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);

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);

• javascript
• operators

Share Improve this question Follow asked Feb 19, 2018 at 23:28 dev-ccdev-cc 45433 silver badges1313 bronze badges 4

• 1 Which part don't you understand? Break it down: 2 + "2" -> "22"; then -"22" -> -22; then ~-22 -> 21. Does this existing Q&A answer your question? If not, could you edit to be more specific about what it is? – jonrsharpe Commented Feb 19, 2018 at 23:30
• 1 Did you try doing this a step at a time in the javascript console of your browser? Try 2 + "2" first, and so on... – lurker Commented Feb 19, 2018 at 23:31
• simple answer, because type coercion, maths and bitwise operations - all these are documented - try mdn documentation for up to date documentation – Jaromanda X Commented Feb 19, 2018 at 23:34
• 1 Check this out https://www.joezimjs./javascript/great-mystery-of-the-tilde/ – MaveRick Commented Feb 19, 2018 at 23:37

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2 Answers 2

Sorted by: Reset to default 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.

• 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 in 21

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 pliment.

So:

• -22 representation is the 2's pliment of +22
• +22 in binary = 0000 0000 0000 0000 0000 0000 0001 0110
• 2's pliment of (22) = (-22) = 1111 1111 1111 1111 1111 1111 1110 1001
• • 1 = 1111 1111 1111 1111 1111 1111 1110 1010
• ~-22 = bitwise not of -22 = 0000 0000 0000 0000 0000 0000 0001 0101
• 10101 in binary = 21 in decimal