(ECMAScript7规范中的ToPrimitive抽象操作)https://note.xiexuefeng.cc/post/valueof-tostring-toprimitive
(关于valueOf、toString和[Symbol.toPrimitive]方法)https://segmentfault.com/a/1190000016325587
(ECMAScript7规范中的ToPrimitive抽象操作)https://note.xiexuefeng.cc/post/valueof-tostring-toprimitive
(关于valueOf、toString和[Symbol.toPrimitive]方法)https://segmentfault.com/a/1190000016325587