UnderscoreJS 0.5.2 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 22262 bytes (21.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8206 bytes (8.0k, 37%).

After GZIP compression these minified files vary in size:

Boot	2967 bytes (2.9k)	CDN
gzip -6 (default)	2690 bytes (2.6k)	local copy
gzip -9	2690 bytes (2.6k)	local copy
Baidu	2684 bytes (2.6k)	CDN
cdnjs	2672 bytes (2.6k)	CDN
libdeflate -12	2614 bytes (2.6k)	local copy
7zip -mx=9 -tgzip	2612 bytes (2.6k)	local copy
zultra	2612 bytes (2.6k)	local copy
pigz -11 -n	2611 bytes (2.5k)	local copy
kzip -s0 -rn -b0	2610 bytes (2.5k)	local copy
Zopfli	2607 bytes (2.5k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.2.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.5.2 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 65 bytes by using my UnderscoreJS 0.5.2 Zopfli version instead of the best available CDN (2.49% smaller than cdnjs, 2607 vs. 2672 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 --mls128 --bsr25 --lazy --ohh

(found February 10, 2019)

Description	Value	Parameter
iterations	100000	--i100000
maximum blocks	8	--mb8
maximum length score	128	--mls128
block splitting recursion	25	--bsr25
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/0.5.2/underscore-min.js --location | md5sum
10f714b13e1699993739cdb4006df282 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.2.min.zopfli.js.gz | md5sum
10f714b13e1699993739cdb4006df282 -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.2/underscore-min.js --location | sha1sum
97ccdbc59522095a58a3bf33196a08c913437e4c -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.2.min.zopfli.js.gz | sha1sum
97ccdbc59522095a58a3bf33196a08c913437e4c -

These CDNs send you the original file:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	2967 bytes	10f714b13e1699993739cdb4006df282	(invalid)
cdnjs	2672 bytes	10f714b13e1699993739cdb4006df282	(invalid)

And some CDNs send you a different file:

CDN	Size (compressed)	MD5 (uncompressed)	Comment / Diff	Timestamp
Baidu	2684 bytes	89b0cf33d7210176b4c42e55faeb652f	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
2607 bytes	-1 byte	zopfli --i100000 --mls128 --bsr25 --lazy --ohh	February 10, 2019 @ 18:09
2608 bytes	-1 byte	zopfli --i10000 --mls512 --bsr14 --lazy --ohh	February 2, 2016 @ 23:08
2609 bytes	-1 byte	zopfli --i1000 --mls2 --bsr20 --lazy --ohh	February 2, 2016 @ 21:00
2610 bytes	–	zopfli --i100 --mls16384 --bsr10 --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 @ 12:48.

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	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
4	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
6	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
7	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
8	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
9	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
10	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
11	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
12	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
13	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
14	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
15	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
16	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
17	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
18	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
19	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
20	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
21	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
22	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
23	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
25	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
30	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609
40	2609	2609	2609	2609	2608	2609	2607	2609	2608	2608	2608	2608	2609	2609	2609

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	2610 bytes	–	100%
1,000	2609 bytes	-1 byte	100%
10,000	2608 bytes	-1 byte	100%
100,000	2607 bytes	-1 byte	40.00%
1,000,000	2607 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)	2610 bytes	+3 bytes (+0.12%)	–
1	2610 bytes	+3 bytes (+0.12%)	–
2	2631 bytes	+24 bytes (+0.92%)	+21 bytes
3	2669 bytes	+62 bytes (+2.38%)	+59 bytes
4	2681 bytes	+74 bytes (+2.84%)	+71 bytes
5	2708 bytes	+101 bytes (+3.87%)	+98 bytes
6	2737 bytes	+130 bytes (+4.99%)	+127 bytes
7	2770 bytes	+163 bytes (+6.25%)	+160 bytes
8	2807 bytes	+200 bytes (+7.67%)	+197 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	2333 bytes	-274 bytes (-10.51%)
Brotli (Wikipedia)	brotli	brotli -q 11	2413 bytes	-194 bytes (-7.44%)
Zstandard (Wikipedia)	zstd	zstd -19	2616 bytes	+9 bytes (+0.35%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	2641 bytes	+34 bytes (+1.30%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	2675 bytes	+68 bytes (+2.61%)
LZMA2 (Wikipedia)	xz	xz -9	2700 bytes	+93 bytes (+3.57%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	3228 bytes	+621 bytes (+23.82%)

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.