And this, of course, evaluates to the string “fail”.
(![]+[])[+[]]+(![]+[])[+!+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]];
— Marcus Lagergren (@lagergren) May 23, 2013
Rewrite to show the four (string)[subscript] uses:
(![]+[]) [+[]]
+(![]+[]) [+!+[]]
+([![]] + [][[]])[+!+[]+[+[]]]
+(![]+[]) [!+[]+!+[]];
Then evaluate each color-coded expression:
![]+[] => "false"
+[] => "0"
+!+[] => "1"
!+[] => 1
That third string really exercises those square brackets!
[![]] + [][[]] => "falseundefined"
01234567890123
Finally, we’re ready to ‘fail’:
[0]+[1]+[10]+[2]
f + a + i + l
Musings 