在Javascript中做什么~~（“双波浪”）？ [英] What does ~~ ("double tilde") do in Javascript?

问题描述

我今天正在查看一个在线游戏物理库，并遇到了~~运算符。我知道单个〜是一个按位NOT，是否会使~~一个NOT的NOT，它会返回相同的值，不是吗？

I was checking out an online game physics library today and came across the ~~ operator. I know a single ~ is a bitwise NOT, would that make ~~ a NOT of a NOT, which would give back the same value, wouldn't it?

推荐答案

它会删除小数点后的所有内容，因为按位运算符会将其操作数隐式转换为带符号的32位整数。这适用于操作数是（浮点）数字还是字符串，结果是数字。

It removes everything after the decimal point because the bitwise operators implicitly convert their operands to signed 32-bit integers. This works whether the operands are (floating-point) numbers or strings, and the result is a number.

换句话说，它产生：

function(x) {
  if(x < 0) return Math.ceil(x);
  else return Math.floor(x);
}

仅当 x 介于 - （2 < sup> 31 ）和2 ³¹ - 1.否则，将发生溢出并且数字将环绕。

only if x is between -(2³¹) and 2³¹ - 1. Otherwise, overflow will occur and the number will "wrap around".

这可能被认为有用的是将函数的字符串参数转换为数字，但由于存在溢出的可能性以及与非整数一起使用不正确，我不会那样使用它，除了code golf（ ie 以牺牲可读性和健壮性为代价，无意义地删除程序源代码中的字节。我会改用 + x 或 Number（x）。

This may be considered useful to convert a function's string argument to a number, but both because of the possibility of overflow and that it is incorrect for use with non-integers, I would not use it that way except for "code golf" (i.e. pointlessly trimming bytes off the source code of your program at the expense of readability and robustness). I would use +x or Number(x) instead.

数字-43.2，例如：

The number -43.2, for example is:

-43.2 ₁₀ = 11111111111111111111111111010101 ₂

-43.2₁₀ = 11111111111111111111111111010101₂

作为签名（二进制补码）32位二进制数。 （JavaScript忽略小数点后的内容。）反转位给出：

as a signed (two's complement) 32-bit binary number. (JavaScript ignores what is after the decimal point.) Inverting the bits gives:

NOT -43 ₁₀ = 00000000000000000000000000101010 ₂ = 42 ₁₀

NOT -43₁₀ = 00000000000000000000000000101010₂ = 42₁₀

再次反转给出：

NOT 42 ₁₀ = 11111111111111111111111111010101 ₂ = -43 ₁₀

NOT 42₁₀ = 11111111111111111111111111010101₂ = -43₁₀

这与 Math.floor（-43.2），因为负数被舍入为零，而不是远离它。 （地板函数，等于-44，总是向下舍入到下一个较低的整数，无论​​数字是正数还是负数。）

This differs from Math.floor(-43.2) in that negative numbers are rounded toward zero, not away from it. (The floor function, which would equal -44, always rounds down to the next lower integer, regardless of whether the number is positive or negative.)

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