I saw this question in a test, but I don't understand how operators work on the statement.
let a = ~-(2 + "2");
console.log(a);
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1Which part don't you understand? Break it down: `2 + "2"` -> `"22"`; then `-"22"` -> `-22`; then `~-22` -> `21`. Does [this existing Q&A](https://stackoverflow.com/q/12299665/3001761) answer your question? If not, could you [edit] to be more specific about what it is? – jonrsharpe Feb 19 '18 at 23:30
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1Did you try doing this a step at a time in the javascript console of your browser? Try `2 + "2"` first, and so on... – lurker Feb 19 '18 at 23:31
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simple answer, because type coercion, maths and bitwise operations - all these are documented - try mdn documentation for up to date documentation – Jaromanda X Feb 19 '18 at 23:34
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1Check this out `https://www.joezimjs.com/javascript/great-mystery-of-the-tilde/` – MaveRick Feb 19 '18 at 23:37
2 Answers
10
~-(2 + "2")
- 0:
2 + "2"(Concatenation) ="22" - 1:
-"22"(Coercion) =-22 - 2:
~-22(Bitwise NOT) = -(-22 + 1) = 21
Bitwise NOTing any number x yields -(x + 1). For example, ~-5 yields 4.
Ele
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According to your response ~-0xffffffff = -(-4294967295 + 1) = 4294967294. Sorry the result is -2 – tdjprog Feb 20 '18 at 00:59
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@tdjprog So is it correct to say that all numbers under `2^32` will behave as described by this answer, and that those above that threshold will overflow in some way and behave differently? – IMSoP Jun 13 '18 at 17:05
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- Step 1:
(2+"2")results in the string"22" - Step 2:
-("22")is the same as (-1)*("22"), and results in the number-22 - Step 3: Bitwise not (
~), which results in21
To understand that last step, you need to know that JavaScript stores numbers as 64-bit floating point numbers, but all bitwise operations are performed on 32-bit signed integers using 2's compliment.
So:
- -22 representation is the 2's compliment of +22
- +22 in binary =
0000 0000 0000 0000 0000 0000 0001 0110 - 2's compliment of (22) = (-22) =
1111 1111 1111 1111 1111 1111 1110 1001 - 1 =
1111 1111 1111 1111 1111 1111 1110 1010
- 1 =
~-22= bitwise not of -22 =0000 0000 0000 0000 0000 0000 0001 010110101in binary =21in decimal
