What's the difference between ES6 Map and WeakMap?

问题

Looking this and this MDN pages it seems like the only difference between Maps and WeakMaps is a missing "size" property for WeakMaps. But is this true? What's the difference between them?

回答1:

From the very same page, section "Why Weak Map?":

The experienced JavaScript programmer will notice that this API could be implemented in JavaScript with two arrays (one for keys, one for values) shared by the 4 API methods. Such an implementation would have two main inconveniences. The first one is an O(n) search (n being the number of keys in the map). The second one is a memory leak issue. With manually written maps, the array of keys would keep references to key objects, preventing them from being garbage collected. In native WeakMaps, references to key objects are held "weakly", which means that they do not prevent garbage collection in case there would be no other reference to the object.

Because of references being weak, WeakMap keys are not enumerable (i.e. there is no method giving you a list of the keys). If they were, the list would depend on the state of garbage collection, introducing non-determinism.

[And that's why they have no size property as well]

If you want to have a list of keys, you should maintain it yourself. There is also an ECMAScript proposal aiming at introducing simple sets and maps which would not use weak references and would be enumerable.

‐ which would be the "normal" Maps. Not mentioned at MDN, but in the harmony proposal, those also have items, keys and values generator methods and implement the Iterator interface.

回答2:

They both behave differently when a object referenced by their keys/values gets deleted. Lets take the below example code:

var map = new Map();
var weakmap = new WeakMap();

(function(){
    var a = {x: 12};
    var b = {y: 12};

    map.set(a, 1);
    weakmap.set(b, 2);
})()

The above IIFE is executed there is no way we can reference {x: 12} and {y: 12} anymore. Garbage collector goes ahead and deletes the key b pointer from “WeakMap” and also removes {y: 12} from memory. But in case of “Map”, the garbage collector doesn’t remove a pointer from “Map” and also doesn’t remove {x: 12} from memory.

Summary: WeakMap allows garbage collector to do its task but not Map.

References: http://qnimate.com/difference-between-map-and-weakmap-in-javascript/

回答3:

Maybe the next explanation will be more clear for someone.

var k1 = {a: 1};
var k2 = {b: 2};

var map = new Map();
var wm = new WeakMap();

map.set(k1, 'k1');
wm.set(k2, 'k2');

k1 = null;
map.forEach(function (val, key) {
    console.log(key, val); // k1 {a: 1}
});

k2 = null;
wm.get(k2); // undefined

As you see, after removing k1 key from the memory we can still access it inside the map. At the same time removing k2 key of WeakMap removes it from wm as well by reference.

That's why WeakMap hasn't enumerable methods like forEach, because there is no such thing as list of WeakMap keys, they are just references to another objects.

回答4:

Another difference (source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap):

Keys of WeakMaps are of the type Object only. Primitive data types as keys are not allowed (e.g. a Symbol can't be a WeakMap key).

Nor can a string, number, or boolean be used as a WeakMap key. A Map can use primitive values for keys.

w = new WeakMap;
w.set('a', 'b'); // Uncaught TypeError: Invalid value used as weak map key

m = new Map
m.set('a', 'b'); // Works

回答5:

WeapMap in javascript does not hold any keys or values, it just manipulate key value using a unique id and define property to key object.

because it define property to key by method Object.definePropert(), key must not be primitive type.

and also because WeapMap does not contain actually key value pairs, we cannot get length property of weakmap.

and also manipulated value is assigned back to key, garbage collector easily can collect key if it in no use.

Sample code for implementation.

if(typeof WeapMap != undefined){
return;
} 
(function(){
   var WeapMap = function(){
      this.__id = '__weakmap__';
   }

   weakmap.set = function(key,value){
       var pVal = key[this.__id];
        if(pVal && pVal[0] == key){
           pVal[1]=value;
       }else{
          Object.defineProperty(key, this.__id, {value:[key,value]});
          return this;
        }
   }

window.WeakMap = WeakMap;
})();

reference of implementation

回答6:

WeakMap keys must be objects, not primitive values.

let weakMap = new WeakMap();

let obj = {};

weakMap.set(obj, "ok"); // works fine (object key)

// can't use a string as the key
weakMap.set("test", "Not ok"); // Error, because "test" is not an object

Why????

Let's see below example.

let user = { name: "User" };

let map = new Map();
map.set(user, "...");

user = null; // overwrite the reference

// 'user' is stored inside the map,
// We can get it by using map.keys()

If we use an object as the key in a regular Map, then while the Map exists, that object exists as well. It occupies memory and may not be garbage collected.

WeakMap is fundamentally different in this aspect. It doesn’t prevent garbage-collection of key objects.

let user = { name: "User" };

let weakMap = new WeakMap();
weakMap.set(user, "...");

user = null; // overwrite the reference

// 'user' is removed from memory!

if we use an object as the key in it, and there are no other references to that object – it will be removed from memory (and from the map) automatically.

WeakMap does not support iteration and methods keys(), values(), entries(), so there’s no way to get all keys or values from it.

WeakMap has only the following methods:

weakMap.get(key)
weakMap.set(key, value)
weakMap.delete(key)
weakMap.has(key)

That is obvious as if an object has lost all other references (like 'user' in the code above), then it is to be garbage-collected automatically. But technically it’s not exactly specified when the cleanup happens.

The JavaScript engine decides that. It may choose to perform the memory cleanup immediately or to wait and do the cleaning later when more deletions happen. So, technically the current element count of a WeakMap is not known. The engine may have cleaned it up or not or did it partially. For that reason, methods that access all keys/values are not supported.

Note:- The main area of application for WeakMap is an additional data storage. Like caching an object until that object gets garbage collected.

来源：https://stackoverflow.com/questions/15604168/whats-the-difference-between-es6-map-and-weakmap

标签

javascript

ecmascript-6

ecmascript-harmony

weakmap