Meaning of ~ in javascript function

Question

I'm reading this javascript function:

if (~['bacon', 'burger'].indexOf(type)) {
    this.res.writeHead(200, { 'Content-Type': 'text/plain' });
    this.res.end('Serving ' + type + ' sandwich!\n');
  }

But I'm not sure what means ~ some one know when I use it or what meaning?

[JS bit-wise operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators) — ajp15243, Apr 30 '14 at 13:56
Bitwise NOT operator -> [MDN reference](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Bitwise_operators) — Jonathan, Apr 30 '14 at 13:57
possible duplicate of [What does a tilde do when it precedes an expression?](http://stackoverflow.com/questions/12299665/what-does-a-tilde-do-when-it-precedes-an-expression) — Cerbrus, Apr 30 '14 at 13:59

score 9 · Accepted Answer · answered Apr 30 '14 at 13:59

~ is the bitwise NOT operator. It toggles every bit of a number.

0 becomes -1.
-1 becomes 0.
No other numbers become zero.

That means that

if (~['bacon', 'burger'].indexOf(type)) {

is a confusing way of writing

if (['bacon', 'burger'].indexOf(type) == -1) {

indexOf returns -1 when it doesn't find the string.

score 3 · Answer 2 · answered Apr 30 '14 at 13:56

3

~ is a Bitwise NOT operator...

answered Apr 30 '14 at 13:56

Amir Popovich

29,350
9
53
99

score 2 · Answer 3 · answered Apr 30 '14 at 14:00

In this instance, the ~ allows that code to turn the return value of .indexOf() — which is a number indicating the position of the searched-for value in the array — into a boolean. In other words, it takes the "where is the value" result and turns it into a "is the value in the list" result.

How? Well, .indexOf() returns -1 when the value is not found, and a number greater than or equal to zero if it is. The ~ operator converts its numeric argument to a 32-bit integer and then inverts every bit. That process happens to turn -1 to 0, and any positive integer to some negative non-zero value, and 0 to -1. When such results are subsequently examined as boolean values, the original -1 will be false (because 0 is "falsy") while the integers greater than or equal to zero will be true (because they're all converted to some non-zero value).

score -1 · Answer 4 · answered Apr 30 '14 at 13:59

-1

Bitwise NOT (~ a) Inverts the bits of its operand.

EXAMPLE

9 (base 10) = 00000000000000000000000000001001 (base 2)
```
       --------------------------------
```
~9 (base 10) = 11111111111111111111111111110110 (base 2) = -10 (base 10)

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators

answered Apr 30 '14 at 13:59

Jebathon

4,310
14
57
108

Don't use someone's comment on the question as an answer. And don't use examples from **[other sites](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators)** without giving credit – Cerbrus Apr 30 '14 at 13:59
Please don't down-vote my answer for no reason at all in the future – Jebathon Apr 30 '14 at 14:04
At the time of my comment / vote, you only had some sample code copied from the dev.mozilla site in your answer. Even now, all you have is that example and a link, without an explanation. _"Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect."_ **[Help center link](http://stackoverflow.com/help/privileges/vote-down)** – Cerbrus Apr 30 '14 at 14:07

Meaning of ~ in javascript function

4 Answers4