WeakMap的key不能是基本类型
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", "Whoops"); // Error, because "test" is not an object
如果key指向了null，则key会被自动回收
let john = { name: "John" }; let weakMap = new WeakMap(); weakMap.set(john, "..."); john = null; // overwrite the reference // john is removed from memory!
WeekMap只有如下的方法
- weakMap.get(key)
- weakMap.set(key, value)
- weakMap.delete(key)
- weakMap.has(key)
什么时候使用这个数据结构，如果key失去了引用，那么就自动回收它，比如用户和用户的访问量，如果用户离开了，那么它的访问量也就没有存在的价值了，此时可以使用这个数据结构进行存储
let visitsCountMap = new WeakMap(); // weakmap: user => visits count // increase the visits count function countUser(user) { let count = visitsCountMap.get(user) || 0; visitsCountMap.set(user, count + 1); }
另外一种用途就是缓存
let cache = new WeakMap(); // calculate and remember the result function process(obj) { if (!cache.has(obj)) { let result = /* calculate the result for */ obj; cache.set(obj, result); } return cache.get(obj); } // 📁 main.js let obj = {/* some object */}; let result1 = process(obj); let result2 = process(obj); // ...later, when the object is not needed any more: obj = null; // Can't get cache.size, as it's a WeakMap, // but it's 0 or soon be 0 // When obj gets garbage collected, cached data will be removed as well
WeakSet可以用来记录哪些人访问了网站
let visitedSet = new WeakSet(); let john = { name: "John" }; let pete = { name: "Pete" }; let mary = { name: "Mary" }; visitedSet.add(john); // John visited us visitedSet.add(pete); // Then Pete visitedSet.add(john); // John again // visitedSet has 2 users now // check if John visited? alert(visitedSet.has(john)); // true // check if Mary visited? alert(visitedSet.has(mary)); // false john = null; // visitedSet will be cleaned automatically
