Javascript 浮點數計算誤差

var x = 5.5555;
var y = 2.2222;
var z = 1.1111;
var w = x+y+z;
alert(w); //8.8888
w -= x;
alert(w); //3.3332999999999995
alert(w.toFixed(4)); //3.3333

輸出結果
8.8888
3.3332999999999995
3.3333