Javascript迭代的技术定义是什么？您如何测试？ [英] What is the technical definition of a Javascript iterable and how do you test for it?

问题描述

我一直在实现一个有用的子类ES6 Set 对象。对于我的许多新方法，我想接受一个可以是另一个Set或者Array的参数，或者我可以重复的任何东西。我已经在我的界面中调用了一个可迭代的，只需使用 .forEach（）（对于一个集合或一个数组，它可以正常工作）示例代码： / p>

  //删除此集中位于otherIterable中的项目
 //返回删除的项目数量
 remove（otherIterable）{
 let cnt = 0; 
 otherIterable.forEach（item => {
 if（this.delete（item））{
 ++ cnt; 
} 
}）; 
 return cnt; 
} 
  

或

  //将所有项目从其他可迭代添加到此集合
 addTo（iterable） {
 iterable.forEach（item => {
 this.add（item）; 
}）; 
} 
  pre> 
 
 
但是，我怀疑我可能不会以ES6定义的方式真正支持任何迭代，所以我对一个Javascript迭代的真正定义感兴趣使用该术语作为ES6规范？
 
 
 
何你可以在ES6 Javascript中测试吗？ 
 
 
 
如何迭代一个通用的迭代？
 
 
 
我在ES6规范中发现了这样的短语： 
 
 
如果参数iterable存在，那么它应该是一个对象
，它实现一个@@ iterator方法，返回一个迭代器对象
生成一个两元素的数组类对象，其第一个元素是一个
值，它将被用作WeakMap键，第二个元素是
与该键相关联的值。 
 
 
但是，它指的是一个 @@ iterator方法似乎可以通过该属性名访问。
解决方案
 
 
 
什么是真正的定义一个使用该术语作为ES6规范的Javascript迭代？
 
 
 §25.1.1。 1 定义了 Iterable 接口。
 
 
 
它们是具有 Symbol.iterator  -keyed方法返回一个有效的迭代器（这反过来是一个预期的行为，因为它应该根据§25.1.1.2）。
 
 
如何在ES6 Javascript中测试？ 
 
 
我们无法测试 @@ iterator 方法返回而不调用它，我们无法测试结果是否符合 Iterator 接口，而不尝试运行它。最好的选择是做
  function looksIntable（o）{
 return typeof o [Symbol.iterator] = =功能; 
} 
  
然而，我通常不会测试这个，但只是让它失败， 
 
 
如何迭代一个通用的迭代？
 
 
 不要使用 forEach 。（实际上，永远不要使用 forEach  ES6中的任何地方）
 
 
 
正确的迭代方式是（... of ...）的 循环。它执行所有的迭代检查（使用摘要的  GetIterator 操作，并运行（甚至关闭）迭代器，并在非迭代值使用时抛出适当的 TypeError  。
 
I've been implementing a useful subclass of the ES6 Set object.  For many of my new methods, I want to accept an argument that can be either another Set or an Array, or really anything that I can iterate.  I've been calling that an "iterable" in my interface and just use .forEach() on it (which works fine for a Set or an Array.  Example code:
// remove items in this set that are in the otherIterable
// returns a count of number of items removed
remove(otherIterable) {
    let cnt = 0;
    otherIterable.forEach(item => {
        if (this.delete(item)) {
            ++cnt;
        }
    });
    return cnt;
}
Or
// add all items from some other iterable to this set
addTo(iterable) {
    iterable.forEach(item => {
        this.add(item);
    });
}
But, I suspect I may be not really supporting any iterable in the way that ES6 defines it so I'm interested in what the real definition of a Javascript iterable is using the term as the ES6 specification does?  

How do you test for it in ES6 Javascript?  

How should you iterate a generic iterable?

I've found phrases like this in the ES6 specification:

  
If the parameter iterable is present, it is expected to be an object
  that implements an @@iterator method that returns an iterator object
  that produces a two element array-like object whose first element is a
  value that will be used as a WeakMap key and whose second element is
  the value to associate with that key.
But, that refers to an @@iterator method which I don't seem to be able to access via that property name.
解决方案

  
What is the real definition of a Javascript iterable using the term as the ES6 specification does?
§25.1.1.1 defines "The Iterable Interface".

They're objects with a Symbol.iterator-keyed method that returns a valid Iterator (which in turn is an object expected to behave as it should according to §25.1.1.2).

  
How do you test for it in ES6 Javascript? 
We cannot test what the @@iterator method returns without calling it, and we cannot test whether the result conforms to the Iterator interface without trying to run it. The best bet would be to do
function looksIterable(o) {
    return typeof o[Symbol.iterator] == "function";
}
however I wouldn't usually test for this but simply let it fail with an exception when it's not iterable.

  
How should you iterate a generic iterable?
Don't use forEach. (In fact, never use forEach anywhere in ES6).

The proper way to iterate is a for (… of …) loop. It does all the checking for iterability (using the abstract GetIterator operation and running (and even closing) the iterator, and throws appropriate TypeErrors when used on non-iterable values.

                        
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