javascript - 了解实现自定义迭代器

Question

我正在尝试理解 Ecmascript 6 迭代器并尝试创建一个行为与 native 数组非常相似的数据结构。

for (let i of [1,2,3]) console.log(i);  //Iterate over data set itself

将输出1,2,3

for (let i of [1,2,3].keys()) console.log(i); //Iterate over a custom iterator from a method

将输出0,1,2，和

var a = [1,2,3];
var keys = [...a.keys()];

将按预期包含 [0,1,2]。

因此，

console.log([1,2,3].keys().next());

将输出Object {value: 0, done: false}

现在我创建了一种新的数据类型，并试图让它以同样的方式运行。

var myDogs = function(dogs) {
 this.dogs = dogs;
 this[Symbol.iterator] = () => {
   let i = -1;
   return {
    next() {
     i++;
     var dog = Object.keys(dogs)[i];
     if (!dog) return {done:true};
     return {value:{ dog, hungry:dogs[dog] }, done:false};
    }
   };
 };
 this.dogsNames = () => {
  return {
   [Symbol.iterator]() {
     let i = -1;
     return {
      next() {
       i++;
       var dog = Object.keys(dogs)[i];
       if (!dog) return {done:true};
       return {value: dog, done:false};
      }
     };
   }
  }
 }
};

var dogs = new myDogs({ buddy: true, hasso: false });

这按预期工作(自定义迭代器 - thank you ):

var dogHungryMap = [...dogs];
dogHungryMap == [{ dog: 'buddy', hungry: true }, { dog: 'hasso': hungry: false }]

迭代器 dogsNames() 按预期几乎 工作。这没关系:

var names = [...dogs.dogsNames()];
names == ["buddy", "hasso"]

但这不是:

dogs.dogsNames().next()
VM19728:2 Uncaught TypeError: dogs.dogsNames(...).next is not a function(…)

为什么以及如何重现原生数组的行为？

Answer 1

因为 dogsNames() 返回一个以迭代器为键的对象。因此，您可以在 dogsNames 上使用 for...of，但要直接访问 next()，您需要访问在对象上声明的迭代器函数，如下所示:

dogsNames()[Symbol.iterator]().next()

Babel REPL Example

为了完整起见，共享 next() 函数的完整代码:

var myDogs = function(dogs) {
 this.dogs = dogs;
 let i = -1;
 var iter = {
   next() {
     i++;
     var dog = Object.keys(dogs)[i];
     if (!dog) return {done:true};
     return {value:{ dog, hungry:dogs[dog] }, done:false};
   }
 }
 this[Symbol.iterator] = () => iter;
 this.next = iter.next;
};
var dogList = new myDogs({
  dog1: "no",
  dog2: "yes"
});
for(const x of dogList) console.log(x);
console.log(dogList.next());
console.log(dogList.next());