意外的右移操作員行為

Question

請查看以下Node.js REPL輸出：

> n=10
10
> console.log(n.toString(2)); //ok ?
1010
undefined
> m=n>>1 
5
> console.log(m.toString(2)); //cool :)
101

這正在按預期方式工作。 現在來看這個：

> n=Number.MAX_VALUE
1.7976931348623157e+308
> m=n>>1
0
> n.toString(16) // to read easier
'fffffffffffff800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'
> m.toString(16) // to read easier
'0'

剛剛發生什么事了？ 我不應該右移

fffffffffffff800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

1位數給我

7ffffffffffffc00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

我怎么變成零呢？

另外，我注意到這不僅僅是V8 / Node問題。 我在Firebug中得到了相同的結果。

Answer 1

Javascript的位級操作僅適用於32位帶符號的值（對於零擴展>>>運算符，則為無符號）。 在進行位操作之前，高於這32位的任何內容都將設置為零。

Answer 2

在JavaScript中，當您計算x >> y ， x會轉換為帶符號的32位整數。 然后將其右移y位置。

您的值0xfffffffffffff800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000因為這是最低的32位 右移將得到零。