UnderscoreJS 1.3.3 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 37390 bytes (36.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 12821 bytes (12.5k, 34%).

After GZIP compression these minified files vary in size:

Boot	4904 bytes (4.8k)	CDN
Baidu	4418 bytes (4.3k)	CDN
gzip -6 (default)	4410 bytes (4.3k)	local copy
gzip -9	4401 bytes (4.3k)	local copy
cdnjs	4396 bytes (4.3k)	CDN
unpkg	4393 bytes (4.3k)	CDN
libdeflate -12	4282 bytes (4.2k)	local copy
7zip -mx=9 -tgzip	4280 bytes (4.2k)	local copy
kzip -s0 -rn -b0	4277 bytes (4.2k)	local copy
zultra	4277 bytes (4.2k)	local copy
pigz -11 -n	4266 bytes (4.2k)	local copy
Zopfli	4259 bytes (4.2k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.3.3 file, ETag caching is available and
if your browser doesn't support GZIP decompression then the uncompressed version will be sent.

Currently best Zopfli settings

Save 134 bytes by using my UnderscoreJS 1.3.3 Zopfli version instead of the best available CDN (3.15% smaller than unpkg, 4259 vs. 4393 bytes):

Download

You can use my super-compressed files for whatever purpose you like as long as you respect the library's original license agreement.
There are no restrictions from my side - but please avoid hot-linking if you run a high-traffic website.

These command-line settings yielded the best compression ratio so far (Linux version of zopfli-krzymod):
zopfli --i1000000 --mb8 --mls1024 --bsr8 --lazy --ohh

(found February 26, 2020)

Description	Value	Parameter
iterations	1000000	--i1000000
maximum blocks	8	--mb8
maximum length score	1024	--mls1024
block splitting recursion	8	--bsr8
lazy matching in LZ77	yes	--lazy
optimized Huffman headers	yes	--ohh
initial random W for iterations	1	--rw1
initial random Z for iterations	2	--rz2

Verify file integrity

After decompression, my uncompressed files are identical to the original ones:

MD5:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.3/underscore-min.js --location | md5sum
0899f764c85bbcc13206e3dd8318e678 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.zopfli.js.gz | md5sum
0899f764c85bbcc13206e3dd8318e678 -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.3/underscore-min.js --location | sha1sum
baa693e82387a328abccf9694c6d2db571ac8aca -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.zopfli.js.gz | sha1sum
baa693e82387a328abccf9694c6d2db571ac8aca -

These CDNs send you the original file:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	4904 bytes	0899f764c85bbcc13206e3dd8318e678	(invalid)
cdnjs	4396 bytes	0899f764c85bbcc13206e3dd8318e678	(invalid)
unpkg	4393 bytes	0899f764c85bbcc13206e3dd8318e678	July 11, 2016 @ 16:49

And some CDNs send you a different file:

CDN	Size (compressed)	MD5 (uncompressed)	Comment / Diff	Timestamp
Baidu	4418 bytes	13223be203b9c732fbb5766b6ac55dd1	only whitespaces differ	(invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available UnderscoreJS versions at minime.stephan-brumme.com:

1.13.6, 1.13.5, 1.13.4, 1.13.3, 1.13.2, 1.13.1, 1.13.0,
1.12.1, 1.12.0,
1.11.0,
1.10.2, 1.10.1, 1.10.0,
1.9.2, 1.9.1, 1.9.0,
1.8.3, 1.8.2, 1.8.1,
1.7.0,
1.6.0,
1.5.2, 1.5.0,
1.4.4, 1.4.3, 1.4.2, 1.4.1, 1.4.0,
1.3.3, 1.3.2, 1.3.1, 1.3.0,
1.2.4, 1.2.3, 1.2.2, 1.2.1, 1.2.0,
1.1.7, 1.1.5, 1.1.4, 1.1.3, 1.1.2, 1.1.1, 1.1.0,
1.0.4, 1.0.3, 1.0.2, 1.0.1, 1.0.0,
0.6.0,
0.5.7, 0.5.6, 0.5.5, 0.5.4, 0.5.3, 0.5.2, 0.5.1, 0.5.0,
0.4.7, 0.4.6, 0.4.5, 0.4.4, 0.4.3, 0.4.2, 0.4.1, 0.4.0,
0.3.3, 0.3.2, 0.3.1, 0.3.0,
0.2.0,
0.1.1, 0.1.0

The project site contains an overview how well these versions were compressed.
Other interesting projects are AngularJS, BackboneJS, Bootstrap, D3, Dojo, Ember, jQuery, Knockout, lodash, React, Socket.IO, ThreeJS and Vue.

Changelog

Best Zopfli parameters so far:

Size	Improvement	Parameters	Found
4259 bytes	-1 byte	zopfli --i1000000 --mls1024 --bsr8 --lazy --ohh	February 26, 2020 @ 19:52
4260 bytes	-1 byte	zopfli --i10000 --mls1024 --bsr8 --lazy --ohh	September 8, 2015 @ 15:07
4261 bytes	-1 byte	zopfli --i10000 --mls512 --bsr12 --lazy --ohh	September 8, 2015 @ 15:01
4262 bytes	-1 byte	zopfli --i1000 --mls512 --bsr10 --lazy --ohh	September 8, 2015 @ 10:53
4263 bytes	-1 byte	zopfli --i1000 --mls512 --bsr14 --lazy --ohh	September 8, 2015 @ 10:53
4264 bytes	–	zopfli --i100 --mls512 --bsr16 --lazy --ohh	September 8, 2015 @ 10:32

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on July 20, 2020 @ 12:52.

Heatmaps

This Zopfli heatmap visualizes how compression changes when modifying the --bsr and --mls parameter.
Cell's contents is the best filesize achieved (in bytes, hover with mouse over cells to see number of iterations).

Good parameters are green, bad are red. The best and worst are bold as well.
The brightness of the blue background color indicates how many iterations were processed:
10,000, 100,000 or 1,000,000.

bsr \ mls	2	4	8	16	32	64	128	256	512	1024	2048	4096	8192	16384	32768
bsr \ mls	2	4	8	16	32	64	128	256	512	1024	2048	4096	8192	16384	32768
2	4269	4269	4277	4268	4267	4277	4277	4265	4269	4268	4268	4267	4269	4268	4268
4	4263	4263	4263	4263	4261	4261	4264	4267	4264	4264	4262	4263	4261	4265	4265
6	4263	4264	4264	4263	4262	4263	4264	4262	4263	4261	4267	4262	4264	4261	4261
7	4275	4275	4275	4275	4261	4263	4264	4261	4261	4264	4261	4262	4261	4261	4261
8	4263	4265	4261	4264	4261	4264	4264	4267	4262	4259	4261	4262	4261	4260	4260
9	4263	4264	4264	4263	4261	4261	4264	4263	4264	4261	4261	4262	4260	4260	4260
10	4263	4264	4264	4263	4263	4263	4260	4264	4261	4263	4261	4264	4261	4261	4261
11	4276	4276	4275	4277	4264	4275	4264	4261	4261	4263	4261	4262	4261	4261	4261
12	4265	4264	4264	4263	4263	4264	4264	4262	4261	4264	4261	4262	4262	4261	4261
13	4264	4264	4262	4263	4264	4263	4264	4267	4261	4264	4262	4262	4260	4261	4261
14	4264	4264	4261	4262	4263	4263	4264	4261	4261	4264	4261	4262	4260	4261	4261
15	4264	4264	4264	4263	4262	4263	4264	4262	4262	4264	4261	4262	4260	4261	4261
16	4263	4265	4264	4263	4261	4263	4264	4264	4261	4264	4262	4262	4260	4260	4260
17	4263	4264	4264	4263	4264	4263	4264	4262	4261	4264	4261	4262	4260	4260	4260
18	4264	4264	4264	4263	4260	4262	4264	4260	4261	4264	4261	4264	4261	4261	4261
19	4264	4264	4264	4264	4263	4263	4264	4261	4263	4261	4261	4262	4261	4261	4261
20	4264	4264	4261	4263	4264	4264	4260	4263	4261	4264	4261	4262	4261	4261	4261
21	4276	4276	4265	4263	4261	4263	4264	4263	4263	4264	4261	4262	4261	4261	4261
22	4263	4264	4261	4263	4263	4263	4264	4263	4261	4264	4261	4262	4261	4261	4261
23	4264	4264	4261	4263	4263	4263	4264	4263	4261	4264	4261	4264	4262	4262	4262
25	4264	4265	4264	4263	4262	4263	4264	4262	4261	4264	4262	4262	4262	4262	4262
30	4265	4265	4264	4264	4262	4264	4264	4264	4261	4261	4261	4262	4261	4262	4262
40	4265	4265	4264	4262	4264	4264	4264	4261	4261	4264	4266	4267	4260	4260	4260

Due to the Monte Carlo design of my search algorithm, not all parameters have reached the same number of iterations yet:

Iterations	Min. Bytes	Reduction	Coverage
100	4264 bytes	–	100%
1,000	4262 bytes	-2 bytes	100%
10,000	4260 bytes	-2 bytes	100%
100,000	4260 bytes	–	6.38%
1,000,000	4259 bytes	-1 byte	0.29%
10,000,000	–	–	–

KZIP has far less options available for tuning/optimization. I only played around with the number of blocks (parameter -n):

Blocks	Min. Bytes	Compared To Best Zopfli	Compared To Best KZIP
(any)	4277 bytes	+18 bytes (+0.42%)	–
1	4278 bytes	+19 bytes (+0.45%)	+1 byte
2	4291 bytes	+32 bytes (+0.75%)	+14 bytes
3	4290 bytes	+31 bytes (+0.73%)	+13 bytes
4	4324 bytes	+65 bytes (+1.53%)	+47 bytes
5	4345 bytes	+86 bytes (+2.02%)	+68 bytes
6	4376 bytes	+117 bytes (+2.75%)	+99 bytes
7	4400 bytes	+141 bytes (+3.31%)	+123 bytes
8	4425 bytes	+166 bytes (+3.90%)	+148 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
However, support for Brotli is constantly growing - for example, your browser actually supports it !

Algorithm	Program	Parameters	Size	Compared To Best Zopfli
RAR (proprietary)	RAR	rar a -m5 -md64m -mc63:128t -mt1	3809 bytes	-450 bytes (-10.57%)
Brotli (Wikipedia)	brotli	brotli -q 11	3937 bytes	-322 bytes (-7.56%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	4240 bytes	-19 bytes (-0.45%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	4281 bytes	+22 bytes (+0.52%)
Zstandard (Wikipedia)	zstd	zstd -19	4304 bytes	+45 bytes (+1.06%)
LZMA2 (Wikipedia)	xz	xz -9	4352 bytes	+93 bytes (+2.18%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	4640 bytes	+381 bytes (+8.95%)

Detailled Analysis

I wrote a DEFLATE decoder in Javascript. Click the button below to start a client-side analysis of the smallest gzipped files (may take a second):

Load and Parse Data

Notes: pigz is a fast open source multi-threaded implementation of gzip written by one of the original authors of gzip.
However, when using compression level 11, pigz actually switches to the slower Zopfli algorithm and isn't multi-threaded anymore.
KrzyMOD's extensions to Zopfli offer the highest level of configuration and is therefore used for my brute-force search.
Ken Silverman wrote the closed-source KZIP compression program and Jonathon Fowler ported it to Linux.
Defluff was created by Joachim Henke; DeflOpt is a tool by Ben Jos Walbeehm.

website made by Stephan Brumme in 2015 and still improving in 2026.
all timestamps are displayed in central european time. see my changelog.
no flash, not even images or external css files - and everything squeezed into a single html file.
which was handsomely compressed before releasing it into the wild internet - obviously.

please visit my homepage and my blog, too.
email: minime (at) stephan-brumme.com

All trademarks are property of their respective owners. You know, the boring legal stuff.