Lazy range implementation with Symbol.iterator - live demo => http://jsbin.com/jeguzo/edit?js,console

lazyRange.js

// Practical Application of a Symbol.iterator
// to build a Lazy Sequence 

const asc = xs => xs.sort((a, b) => a - b)

class Range {
  constructor(to, from = 0, step = 1) {
    this.step = step
    
    if (typeof to === 'undefined' || to === null) {
      this.infinite = true
      this._from = from
    } else {
      let [f, t] = asc([from, to])
      this._from = f
      this._to = t 
    }
  }
  
  from (n) {
    this._from = n
    return this
  } 
  
  to (n) {
    this._to = n
    this.infinite = false
    return this
  }
  
  * [Symbol.iterator] () {
    let current = this._from
    const keepGoing = () => this.infinite || current <= this._to
    
    while (keepGoing()) {
      yield current
      current += this.step
    }
  }

  toArray () {
    return Array.from(this)
  }
}

// static `factory` helper
const range = (...args) => new Range(...args)

// tests
console.clear()

console.log(range(10, 20))
console.log(range(10, 20, 3))
console.log(range().from(1000))

const s1 = range().to(20).from(10)

console.log(s1.toArray())
console.log(range().toArray().length) //=> long random number above 50k

// Lazily logs numbers from 10 to 20
for (let i of range(10, 20)) {
  console.log(i)
}