Symbol.for()不会每次调用就返回一个新的 Symbol 类型的值,而是会先检查给定的key是否已经存在,如果不存在才会新建一个值
`
{
constFOO_KEY=Symbol.for('foo');
functionA(){
this.foo='hello';
}
if(!FOO_KEY){
FOO_KEY=newA();
}
returnFOO_KEY;
}
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