Express a[i]+-~j as a[i]-~j?

[Vogtinator]

I'm wondering why a[i]-~j isn't used instead of a[i]+-~j.
It's that obvious that I'm probably missing something, but what?

[xem]

Hello! :)
It's not a real substraction.
Try to execute that in your JS console:
a="ABCD";for(j in a)for(i in a)console.log((y=a[i]+-~j) + " vs " + (y=a[i]-~j))
you'll get:

A1 vs NaN 
B1 vs NaN 
C1 vs NaN 
D1 vs NaN 
A2 vs NaN 
B2 vs NaN 
C2 vs NaN 
D2 vs NaN 
A3 vs NaN 
B3 vs NaN 
C3 vs NaN 
D3 vs NaN 
A4 vs NaN 
B4 vs NaN 
C4 vs NaN 
D4 vs NaN

you should see this expression as a concatenation of the string a[i], and -~j which is a shorter way to write (j+1)

[Vogtinator]

Ok, so it's actually string concatenation.

Thanks for the explanation!