如何可靠地检查对象是 EcmaScript 6 Map/Set?

2022-01-17 00:00:00 dictionary set collections javascript ecmascript-6

我只想检查一个对象是 Map 或 Set 而不是 Array.

I just want to check that an object is a Map or Set and not an Array.

要检查一个数组，我正在使用 lodash 的 _.isArray.

to check an Array I'm using lodash's _.isArray.

function myFunc(arg) {
  if (_.isArray(arg)) {
    // doSomethingWithArray(arg)
  }

  if (isMap(arg)) {
    // doSomethingWithMap(arg)
  }

  if (isSet(arg)) {
    // doSomethingWithSet(arg)
  }
}

如果我要实现 isMap/isSet，它需要是什么样的?如果可能的话，我希望它能够捕获 Map/Set 的子类.

If I were to implement isMap/isSet, what does it need to look like? I'd like for it to be able to catch subclasses of Map/Set if possible as well.

推荐答案

这种情况类似于ES5之前的方法来正确可靠地检测数组.有关实现 isArray 的可能陷阱.

The situation is similar to pre-ES5 methods to detect arrays properly and reliably. See this great article for the possible pitfalls of implementing isArray.

我们可以使用

  • obj.constructor == Map/Set，但这对子类实例不起作用(并且很容易被欺骗)
  • obj instanceof Map/Set，但这仍然不能跨领域工作(并且可能被原型修改所欺骗)
  • obj[Symbol.toStringTag] == "Map"/"Set"，但是这很容易被再次欺骗.
  • obj.constructor == Map/Set, but that doesn't work on subclass instances (and can easily be deceived)
  • obj instanceof Map/Set, but that still doesn't work across realms (and can be deceived by prototype mangling)
  • obj[Symbol.toStringTag] == "Map"/"Set", but that can trivially be deceived again.

确实，我们需要测试一个对象是否有一个 [[MapData]]/[[SetData]] 内部槽.这不是那么容易访问 - 它是内部的.不过，我们可以使用 hack:

To be really sure, we'd need to test whether an object has a [[MapData]]/[[SetData]] internal slot. Which is not so easily accessible - it's internal. We can use a hack, though:

function isMap(o) {
    try {
        Map.prototype.has.call(o); // throws if o is not an object or has no [[MapData]]
        return true;
    } catch(e) {
        return false;
    }
}
function isSet(o) {
    try {
        Set.prototype.has.call(o); // throws if o is not an object or has no [[SetData]]
        return true;
    } catch(e) {
        return false;
    }
}

对于一般用途，我推荐 instanceof - 它简单、易懂、高效，适用于大多数合理的情况.或者你马上去鸭子打字，只检查对象是否有 has/get/set/delete/add/delete 方法.

For common use, I'd recommend instanceof - it's simple, understandable, performant, and works for most reasonable cases. Or you go for duck typing right away and only check whether the object has has/get/set/delete/add/delete methods.

相关文章