瞭解實現自定義迭代器

Question

我想了解Ecmascript 6迭代器，並試圖創建一個行爲非常像本地數組的數據結構。

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

將輸出1,2,3

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

將輸出0,1,2，和

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

將包含[0,1,2]如預期。

因此，

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

將輸出Object {value: 0, done: false}

現在，我創建了一個新的數據類型，並試圖使其行爲方式相同。

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 }); 

可正常工作（自定義迭代器 - thank you）：

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

dogsNames()工作幾乎如預期的迭代器。這是OK：

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

但這並不：

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

爲什麼，我怎麼能重現本地陣列的行爲？

Answer 1

dogNames必須返回一個迭代（一些實現next方法），這也是一個可迭代（一些implemen方法）（並返回它自己）。 Every built-in iterator does that。

快速測試驗證：

var it = [].keys(); 
it[Symbol.iterator]() === it // true 

所以，你可以更改您的代碼

this.dogsNames =() => { 
    let i = -1; 
    return { 

    [Symbol.iterator]() { 
     return this; 
    }, 

    next() { 
     i++; 
     var dog = Object.keys(dogs)[i]; // should probably also be put in the outer function 
     if (!dog) return {done:true}; 
     return {value: dog, done:false}; 
    }, 

    }; 
}; 

Answer 2

因爲dogsNames()返回一個具有迭代器作爲關鍵字的對象。所以，你可以使用for...of在dogsNames，但訪問next()直接您需要訪問聲明的對象上，像這樣的迭代器功能：

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

Babel REPL Example

以及物品是否完整，未來共享的完整代碼（ ）函數：

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());