update weakmap and weakset

5.7weak/t.js

@@ -0,0 +1,48 @@
+console.log("js garbage recyle");
+let john = {name: "John"};
+john = null;
+console.log(john);
+
+john = {name: "John"};
+array = [john];
+john = null;
+console.log(`array[0]: ${array[0].name}`);
+
+console.log("==========WeakMap==========");
+console.log("WeakMap must use obj as keys in map, the string or other primitive values are not allowed");
+let weakMap = new WeakMap();
+let obj = {};
+weakMap.set(obj, "ok");
+console.log(`weakMap.set(obj, "ok");`)
+console.log(`weakMap.get(obj): ${weakMap.get(obj)}`)
+//weakMap.set("test", "Whoops");
+console.log("If we use an object as the key, and this object has no other references to that obj, it will be removed from the memory automatically");
+
+console.log(`obj = null`)
+obj = null;
+console.log(`obj has been removed from the memory`)
+console.log(`obj cannot be used in map as well`)
+
+console.log(`weakmap only has four methods:
+    weakMap.set(key, value)
+    weakMap.get(key)
+    weakMap.delete(key)
+    weakMap.has(key)`)
+
+console.log(`Useage: if the obj dies, the secret disappears:
+    weakMap.set(john, "secret documents);`)
+
+let visitsCountMap = new Map();
+
+function countUser(user){
+    let count = visitsCountMap.get(user) || 0;
+    visitsCountMap.set(user, count + 1);
+}
+
+console.log(`weakset only has three methods:
+    weakSet.add(key)
+    weakSet.delete(key)
+    weakSet.has(key)`)
+
+console.log(`both weakset and weakmap has no length/size method`)
+