fix:向0取整 fix(-3.1) = -3; fix(3.1) = 3;floor:向负无穷取整 floor(-3.1) = -4; floor(3.1) = 3;ceil:向正无穷取整 ceil(-3.1) = -3; ceil(3.1) = 4;round:四舍五入 round(-3.1) = -3; round(3.1) = 3;