snap.data()返回原始JavaScript对象，其属性包含文档中字段的值.它不包含名为 ref 的属性(除非您有一个文档字段也称为ref).

snap.data() returns a raw JavaScript object whose properties contain the values of the fields in the document. It does not contain a property called ref (unless you had a document field also called ref).

如果需要写回已更改的文档，请使用 snap.ref 中提供的DocumentReference类型对象.另请参阅传递给 DocumentSnapshot 类型对象的API文档.功能.

If you need to write back to the document that changed, use the DocumentReference type object provided in snap.ref. See also the API documentation for the DocumentSnapshot type object passed to the function.

snap.ref.set(...)