我刚刚发现了这个特点:
Map: Map对象是简单的键/值映射。
这让我很困惑。常规JavaScript对象是字典，那么Map与字典有什么不同呢?从概念上讲，它们是相同的(根据Stack Overflow的另一个问题)
文档也没有帮助:
Map对象是键/值对的集合，其中键和值都可以是任意的ECMAScript语言值。不同的键值只能出现在Map集合中的一个键/值对中。使用创建Map时选择的比较算法进行区分的不同键值。
Map对象可以按插入顺序迭代其元素。Map对象必须使用哈希表或其他机制来实现，这些机制提供的访问时间平均与集合中元素的数量呈次线性关系。本Map对象规范中使用的数据结构仅用于描述Map对象所需的可观察语义。它并不是一个可行的实现模型。
听起来还是像个物件，显然我错过了什么。
为什么JavaScript获得一个(受良好支持的)Map对象?它能做什么?
当前回答
TL;DR:你不能在对象中使用键' length'来存储值，但在Map中可以。
对于我来说，不使用object而使用Map的实际考虑是，从实际的角度来看，object的键不是字符串或数字，而是字符串或数字的子集。具体来说，使用与原型的属性名或众所周知的属性冲突的键可能会导致问题。例如，存储具有键长度的值可能会使使用length键来确定给定对象是否为数组的代码混淆。
其他回答
这是我记住它的一个简单方法:KOI
Keys. Object key is strings or symbols. Map keys can also be numbers (1 and "1" are different), objects, NaN, etc. It uses === to distinguish between keys, with one exception NaN !== NaN but you can use NaN as a key. Order. The insertion order is remembered. So [...map] or [...map.keys()] has a particular order. Interface. Object: obj[key] or obj.a (in some language, [] and []= are really part of the interface). Map has get(), set(), has(), delete() etc. Note that you can use map[123], but that is using it as a plain JavaScript object.
根据MDN:
Map对象可以按插入顺序迭代其元素——for..of循环每次迭代将返回一个[key, value]数组。
and
Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Because of this, Objects have been used as Maps historically; however, there are important differences between Objects and Maps that make using a Map better. An Object has a prototype, so there are default keys in the map. However, this can be bypassed using map = Object.create(null). The keys of an Object are Strings, where they can be any value for a Map. You can get the size of a Map easily while you have to manually keep track of size for an Object.
Map
按顺序迭代是开发人员一直想要的功能，部分原因是它可以确保在所有浏览器中都具有相同的性能。所以对我来说，这是一个大问题。
myMap.has(key)方法将特别方便，还有myMap.has(key)方法。大小属性。
除了上面提到的可用性差异，如果你对大型对象的性能差异更感兴趣，普通对象在chrome上设置、更新和删除大量数据时似乎要快2倍。
Experiment here: https://perf.link/#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
除了按定义良好的顺序可迭代，以及能够使用任意值作为键(除了-0)之外，map还很有用，原因如下:
The spec enforces map operations to be sublinear on average. Any non-stupid implementation of object will use a hash table or similar, so property lookups will probably be constant on average. Then objects could be even faster than maps. But that is not required by the spec. Objects can have nasty unexpected behaviors. For example, let's say you didn't set any foo property to a newly created object obj, so you expect obj.foo to return undefined. But foo could be built-in property inherited from Object.prototype. Or you attempt to create obj.foo by using an assignment, but some setter in Object.prototype runs instead of storing your value. Maps prevent these kind of things. Well, unless some script messes up with Map.prototype. And Object.create(null) would work too, but then you lose the simple object initializer syntax.
这两个技巧可以帮助你决定是使用Map还是Object:
当键在运行时之前未知时，以及当 所有键都是相同的类型，所有值都是相同的类型。 如果需要将原始值存储为键，则使用映射 因为object将每个键都视为字符串，要么是数字值， 布尔值或任何其他基本值。 当存在操作单个元素的逻辑时使用对象。
来源:键集合
