Why?

Question

Why JS don't add numbers, he add chars?(sometimes)

Answer 1

""+1+2 == 12 because priority is like this :
((""+1)+2) == "1"+2 == "12"
as you can see js does auto typecasting with priority from left to right

1+2 == 2 (no strings in operations)

perhaps the values you're trying to add are not all pure numbers.

Answer 2

http://speakingjs.com/es5/ch11.html
all about js numbers