<s>
Google	B-Application
Closure	O
Tools	O
is	O
a	O
set	O
of	O
tools	O
to	O
help	O
developers	B-Operating_System
build	O
rich	O
web	B-Application
applications	I-Application
with	O
JavaScript	B-Language
.	O
</s>
<s>
It	O
was	O
developed	O
by	O
Google	B-Application
for	O
use	O
in	O
their	O
web	B-Application
applications	I-Application
such	O
as	O
Gmail	B-Application
,	O
Google	B-Application
Docs	I-Application
and	O
Google	B-Application
Maps	I-Application
.	I-Application
</s>
<s>
The	O
Closure	O
Compiler	O
is	O
a	O
tool	O
for	O
making	O
JavaScript	B-Language
downloads	O
run	O
faster	O
,	O
at	O
the	O
expense	O
of	O
human	O
readability	O
.	O
</s>
<s>
It	O
does	O
not	O
compile	O
from	O
JavaScript	B-Language
to	O
machine	B-Language
code	I-Language
,	O
but	O
rather	O
compiles	O
from	O
JavaScript	B-Language
to	O
more	O
efficient	O
JavaScript	B-Language
.	O
</s>
<s>
It	O
parses	B-Language
JavaScript	B-Language
,	O
analyzes	O
it	O
,	O
removes	O
dead	O
code	O
and	O
rewrites	O
and	O
minifies	B-Language
what	O
is	O
left	O
.	O
</s>
<s>
It	O
also	O
checks	O
syntax	O
,	O
variable	O
references	O
,	O
and	O
types	O
and	O
warns	O
about	O
common	O
JavaScript	B-Language
pitfalls	O
.	O
</s>
<s>
It	O
supports	O
transpiling	B-Language
modern	O
ECMAScript	B-Language
code	O
to	O
ECMAScript	B-Language
5	O
,	O
so	O
that	O
programmers	O
can	O
write	O
JavaScript	B-Language
that	O
uses	O
those	O
features	O
,	O
and	O
run	O
it	O
in	O
browsers	B-Application
or	O
other	O
environments	O
that	O
do	O
not	O
yet	O
support	O
them	O
.	O
</s>
<s>
(	O
the	O
Traceur	O
Compiler	O
is	O
another	O
Google	B-Application
project	O
that	O
supports	O
transpiling	B-Language
ES6	O
to	O
ES3	O
.	O
)	O
</s>
<s>
Tool	O
set	O
provide	O
a	O
command	B-Application
line	I-Application
tools	I-Application
used	O
to	O
optimize	O
and	O
compile	O
.js	B-Language
files	O
:	O
</s>
<s>
npm	B-Language
package	O
google-closure-compiler	O
which	O
provides	O
as	O
optional	O
dependencies	O
three	O
compilers	O
:	O
native	O
(	O
binary	O
executable	O
)	O
,	O
JAVA	B-Language
and	O
JavaScript-based	O
one	O
;	O
which	O
are	O
installed	O
at	O
once	O
.	O
</s>
<s>
Interactively	O
in	O
the	O
browser	B-Application
:	O
</s>
<s>
The	O
Closure	O
Compiler	O
service	O
website	O
provides	O
a	O
form	O
for	O
a	O
user	O
to	O
input	O
a	O
URL	O
pointing	O
to	O
a	O
JavaScript	B-Language
source	O
or	O
enter	O
JavaScript	B-Language
source	O
code	O
in	O
a	O
text	O
box	O
.	O
</s>
<s>
The	O
website	O
will	O
display	O
with	O
the	O
optimized	O
JavaScript	B-Language
on	O
right	O
side	O
for	O
the	O
user	O
to	O
copy	O
.	O
</s>
<s>
Parameters	O
include	O
the	O
string	O
of	O
JavaScript	B-Language
to	O
be	O
optimized	O
(	O
or	O
a	O
URL	O
pointing	O
to	O
it	O
)	O
,	O
the	O
optimization	O
level	O
,	O
whether	O
to	O
include	O
errors	O
and	O
warnings	O
,	O
and	O
the	O
output	O
format	O
(	O
JSON	B-General_Concept
,	O
XML	O
,	O
or	O
text	O
)	O
.	O
</s>
<s>
The	O
Closure	O
compiler	O
also	O
supports	O
type	O
checking	O
via	O
type	O
annotations	O
that	O
must	O
be	O
written	O
in	O
JSDoc	B-Language
comments	O
.	O
</s>
<s>
Programming	O
languages	O
that	O
transpile	B-Language
to	O
JavaScript	B-Language
benefit	O
from	O
Closure	O
Tools	O
.	O
</s>
<s>
For	O
example	O
,	O
Closure	O
Compiler	O
helps	O
to	O
make	O
ClojureScript	B-Language
practical	O
by	O
making	O
the	O
compiled	O
JavaScript	B-Language
code	O
more	O
efficient	O
.	O
</s>
<s>
The	O
Closure	O
Library	O
is	O
a	O
JavaScript	B-Language
library	I-Language
,	O
written	O
specifically	O
to	O
take	O
advantage	O
of	O
the	O
Closure	O
Compiler	O
,	O
based	O
on	O
a	O
modular	B-Architecture
architecture	I-Architecture
.	O
</s>
<s>
It	O
provides	O
cross-browser	B-Architecture
functions	O
for	O
DOM	B-General_Concept
manipulations	O
and	O
events	O
,	O
Ajax	B-General_Concept
and	O
JSON	B-General_Concept
,	O
as	O
well	O
as	O
more	O
high-level	O
objects	O
such	O
as	O
User	B-Application
Interface	I-Application
widgets	O
and	O
Controls	O
.	O
</s>
<s>
Closure	O
Templates	B-Language
are	O
a	O
templating	B-Language
system	I-Language
for	O
dynamically	O
generating	O
HTML	B-Language
in	O
both	O
Java	B-Language
and	O
JavaScript	B-Language
.	O
</s>
<s>
Because	O
the	O
language	O
was	O
apparently	O
referred	O
to	O
as	O
"	O
Soy	O
"	O
internal	O
to	O
Google	B-Application
,	O
and	O
"	O
Soy	O
"	O
remains	O
in	O
some	O
of	O
the	O
documentation	O
and	O
classes	O
,	O
sometimes	O
Closure	O
Templates	B-Language
are	O
referred	O
to	O
as	O
"	O
Soy	O
Templates	B-Language
"	O
.	O
</s>
<s>
This	O
is	O
a	O
compiler	O
which	O
provides	O
an	O
extended	O
version	O
of	O
CSS	B-Language
,	O
which	O
is	O
compiled	O
down	O
to	O
ordinary	O
CSS	B-Language
.	O
</s>
<s>
Internally	O
in	O
Google	B-Application
,	O
this	O
extended	O
version	O
of	O
CSS	B-Language
is	O
referred	O
to	O
as	O
GSS	O
.	O
</s>
