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Showing content from https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Symbol/keyFor below:

Symbol.keyFor() - JavaScript | MDN

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Symbol.keyFor(sym) 方法用来获取全局 symbol 注册表中与某个 symbol 关联的键。

sym

必选参数，需要查找键值的某个 Symbol。

如果全局注册表中查找到该 symbol，则返回该 symbol 的 key 值，返回值为字符串类型。否则返回 undefined

// 创建一个全局 Symbol
var globalSym = Symbol.for("foo");
Symbol.keyFor(globalSym); // "foo"

var localSym = Symbol();
Symbol.keyFor(localSym); // undefined，

// 以下 Symbol 不是保存在全局 Symbol 注册表中
Symbol.keyFor(Symbol.iterator); // undefined

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