实现迭代器

iterator.js

var util = {
    isType: function(value, type) {
        return Object.prototype.toString.call(value).toLowerCase().slice(8, -1) === type;
    }
};

function Iterator(list) {
  list = util.isType(list, 'array') ? list : [list];
  this._array = list;
  this._index = 0;
}

Iterator.prototype.next = function() {
  return this._index < this._array.length ? {value: this._array[this._index++], done: false} : {value: undefined, done: true};
}

Iterator.prototype.map = function(fn) {
  var newIterator = new Iterator(this._array);
  var _self = this;
  return {
    next: function() {
      var {done, value} = newIterator.next();
      return {
        done: done,
        value: done ? undefined : fn.call(_self, value)
      }
    }
  }
}