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Showing content from http://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/WeakMap/get below:

WeakMap.prototype.get() - JavaScript | MDN

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WeakMap 实例的 get() 方法返回该 WeakMap 中的指定元素。

const weakmap1 = new WeakMap();
const object1 = {};
const object2 = {};

weakmap1.set(object1, 42);

console.log(weakmap1.get(object1));
// Expected output: 42

console.log(weakmap1.get(object2));
// Expected output: undefined

key

要从 WeakMap 对象返回的元素的键。

该 WeakMap 对象中与指定键相关联的元素。如果找不到该键，则返回 undefined。如果 key 不是对象或非全局注册的符号，则始终返回 undefined。

const wm = new WeakMap();
wm.set(window, "foo");

wm.get(window); // 返回 "foo"。
wm.get("baz"); // 返回 undefined。

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