What does ~~ (“double tilde”) do in Javascript?

Question

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

Answer 1

Just a bit of a warning. The other answers here got me into some trouble.

The intent is to remove anything after the decimal point of a floating point number, but it has some corner cases that make it a bug hazard. I'd recommend avoiding ~~.

First, ~~ doesn't work on very large numbers.

~~1000000000000 == -727279968

As an alternative, use Math.trunc() (as Gajus mentioned, Math.trunc() returns the integer part of a floating point number but is only available in ECMAScript 6 compliant JavaScript). You can always make your own Math.trunc() for non-ECMAScript-6 environments by doing this:

if(!Math.trunc){
    Math.trunc = function(value){
        return Math.sign(value) * Math.floor(Math.abs(value));
    }
}

I wrote a blog post on this for reference: http://bitlords.blogspot.com/2016/08/the-double-tilde-x-technique-in.html

Answer 2

Given ~N is -(N+1), ~~N is then -(-(N+1) + 1). Which, evidently, leads to a neat trick.

Answer 3

The first ~ operator forces the operand to an integer (possibly after coercing the value to a string or a boolean), then inverts the lowest 31 bits. Officially ECMAScript numbers are all floating-point, but some numbers are implemented as 31-bit integers in the SpiderMonkey engine.

You can use it to turn a 1-element array into an integer. Floating-points are converted according to the C rule, ie. truncation of the fractional part.

The second ~ operator then inverts the bits back, so you know that you will have an integer. This is not the same as coercing a value to boolean in a condition statement, because an empty object {} evaluates to true, whereas ~~{} evaluates to false.

js>~~"yes"
0
js>~~3
3
js>~~"yes"
0
js>~~false
0
js>~~""
0
js>~~true
1
js>~~"3"
3
js>~~{}
0
js>~~{a:2}
0
js>~~[2]
2
js>~~[2,3]
0
js>~~{toString: function() {return 4}}
4
js>~~NaN
0
js>~~[4.5]
4
js>~~5.6
5
js>~~-5.6
-5

Answer 4

Converting Strings to Numbers

console.log(~~-1);    // -1
console.log(~~0);     // 0
console.log(~~1);     // 1
console.log(~~"-1");  // -1
console.log(~~"0");   // 0
console.log(~~"1");   // 1
console.log(~~true);  // 1
console.log(~~false); // 0

~-1 is 0

if (~someStr.indexOf("a")) {
  // Found it
} else  {
  // Not Found
}

source

Answer 5

Tilde(~) has an algorihm -(N+1)

For examle:

~0 = -(0+1) = -1
~5 = -(5+1) = -6
~-7 = -(-7+1) = 6

Double tilde is -(-(N+1)+1)

For example:

~~5 = -(-(5+1)+1) = 5
~~-3 = -(-(-3+1)+1) = -3

Triple tilde is -(-(-(N+1)+1)+1)

For example:

~~~2 = -(-(-(2+1)+1)+1) = -3
~~~3 = -(-(-(3+1)+1)+1) = -4

Answer 6

Here is an example of how this operator can be used efficiently, where it makes sense to use it:

leftOffset = -(~~$('html').css('padding-left').replace('px', '') + ~~$('body').css('margin-left').replace('px', '')),

Source:

See section Interacting with points