摘要：心血來潮地跑到做了個，結果當然是慘不忍睹，發現自己對不少的基礎知識的理解是模模糊糊，甚至是錯的。讀到又琢磨了一會兒，才搞明白為啥補碼這么反直覺。二補碼只能腦補，或者用代碼打印腦補的內容。寫了半天，一言以蔽之相當于調用參考鏈接

心血來潮地跑到Upworks做了個JavaScript Test，結果當然是慘不忍睹，發現自己對不少JavaScript的基礎知識的理解是模模糊糊，甚至是錯的。

比如這題：

~-(2+"2")

這個表達式的值是21，我腦補了好久也得不到這個答案，這才發現，我完全不理解Bitwise NOT操作符。

不懂就補嘍。

這回還真是“補”，回想起大學那會兒，學到“補碼”時，腦子里就是一團霧，原來那團迷霧到今天也沒散，真他么黏糊。

讀到 Why is ~5 === -6 in JavaScript?：

It does indeed perform a bit-wise NOT, the negative number is in two"s complement. So the value 1010 is -6.
Two"s complement basically works by the very left-most bit signifies a negative number and is taken as a negative value. All other 1 bits are added to this number. For example:
1010 => (-8 +0 +2 +0) => -6
1111 => (-8 +4 +2 +1) => -1

又琢磨了一會兒，才搞明白為啥補碼這么反直覺。

“二補碼”只能腦補，或者用代碼打印腦補的內容。

在JavaScript里，如果用number.toString(2)，結果是這樣：

Decimal:   5  |  Binary: 00000000000000000000000000000101
Decimal:   4  |  Binary: 00000000000000000000000000000100
Decimal:   3  |  Binary: 00000000000000000000000000000011
Decimal:   2  |  Binary: 00000000000000000000000000000010
Decimal:   1  |  Binary: 00000000000000000000000000000001
Decimal:   0  |  Binary: 00000000000000000000000000000000
Decimal:  -0  |  Binary: 00000000000000000000000000000000
Decimal:  -1  |  Binary: 000000000000000000000000000000-1
Decimal:  -2  |  Binary: 00000000000000000000000000000-10
Decimal:  -3  |  Binary: 00000000000000000000000000000-11
Decimal:  -4  |  Binary: 0000000000000000000000000000-100
Decimal:  -5  |  Binary: 0000000000000000000000000000-101

這個結果符合直覺，但加法器的實現只會使用二補碼，下面是加法器實際使用的“二補碼”：

Decimal:   5  |  Binary: 00000000000000000000000000000101
Decimal:   4  |  Binary: 00000000000000000000000000000100
Decimal:   3  |  Binary: 00000000000000000000000000000011
Decimal:   2  |  Binary: 00000000000000000000000000000010
Decimal:   1  |  Binary: 00000000000000000000000000000001
Decimal:   0  |  Binary: 00000000000000000000000000000000
Decimal:  -0  |  Binary: 00000000000000000000000000000000
Decimal:  -1  |  Binary: 11111111111111111111111111111111
Decimal:  -2  |  Binary: 11111111111111111111111111111110
Decimal:  -3  |  Binary: 11111111111111111111111111111101
Decimal:  -4  |  Binary: 11111111111111111111111111111100
Decimal:  -5  |  Binary: 11111111111111111111111111111011

理解了二補碼，再來看-22怎么被補成21的：

Decimal: -22  |  Binary: 11111111111111111111111111101010
->
Decimal:  21  |  Binary: 00000000000000000000000000010101

Python對此解釋得更直接：

~ x
Returns the complement of x - the number you get by switching each 1 for a 0 and each 0 for a 1. 
This is the same as -x - 1.

我連補碼都沒鬧明白，竟然過了關還Score top 30%，可見其他人都沒作弊，我真是無恥的程序員。

寫了半天，一言以蔽之：

~x 相當于調用

function twosComplement(x){
    return 0 -x - 1;
}

參考鏈接：

Why is ~5 === -6 in JavaScript?

Why does bitwise “not 1” equal -2?

MDN Bitwise operators

Python"s bitwise operators.

Two"s Complement