UnderscoreJS 1.5.0 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 42933 bytes (41.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 14241 bytes (13.9k, 33%).

After GZIP compression these minified files vary in size:

Boot	5494 bytes (5.4k)	CDN
gzip -6 (default)	4947 bytes (4.8k)	local copy
Baidu	4940 bytes (4.8k)	CDN
cdnjs	4938 bytes (4.8k)	CDN
gzip -9	4933 bytes (4.8k)	local copy
zultra	4815 bytes (4.7k)	local copy
7zip -mx=9 -tgzip	4812 bytes (4.7k)	local copy
libdeflate -12	4809 bytes (4.7k)	local copy
kzip -s0 -rn -b1	4806 bytes (4.7k)	local copy
pigz -11 -n	4803 bytes (4.7k)	local copy
Zopfli	4795 bytes (4.7k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.0.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.5.0 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 143 bytes by using my UnderscoreJS 1.5.0 Zopfli version instead of the best available CDN (2.98% smaller than cdnjs, 4795 vs. 4938 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 --mls2048 --bsr4 --lazy --ohh

(found February 28, 2020)

Description	Value	Parameter
iterations	1000000	--i1000000
maximum blocks	8	--mb8
maximum length score	2048	--mls2048
block splitting recursion	4	--bsr4
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/1.5.0/underscore-min.js --location | md5sum
cc07a4658799e1512b086467e7ef5ca5 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.0.min.zopfli.js.gz | md5sum
cc07a4658799e1512b086467e7ef5ca5 -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.5.0/underscore-min.js --location | sha1sum
44a4cae62496c4f713ae3a6f1d2b32cf42933069 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.0.min.zopfli.js.gz | sha1sum
44a4cae62496c4f713ae3a6f1d2b32cf42933069 -

These CDNs send you the original file:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	5494 bytes	cc07a4658799e1512b086467e7ef5ca5	(invalid)
cdnjs	4938 bytes	cc07a4658799e1512b086467e7ef5ca5	(invalid)

And some CDNs send you a different file:

CDN	Size (compressed)	MD5 (uncompressed)	Comment / Diff	Timestamp
Baidu	4940 bytes	7c9deae1501c50f661db5702d361a122	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
4795 bytes	-2 bytes	zopfli --i1000000 --mls2048 --bsr4 --lazy --ohh	February 28, 2020 @ 13:22
4797 bytes	-1 byte	zopfli --i10000 --mls2048 --bsr4 --lazy --ohh	September 8, 2015 @ 13:14
4798 bytes	-1 byte	zopfli --i1000 --mls2048 --bsr9 --lazy --ohh	September 8, 2015 @ 10:47
4799 bytes	–	zopfli --i100 --mls2048 --bsr4 --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	4801	4801	4801	4801	4801	4801	4801	4799	4801	4797	4798	4800	4801	4802	4802
4	4801	4801	4802	4799	4799	4800	4800	4797	4798	4798	4795	4801	4800	4800	4800
6	4800	4800	4799	4801	4797	4797	4800	4798	4798	4798	4799	4801	4801	4800	4800
7	4799	4799	4799	4801	4800	4801	4800	4801	4801	4797	4799	4797	4797	4798	4798
8	4801	4801	4799	4801	4799	4800	4802	4802	4801	4798	4798	4800	4800	4800	4800
9	4803	4803	4801	4800	4799	4800	4800	4798	4801	4798	4797	4801	4800	4798	4798
10	4800	4800	4799	4802	4801	4801	4801	4801	4798	4798	4797	4801	4797	4800	4800
11	4800	4800	4800	4800	4799	4801	4802	4801	4799	4798	4798	4800	4808	4800	4800
12	4799	4799	4800	4801	4799	4798	4801	4799	4800	4798	4799	4797	4800	4800	4800
13	4799	4799	4800	4801	4801	4800	4800	4801	4801	4800	4797	4801	4801	4800	4800
14	4800	4800	4800	4801	4799	4800	4802	4798	4800	4798	4798	4799	4800	4799	4799
15	4800	4800	4799	4799	4800	4803	4802	4801	4798	4797	4799	4799	4799	4798	4798
16	4800	4800	4800	4799	4799	4803	4800	4801	4798	4798	4798	4801	4801	4800	4800
17	4801	4801	4799	4800	4801	4797	4800	4798	4799	4798	4798	4800	4800	4801	4801
18	4800	4800	4802	4799	4799	4801	4800	4801	4802	4797	4799	4799	4798	4798	4798
19	4800	4800	4799	4800	4799	4799	4801	4797	4798	4798	4797	4801	4797	4798	4798
20	4799	4799	4799	4807	4801	4800	4802	4799	4799	4798	4797	4800	4799	4800	4800
21	4800	4800	4800	4801	4801	4801	4800	4799	4800	4797	4798	4801	4800	4800	4800
22	4800	4800	4800	4800	4799	4801	4802	4801	4798	4798	4797	4801	4800	4798	4798
23	4800	4800	4802	4800	4800	4803	4801	4797	4798	4798	4797	4801	4800	4798	4798
25	4800	4800	4799	4801	4801	4803	4800	4801	4799	4800	4799	4797	4800	4800	4800
30	4800	4800	4799	4800	4799	4799	4802	4801	4800	4800	4798	4801	4800	4800	4800
40	4800	4800	4799	4801	4798	4801	4800	4798	4799	4798	4797	4799	4800	4800	4800

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	4799 bytes	–	100%
1,000	4798 bytes	-1 byte	100%
10,000	4797 bytes	-1 byte	100%
100,000	4797 bytes	–	10.43%
1,000,000	4795 bytes	-2 bytes	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)	4806 bytes	+11 bytes (+0.23%)	–
1	4806 bytes	+11 bytes (+0.23%)	–
2	4825 bytes	+30 bytes (+0.63%)	+19 bytes
3	4826 bytes	+31 bytes (+0.65%)	+20 bytes
4	4857 bytes	+62 bytes (+1.29%)	+51 bytes
5	4891 bytes	+96 bytes (+2.00%)	+85 bytes
6	4929 bytes	+134 bytes (+2.79%)	+123 bytes
7	4959 bytes	+164 bytes (+3.42%)	+153 bytes
8	4986 bytes	+191 bytes (+3.98%)	+180 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	4262 bytes	-533 bytes (-11.12%)
Brotli (Wikipedia)	brotli	brotli -q 11	4465 bytes	-330 bytes (-6.88%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	4790 bytes	-5 bytes (-0.10%)
Zstandard (Wikipedia)	zstd	zstd -19	4807 bytes	+12 bytes (+0.25%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	4837 bytes	+42 bytes (+0.88%)
LZMA2 (Wikipedia)	xz	xz -9	4864 bytes	+69 bytes (+1.44%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	5113 bytes	+318 bytes (+6.63%)

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.