JavaScript adding a string to a number

What you are talking about is a unary plus. It is different than the plus that is used with string concatenation or addition.

If you want to use a unary plus to convert and have it added to the previous value, you need to double up on it.

> 3 + 4 + "5"
"75"
> 3 + 4 + +"5"
12

Edit:

You need to learn about order of operations:

+ and - have the same precedence and are associated to the left:

 > 4 - 3 + 5
 (4 - 3) + 5
 1 + 5
 6

+ associating to the left again:

> 3 + 4 + "5"
(3 + 4) + "5"
7 + "5"
75

unary operators normally have stronger precedence than binary operators:

> 3 + 4 + +"5"
(3 + 4) + (+"5")
7 + (+"5")
7 + 5
12

You could also use parseInt() or parseFloat(), like this:

> 1 + 2 + "3"
"33"
> 1 + 2 + parseInt(3)
6

I think that's alot cleaner than using +"3", but that is just my opinion.

The answer can be found in Ecma262.pdf section 11.6.1:

If Type(lprim) is String or Type(rprim) is String, then a. Return the String that is the result of concatenating ToString( lprim) followed by ToString(rprim).

So that will resolve all operations according to precedence, so that as soon the string is found any number, the number is converted to string.

4 + 3 + "5"
"75"
4 + 3 + "5" + 3
"753"

To read the whole standard, go here.