text: 'use strict';
  // https://github.com/tc39/proposal-iterator-helpers
  var $ = require('../internals/export');
  var call = require('../internals/function-call');
  var aCallable = require('../internals/a-callable');
  var anObject = require('../internals/an-object');
  var getIteratorMethod = require('../internals/get-iterator-method');
  var createIteratorProxy = require('../internals/iterator-create-proxy');
  var iteratorClose = require('../internals/iterator-close');
  
  var IteratorProxy = createIteratorProxy(function () {
    var iterator = this.iterator;
    var mapper = this.mapper;
    var result, mapped, iteratorMethod, innerIterator;
  
    while (true) {
      try {
        if (innerIterator = this.innerIterator) {
          result = anObject(call(this.innerNext, innerIterator));
          if (!result.done) return result.value;
          this.innerIterator = this.innerNext = null;
        }
  
        result = anObject(call(this.next, iterator));
  
        if (this.done = !!result.done) return;
  
        mapped = mapper(result.value);
        iteratorMethod = getIteratorMethod(mapped);
  
        if (!iteratorMethod) {
          throw TypeError('.flatMap callback should return an iterable object');
        }
  
        this.innerIterator = innerIterator = anObject(call(iteratorMethod, mapped));
        this.innerNext = aCallable(innerIterator.next);
      } catch (error) {
        iteratorClose(iterator, 'throw', error);
      }
    }
  });
  
  $({ target: 'Iterator', proto: true, real: true }, {
    flatMap: function flatMap(mapper) {
      return new IteratorProxy({
        iterator: anObject(this),
        mapper: aCallable(mapper),
        innerIterator: null,
        innerNext: null
      });
    }
  });
  