun(value)
      }
    })
  }

  static cycle(iterable) {
    return new Stream(function* () {
      while (true) {
        yield* genIterator(iterable)
      }
    })
  }
}