javascript - 如何在 Javascript 中观察 WeakMap 的垃圾收集？

Question

这是我对WeakMap的理解“对集合中对象的引用被弱保存。如果没有对存储在 WeakMap 中的对象的其他引用，它们可以被垃圾回收。”

为什么在删除引用后，WeakMap 中仍会出现以下键/值对？ WeakMap 不应该为空吗？

let dog1 = {name: 'Snickers'};
let dog2 = {name: 'Sunny'};

var strong = new Map();
var weak = new WeakMap();


strong.set(dog1, 'Snickers is the best!');
strong.set(dog2, 'Sunny is the 2nd best!');
weak.set(dog1, 'Snickers is the best!');
weak.set(dog2, 'Sunny is the 2nd best!');

dog1 = null;
dog2 = null;

console.log(strong);
console.log(weak);

/*
Output
Map(2) {{…} => "Snickers is the best!", {…} => "Sunny is the 2nd best!"}
WeakMap {{…} => "Snickers is the best!", {…} => "Sunny is the 2nd best!"}
*/

setTimeout(function(){
console.log("1200ms later... waiting for garbarge collection");
console.log(strong);
console.log(weak);
}, 1200);

/*
Output
Map(2) {{…} => "Snickers is the best!", {…} => "Sunny is the 2nd best!"}
WeakMap {{…} => "Snickers is the best!", {…} => "Sunny is the 2nd best!"}
*/

Answer 1

在修复了@Amadan @Bergi @Geeganage 指出的错误后，以下代码在 Safari 和 Chrome 中提供了所需的输出，垃圾收集在几个新添加的超时之一之后运行。导致最终的 WeakMap 没有保留任何引用。

let dog1 = {name: 'Snickers'};
let dog2 = {name: 'Sunny'};

var strong = new Map();
var weak = new WeakMap();


strong.set(dog2, 'Sunny is the 2nd best!');
weak.set(dog1, 'Snickers is the best!');

dog1 = null;
dog2 = null;

console.log(strong);
console.log(weak);

/*
Output
[Log] Map {{name: "Sunny"} => "Sunny is the 2nd best!"} (1)
[Log] WeakMap {{name: "Snickers"} => "Snickers is the best!"} (1)
*/

setTimeout(function(){
console.log("1200ms later... waiting for garbarge collection");
console.log(strong);
console.log(weak);
}, 1200);

setTimeout(function(){
console.log("3200ms later... waiting for garbarge collection");
console.log(strong);
console.log(weak);
}, 3200);

setTimeout(function(){
console.log("6200ms later... waiting for garbarge collection");
console.log(strong);
console.log(weak);
}, 6200);

setTimeout(function(){
console.log("12200ms later... waiting for garbarge collection");
console.log(strong);
console.log(weak);
}, 12200);

/* 
Output (eventually)
[Log] Map {{name: "Sunny"} => "Sunny is the 2nd best!"} (1)
[Log] WeakMap {} (0)
*/

由于垃圾回收不能保证运行，如果你在滚动一个 JS 繁重的网站时将代码粘贴到控制台中，GC 可能实际上会在第一次超时后运行。