Why does ++[[]][+[]]+[+[]] return the string "10"?

Answer 1

This is posted as a possible hint. I got this far before getting pulled away:

var z = ++[[]][+[]]+[+[]];

Using typeof and alerts, here's (I think) the right hand side (after ++[[]])
[] object: (empty array)
+[] number: empty value coerced by leading sign to 0
[+[]] object: Array with 1 element: 0
+ [+[]] number: leading sign coerces single value, BUT
[+[]] + [+[]] string: + coerces concatenation context between OBJECTS

The left hand side prefix and [[]] appears to somehow select the left-hand side of the string and increment the first digit, because trying to separate them reports "invalid left side".

This has similar functionality if you flip it so it works for the right hand side (notice [[]] is moved to the right and now postfix addition):
[+[]]+[+[]][[]]++
Output: 0NaN

I know at least one javascript obfuscator leverages this syntax,
...if you want to dig at what I used to generate this: https://code.sololearn.com/Wo4ysbH3jwcn/?ref=app
(See code's app comments)

Answer 2

Hmm...

https://stackoverflow.com/questions/7202157/why-does-return-the-string-10?noredirect=1&lq=1