UnderscoreJS 0.4.2 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 18318 bytes (17.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 6894 bytes (6.7k, 38%).

After GZIP compression these minified files vary in size:

Boot	2479 bytes (2.4k)	CDN
gzip -9	2252 bytes (2.2k)	local copy
gzip -6 (default)	2251 bytes (2.2k)	local copy
Baidu	2237 bytes (2.2k)	CDN
cdnjs	2237 bytes (2.2k)	CDN
libdeflate -12	2185 bytes (2.1k)	local copy
7zip -mx=9 -tgzip	2184 bytes (2.1k)	local copy
kzip -s0 -rn -b1	2183 bytes (2.1k)	local copy
pigz -11 -n	2181 bytes (2.1k)	local copy
zultra	2180 bytes (2.1k)	local copy
Zopfli	2178 bytes (2.1k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.2.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.4.2 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 59 bytes by using my UnderscoreJS 0.4.2 Zopfli version instead of the best available CDN (2.71% smaller than cdnjs, 2178 vs. 2237 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 --i100000 --mb8 --mls64 --bsr10 --lazy --ohh

(found February 10, 2019)

Description	Value	Parameter
iterations	100000	--i100000
maximum blocks	8	--mb8
maximum length score	64	--mls64
block splitting recursion	10	--bsr10
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/0.4.2/underscore-min.js --location | md5sum
bb499dd5791a21ff5fb048ef3fc5fcfc -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.2.min.zopfli.js.gz | md5sum
bb499dd5791a21ff5fb048ef3fc5fcfc -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.2/underscore-min.js --location | sha1sum
ed05b67669d5679fcae2bbd6d659fb2ce1f11d3c -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.2.min.zopfli.js.gz | sha1sum
ed05b67669d5679fcae2bbd6d659fb2ce1f11d3c -

All listed CDNs deliver identical contents:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	2479 bytes	bb499dd5791a21ff5fb048ef3fc5fcfc	(invalid)
Baidu	2237 bytes	bb499dd5791a21ff5fb048ef3fc5fcfc	(invalid)
cdnjs	2237 bytes	bb499dd5791a21ff5fb048ef3fc5fcfc	(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
2178 bytes	-1 byte	zopfli --i100000 --mls64 --bsr10 --lazy --ohh	February 10, 2019 @ 03:19
2179 bytes	–	zopfli --i100 --mls256 --bsr10 --lazy --ohh	February 2, 2016 @ 20:58

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 @ 12:48.

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	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
4	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
6	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
7	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
8	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
9	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
10	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
11	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
12	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
13	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
14	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
15	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
16	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
17	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
18	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
19	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
20	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
21	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
22	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
23	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
25	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
30	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180
40	2179	2179	2179	2178	2178	2178	2179	2179	2179	2179	2179	2180	2180	2180	2180

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	2179 bytes	–	100%
1,000	2179 bytes	–	100%
10,000	2179 bytes	–	100%
100,000	2178 bytes	-1 byte	73.33%
1,000,000	2178 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)	2183 bytes	+5 bytes (+0.23%)	–
1	2183 bytes	+5 bytes (+0.23%)	–
2	2208 bytes	+30 bytes (+1.38%)	+25 bytes
3	2235 bytes	+57 bytes (+2.62%)	+52 bytes
4	2266 bytes	+88 bytes (+4.04%)	+83 bytes
5	2292 bytes	+114 bytes (+5.23%)	+109 bytes
6	2313 bytes	+135 bytes (+6.20%)	+130 bytes
7	2343 bytes	+165 bytes (+7.58%)	+160 bytes
8	2378 bytes	+200 bytes (+9.18%)	+195 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	1959 bytes	-219 bytes (-10.06%)
Brotli (Wikipedia)	brotli	brotli -q 11	1980 bytes	-198 bytes (-9.09%)
Zstandard (Wikipedia)	zstd	zstd -19	2201 bytes	+23 bytes (+1.06%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	2242 bytes	+64 bytes (+2.94%)
LZMA2 (Wikipedia)	xz	xz -9	2264 bytes	+86 bytes (+3.95%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	2394 bytes	+216 bytes (+9.92%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	2863 bytes	+685 bytes (+31.45%)

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.