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`)