What does ~~ do in JavaScript/node.js?

Question

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What is the “double tilde” (~~) operator in JavaScript?

I found this snip of code in a node.js library's source. What effect does ~~ have on the input variable?

inArray[3] = ~~input;

It's also used in other ways:

return ~~ ((a - b) / 864e5 / 7 + 1.5);

Makes code confusing to read -_-. I hate how people are doing this recently; it makes me very angry. — Domenic, Apr 10 '12 at 18:34
@jasonbar can't search for ~~, but I did look :( For some reason "double tilde" never occurred to me! :) — Kato, Apr 10 '12 at 18:39
@Kato: Just Google for a list of operators, then when you find a page of them, hit Ctrl-F (or Cmd-F), ~. :p — Elliot Bonneville, Apr 10 '12 at 18:39
@ElliotBonneville I started on Google with operators in mind. Sometimes it's just easier to ask Humans. ;) — Kato, Apr 10 '12 at 18:42
What library is that by the way? Curious as to the purpose of `return ~~ ((a - b) / 864e5 / 7 + 1.5);`. Probably a fast estimation of some sort but still — Alex Turpin, Apr 10 '12 at 19:06
@Xeon06 [moment.js](https://github.com/timrwood/moment), the code is [here](https://github.com/timrwood/moment/blob/master/moment.js) on line 155 — Kato, Apr 10 '12 at 20:25

score 16 · Accepted Answer · answered Apr 10 '12 at 18:35

16

The ~ operator flips the bits of its operand. Using it twice flips the bits, then flips them again, returning a standard Javascript value equivalent to the operand, but in integer form. It's shorthand for parseInt(myInt).

answered Apr 10 '12 at 18:35

Elliot Bonneville

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Thanks! I figured it would be something of the sort. For some reason, I couldn't figure out how to Google it; must needs more tea :) – Kato Apr 10 '12 at 18:40
No problem, see my comment on your question for how to Google operators. :p – Elliot Bonneville Apr 10 '12 at 18:41
@Kato Search google for "tilde JavaScript" and press I Feel lucky: http://dreaminginjavascript.wordpress.com/2008/07/04/28/ – Andreas Louv Apr 10 '12 at 18:43
@AndreasAL roger, searched for "tea plus javascript" and feel much luckier. – Kato Apr 10 '12 at 18:44

score 10 · Answer 2 · edited May 23 '17 at 12:15

10

It's a hackish way to truncate a value, a bit like what Math.floor does, except this behaves differently for negative numbers. For example, truncating -15.9 (~~-15.9) gives -15, but flooring it will always round towards the lowest number, so Math.floor(-15.9) will give 16.

Another way to do it is to OR with zero.

var a = 15.9 | 0; //a = 15

edited May 23 '17 at 12:15

Community

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answered Apr 10 '12 at 18:35

Alex Turpin

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re: Differently for negative values -- how so? – Kato Apr 10 '12 at 18:39
@Kato added explanation to post. – Alex Turpin Apr 10 '12 at 18:43
It looks like ~~ is quite a bit faster than Math.floor; not that it's enough to matter in most cases. – Kato Apr 10 '12 at 18:57
@Kato yeah, it sounds like premature optimization to me to use this everwhere. – Alex Turpin Apr 10 '12 at 19:05
i prefer `~~` since `~~idx === idx` iff `idx` is integer. using `|0` you would need braces, i.e. `(idx|0) === idx` – Janus Troelsen Jan 10 '15 at 14:41

score 2 · Answer 3 · answered Apr 10 '12 at 18:34

2

It converts the value to an integer.

answered Apr 10 '12 at 18:34

ThiefMaster

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What does ~~ do in JavaScript/node.js?

3 Answers3