UnderscoreJS 0.4.0 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 17709 bytes (17.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 6494 bytes (6.3k, 37%).

After GZIP compression these minified files vary in size:

Boot	2372 bytes (2.3k)	CDN
gzip -9	2158 bytes (2.1k)	local copy
gzip -6 (default)	2157 bytes (2.1k)	local copy
Baidu	2145 bytes (2.1k)	CDN
cdnjs	2145 bytes (2.1k)	CDN
libdeflate -12	2092 bytes (2.0k)	local copy
kzip -s0 -rn -b1	2090 bytes (2.0k)	local copy
7zip -mx=9 -tgzip	2088 bytes (2.0k)	local copy
pigz -11 -n	2088 bytes (2.0k)	local copy
zultra	2088 bytes (2.0k)	local copy
Zopfli	2087 bytes (2.0k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.0.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.4.0 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 58 bytes by using my UnderscoreJS 0.4.0 Zopfli version instead of the best available CDN (2.78% smaller than cdnjs, 2087 vs. 2145 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 --i1000 --mb8 --mls16384 --bsr23 --lazy --ohh

(found February 2, 2016)

Description	Value	Parameter
iterations	1000	--i1000
maximum blocks	8	--mb8
maximum length score	16384	--mls16384
block splitting recursion	23	--bsr23
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/0.4.0/underscore-min.js --location | md5sum
a45d849bcc6d5d3d29e235e4c83528a1 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.0.min.zopfli.js.gz | md5sum
a45d849bcc6d5d3d29e235e4c83528a1 -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.0/underscore-min.js --location | sha1sum
efe8ae9b678eb2e4707d9c66b29b97bd48e7e8c9 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.0.min.zopfli.js.gz | sha1sum
efe8ae9b678eb2e4707d9c66b29b97bd48e7e8c9 -

All listed CDNs deliver identical contents:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	2372 bytes	a45d849bcc6d5d3d29e235e4c83528a1	(invalid)
Baidu	2145 bytes	a45d849bcc6d5d3d29e235e4c83528a1	(invalid)
cdnjs	2145 bytes	a45d849bcc6d5d3d29e235e4c83528a1	(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
2087 bytes	-1 byte	zopfli --i1000 --mls16384 --bsr23 --lazy --ohh	February 2, 2016 @ 21:00
2088 bytes	–	zopfli --i100 --mls8192 --bsr14 --lazy --ohh	February 2, 2016 @ 20:57

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 @ 12:50.

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	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
4	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
6	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
7	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
8	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
9	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
10	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
11	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
12	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
13	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
14	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
15	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
16	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
17	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
18	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
19	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
20	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
21	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
22	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
23	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
25	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
30	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087
40	2087	2087	2087	2087	2087	2088	2087	2087	2087	2087	2087	2087	2087	2087	2087

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	2088 bytes	–	100%
1,000	2087 bytes	-1 byte	100%
10,000	2087 bytes	–	100%
100,000	2087 bytes	–	93.33%
1,000,000	2087 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)	2090 bytes	+3 bytes (+0.14%)	–
1	2090 bytes	+3 bytes (+0.14%)	–
2	2116 bytes	+29 bytes (+1.39%)	+26 bytes
3	2137 bytes	+50 bytes (+2.40%)	+47 bytes
4	2170 bytes	+83 bytes (+3.98%)	+80 bytes
5	2196 bytes	+109 bytes (+5.22%)	+106 bytes
6	2229 bytes	+142 bytes (+6.80%)	+139 bytes
7	2255 bytes	+168 bytes (+8.05%)	+165 bytes
8	2291 bytes	+204 bytes (+9.77%)	+201 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	1889 bytes	-198 bytes (-9.49%)
Brotli (Wikipedia)	brotli	brotli -q 11	1893 bytes	-194 bytes (-9.30%)
Zstandard (Wikipedia)	zstd	zstd -19	2115 bytes	+28 bytes (+1.34%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	2152 bytes	+65 bytes (+3.11%)
LZMA2 (Wikipedia)	xz	xz -9	2172 bytes	+85 bytes (+4.07%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	2256 bytes	+169 bytes (+8.10%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	2788 bytes	+701 bytes (+33.59%)

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.