摘要：心血來潮地跑到做了個(gè)，結(jié)果當(dāng)然是慘不忍睹，發(fā)現(xiàn)自己對(duì)不少的基礎(chǔ)知識(shí)的理解是模模糊糊，甚至是錯(cuò)的。讀到又琢磨了一會(huì)兒，才搞明白為啥補(bǔ)碼這么反直覺。二補(bǔ)碼只能腦補(bǔ)，或者用代碼打印腦補(bǔ)的內(nèi)容。寫了半天，一言以蔽之相當(dāng)于調(diào)用參考鏈接

心血來潮地跑到Upworks做了個(gè)JavaScript Test，結(jié)果當(dāng)然是慘不忍睹，發(fā)現(xiàn)自己對(duì)不少JavaScript的基礎(chǔ)知識(shí)的理解是模模糊糊，甚至是錯(cuò)的。

比如這題：

~-(2+"2")

這個(gè)表達(dá)式的值是21，我腦補(bǔ)了好久也得不到這個(gè)答案，這才發(fā)現(xiàn)，我完全不理解Bitwise NOT操作符。

不懂就補(bǔ)嘍。

這回還真是“補(bǔ)”，回想起大學(xué)那會(huì)兒，學(xué)到“補(bǔ)碼”時(shí)，腦子里就是一團(tuán)霧，原來那團(tuán)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(huì)兒，才搞明白為啥補(bǔ)碼這么反直覺。

“二補(bǔ)碼”只能腦補(bǔ)，或者用代碼打印腦補(bǔ)的內(nèi)容。

在JavaScript里，如果用number.toString(2)，結(jié)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(gè)結(jié)果符合直覺，但加法器的實(shí)現(xiàn)只會(huì)使用二補(bǔ)碼，下面是加法器實(shí)際使用的“二補(bǔ)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(bǔ)碼，再來看-22怎么被補(bǔ)成21的：

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

Python對(duì)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(bǔ)碼都沒鬧明白，竟然過了關(guān)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(dāng)于調(diào)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