immutable-fib-lazy-seq.js

const Immutable = require('immutable')
const Range = Immutable.Range

const fib = n => {
  switch (n) {
    case 0:
      return 1;
    case 1:
      return 1;
    default:
      return fib(n - 1) + fib(n - 2)
  }
}

const Fib = Range(0, Infinity).map(fib)

console.log(Fib.get(20))