var a = {"x": 1};var b = a;console.log(b.x); a.x = 2;console.log(b.x);a = {"x":3};console.log(b.x);a.x = 4;console.log(b.x);上面四項(xiàng)輸出打印a.x，輸出結(jié)果分別是1 2 2 2問(wèn)題：1.對(duì)象賦值是引用傳遞，但是var a = {"x": 1}，等價(jià)于var a = {x: 1}嗎？字符串做堆的指向棧?引用用可以用字符串來(lái)訪問(wèn)？  能不能分別解釋一下上面四項(xiàng)輸出？