How does this js code output 10??

Question

alert(++[[]][+[]]+[+[]]);

Answer 1

I don't know how it works but is called jsfuck:

https://en.m.wikipedia.org/wiki/JSFuck

http://www.jsfuck.com/

Answer 2

[] is an empty array, if you put + in front of it, the compiler converts it to Integer:
+[] => 0
++[[]][+[]] respectively, this is the first value of the empty array to which 1 was added.
As a result, '1' + '0' => 10