# 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?

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.)

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
``````

In ECMAScript 6, the equivalent of `~~` is Math.trunc:

Returns the integral part of a number by removing any fractional digits. It does not round any numbers.

``````Math.trunc(13.37)   // 13
Math.trunc(42.84)   // 42
Math.trunc(0.123)   //  0
Math.trunc(-0.123)  // -0
Math.trunc("-1.123")// -1
Math.trunc(NaN)     // NaN
Math.trunc("foo")   // NaN
Math.trunc()        // NaN
``````

The polyfill:

``````function trunc(x) {
return x < 0 ? Math.ceil(x) : Math.floor(x);
}
``````

The `~` seems to do `-(N+1)`. So `~2 == -(2 + 1) == -3` If you do it again on -3 it turns it back: `~-3 == -(-3 + 1) == 2` It probably just converts a string to a number in a round-about way.

Also, more detailed info is available here: http://dreaminginjavascript.wordpress.com/2008/07/04/28/

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

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

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

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  {
}
``````

source

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
``````