Shane Tomlinson Shane Tomlinson - 3 months ago 22
Javascript Question

What does ~~ ("double tilde") do in Javascript?

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?

Answer

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.

In other words, it yields:

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

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

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.


How this is the NOT of the NOT

The number -43.2, for example is:

-43.210 = 111111111111111111111111110101012

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

NOT -4310 = 000000000000000000000000001010102 = 4210

Inverting again gives:

NOT 4210 = 111111111111111111111111110101012 = -4310

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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