<s>
JSFuck	B-Language
is	O
an	O
esoteric	O
subset	O
of	O
JavaScript	B-Language
,	O
where	O
code	O
is	O
written	O
using	O
only	O
six	O
characters	O
:	O
[	O
,	O
]	O
,	O
(	O
,	O
)	O
,	O
!,	O
and	O
+	O
.	O
</s>
<s>
The	O
name	O
is	O
derived	O
from	O
Brainfuck	B-Language
,	O
an	O
esoteric	O
programming	O
language	O
that	O
also	O
uses	O
a	O
minimalistic	O
alphabet	O
of	O
only	O
punctuation	O
.	O
</s>
<s>
Unlike	O
Brainfuck	B-Language
,	O
which	O
requires	O
its	O
own	O
compiler	B-Language
or	O
interpreter	B-Application
,	O
JSFuck	B-Language
is	O
valid	O
JavaScript	B-Language
code	O
,	O
meaning	O
that	O
JSFuck	B-Language
programs	O
can	O
be	O
run	O
in	O
any	O
web	B-Application
browser	I-Application
or	O
engine	B-Language
that	O
interprets	O
JavaScript	B-Language
.	O
</s>
<s>
JSFuck	B-Language
is	O
able	O
to	O
recreate	O
all	O
JavaScript	B-Language
functionality	O
using	O
such	O
a	O
limited	O
set	O
of	O
characters	O
because	O
JavaScript	B-Language
is	O
a	O
weakly	O
typed	O
programming	O
language	O
,	O
and	O
it	O
allows	O
the	O
evaluation	O
of	O
any	O
expression	O
as	O
any	O
type	O
.	O
</s>
<s>
In	O
July	O
2009	O
,	O
Yosuke	O
Hasegawa	O
created	O
a	O
web	O
application	O
called	O
jjencode	O
which	O
could	O
encode	O
arbitrary	O
JavaScript	B-Language
into	O
an	O
obfuscated	O
form	O
utilizing	O
only	O
the	O
18	O
symbols	O
[]( 	O
)	O
!+,\"$.:;_{}~=	O
.	O
</s>
<s>
In	O
2012	O
,	O
Martin	O
Kleppe	O
created	O
a	O
"	O
jsfuck	B-Language
"	O
project	O
on	O
GitHub	B-Application
,	O
and	O
a	O
JSFuck.com	O
website	O
with	O
a	O
web	O
app	O
using	O
that	O
implementation	O
of	O
the	O
encoder	O
.	O
</s>
<s>
JSFuck	B-Language
can	O
be	O
used	O
to	O
bypass	O
detection	O
of	O
malicious	O
code	O
submitted	O
on	O
websites	O
,	O
e.g.	O
</s>
<s>
Another	O
potential	O
use	O
of	O
JSFuck	B-Language
lies	O
in	O
code	O
obfuscation	O
.	O
</s>
<s>
An	O
optimized	O
version	O
of	O
JSFuck	B-Language
has	O
been	O
used	O
to	O
encode	O
jQuery	B-Language
,	O
a	O
JavaScript	B-Language
library	I-Language
,	O
into	O
a	O
fully	O
functional	O
version	O
written	O
with	O
just	O
the	O
six	O
characters	O
.	O
</s>
<s>
JSFuck	B-Language
code	O
is	O
extremely	O
"	O
verbose	O
"	O
:	O
In	O
JavaScript	B-Language
,	O
the	O
code	O
alert( 	O
"	O
Hello	O
World	O
!	O
</s>
<s>
In	O
JSFuck	B-Language
,	O
the	O
same	O
code	O
has	O
a	O
length	O
of	O
4325	O
characters	O
.	O
</s>
<s>
Certain	O
single	O
characters	O
require	O
far	O
more	O
than	O
1000	O
characters	O
when	O
expanded	O
as	O
JSFuck	B-Language
.	O
</s>
<s>
The	O
number	O
0	O
is	O
created	O
by	O
+[],	O
where	O
 [  ] 	O
is	O
the	O
empty	O
array	B-Data_Structure
and	O
+	O
is	O
the	O
unary	O
plus	O
,	O
used	O
to	O
convert	O
the	O
right	O
side	O
to	O
a	O
numeric	O
value	O
(	O
zero	O
here	O
)	O
.	O
</s>
<s>
 [  ] 	O
or	O
!+[]	O
in	O
JSFuck	B-Language
)	O
is	O
converted	O
into	O
the	O
numeric	O
value	O
1	O
by	O
the	O
prepended	O
plus	O
sign	O
.	O
</s>
<s>
in	O
JavaScript	B-Language
true	O
+	O
true	O
=	O
2	O
and	O
true	O
=	O
!	O
</s>
<s>
For	O
example	O
,	O
the	O
string	O
"	O
10	O
"	O
can	O
be	O
expressed	O
in	O
JavaScript	B-Language
as	O
 [ 1 ] 	O
+	O
 [ 0 ] 	O
.	O
</s>
<s>
By	O
replacing	O
the	O
digits	O
with	O
the	O
respective	O
JSFuck	B-Language
expansions	O
,	O
this	O
yields	O
 [ +!+ [  ]  ] + [ + [  ]  ] 	O
.	O
</s>
<s>
Some	O
letters	O
can	O
be	O
obtained	O
in	O
JSFuck	B-Language
by	O
accessing	O
single	O
characters	O
in	O
the	O
string	O
representations	O
of	O
simple	O
boolean	O
or	O
numeric	O
values	O
like	O
"	O
false	O
"	O
,	O
"	O
true	O
"	O
,	O
"	O
NaN	O
"	O
,	O
"	O
undefined	O
"	O
with	O
an	O
indexer	O
(	O
a	O
number	O
in	O
square	O
brackets	O
)	O
.	O
</s>
<s>
the	O
boolean	O
constant	O
false	O
plus	O
an	O
empty	O
array	B-Data_Structure
.	O
</s>
<s>
 [  ] 	O
(	O
negation	O
applied	O
to	O
an	O
empty	O
array	B-Data_Structure
)	O
.	O
</s>
<s>
Proof	O
:	O
In	O
JavaScript	B-Language
,	O
alert( ( 	O
!	O
 [  ] + [  ] 	O
)	O
[ +	O
!!	O
</s>
<s>
The	O
Function	O
constructor	O
can	O
be	O
used	O
to	O
trigger	O
execution	O
of	O
JavaScript	B-Language
code	O
contained	O
in	O
a	O
string	O
as	O
if	O
it	O
were	O
native	O
JavaScript	B-Language
.	O
</s>
<s>
The	O
Function	O
constructor	O
can	O
be	O
retrieved	O
in	O
JSFuck	B-Language
by	O
accessing	O
the	O
constructor	O
property	O
of	O
a	O
well	O
known	O
function	O
,	O
such	O
as	O
 [  ]  [ "filter" ] 	O
(	O
Array.prototype.filter	O
)	O
or	O
 [  ]  [ "flat" ] 	O
(	O
Array.prototype.flat	O
)	O
in	O
modern	O
browsers	B-Application
.	O
</s>
<s>
The	O
characters	O
with	O
the	O
shortest	O
JSFuck	B-Language
expansions	O
are	O
listed	O
below	O
.	O
</s>
<s>
Lacking	O
the	O
distinct	O
features	O
of	O
"	O
usual	O
"	O
JavaScript	B-Language
,	O
obfuscation	O
techniques	O
like	O
JSFuck	B-Language
can	O
assist	O
malicious	O
JavaScript	B-Language
code	O
in	O
bypassing	O
intrusion	O
prevention	O
systems	O
or	O
content	O
filters	O
.	O
</s>
<s>
For	O
instance	O
,	O
the	O
lack	O
of	O
alphanumeric	O
characters	O
in	O
JSFuck	B-Language
and	O
a	O
flawed	O
content	O
filter	O
allowed	O
sellers	O
to	O
embed	O
arbitrary	O
JSFuck	B-Language
scripts	O
in	O
their	O
eBay	O
auction	O
pages	O
.	O
</s>
