js自由控制数据小数位

//示例1
var num = 1.9999999995;
document.write(num.toFixed(2));  //显示2.00

//示例2
var num = 1 - 0.8;
document.write(num); //显示结果 1.99999999995
document.write(num.toFixed(2)); //2.00