Babel-transpiled generator(fibo)

fibo-babel.js

"use strict";

var _marked = [fibo].map(regeneratorRuntime.mark);

function fibo() {
  var a, b;
  return regeneratorRuntime.wrap(function fibo$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
          a = 0, b = 1;

        case 1:
          if (!true) {
            _context.next = 8;
            break;
          }

          _context.next = 4;
          return a;

        case 4:
          b = a + b;
          a = b - a;
          _context.next = 1;
          break;

        case 8:
        case "end":
          return _context.stop();
      }
    }
  }, _marked[0], this);
}

fibo.js

function* fibo() {
  let a=0, b=1
  while (true) {
    yield a
    b = a + b
    a = b - a
  }
}