Why '1' + '2' - 3 gives 9?

Question

I am new to javascript, and I tried to run the below in Brave browser:

'1' + '2' - 3;

The browser replied with the value 9, which I don't understand.

Because you are adding two strings together "1" + "2" = "12" then Javascript sees you are doing `-2` and treats the left hand side as a number (type coercion), so it becomes `12 - 3` = `9`...not answering officially as this has been answered many times and this Q should be closed. — Halfpint, Oct 13 '21 at 12:16
`+` is overridden as concatenation for strings but `-` doesn't make sense in string context so JS coerces the string to a number to perform mathematical subtraction. — phuzi, Oct 13 '21 at 12:16

score 2 · Answer 1 · answered Oct 13 '21 at 12:16

2

'1' + '2' is concatenation of strings and result is '12'. '12' - 3 is math operation 12 - 3 and result is 9.

answered Oct 13 '21 at 12:16

Aleks D

score 2 · Answer 2 · answered Oct 13 '21 at 12:17

You shoud do the intermediate steps and see for yourself.

Try:

a = '1' + '2';
console.log(a);
b = a - 3;
console.log(b);   // prints 9

The + operator is described as for purpose of sum of numeric operands or string concatenation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Addition

score 0 · Answer 3 · answered Oct 13 '21 at 12:16

0

That is because your first two numbers are strings. you would get a correct answer if you used:

1 + 2 - 3

Or if you cannot avoid doing a calculation on a string, convert it to a number first:

parseInt('1') + parseInt('2') - 3

answered Oct 13 '21 at 12:16

Toiletduck

score 0 · Answer 4 · edited Oct 13 '21 at 14:07

0

'1'+ '2' = '12' is a string but it is converted to an integer before being substracted by -3, so 12-3.

edited Oct 13 '21 at 14:07

Taher

answered Oct 13 '21 at 12:16

itsmejohn

4 Answers4