obj[sAttr0] = () => {    return ++obj[sAttr0];};這段函數怎么理解？console.log(obj[sAttr0]());為什么輸出NaNreturn ++obj[sAttr0];這里有什么用let obj = {};let sAttr0 = Symbol("hehe");let sAttr1 = Symbol("meme");obj[sAttr1] = 1;console.log(sAttr1);console.log(obj[sAttr1]);obj[sAttr0] = () => {    return ++obj[sAttr0];};console.log(obj[sAttr0]);console.log(obj[sAttr0]());