一个console.log的问题

Question

这里的 aa 为什么值不一样呢?var abc = {aa: null, bb: null, cc: null, dd: null, aa1: null, bb1: null, cc1: null, dd1: null};function set_t(board, loc, tof) {    board[loc] = tof;    return board}console.log(abc)set_t(abc, "aa", 2);