UnderscoreJS 1.4.0 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 40529 bytes (39.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 13403 bytes (13.1k, 33%).

After GZIP compression these minified files vary in size:

Boot	5140 bytes (5.0k)	CDN
gzip -6 (default)	4643 bytes (4.5k)	local copy
Baidu	4637 bytes (4.5k)	CDN
cdnjs	4635 bytes (4.5k)	CDN
gzip -9	4626 bytes (4.5k)	local copy
unpkg	4615 bytes (4.5k)	CDN
zultra	4509 bytes (4.4k)	local copy
7zip -mx=9 -tgzip	4506 bytes (4.4k)	local copy
libdeflate -12	4502 bytes (4.4k)	local copy
kzip -s0 -rn -b0	4499 bytes (4.4k)	local copy
pigz -11 -n	4494 bytes (4.4k)	local copy
Zopfli	4488 bytes (4.4k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.0.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.4.0 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 127 bytes by using my UnderscoreJS 1.4.0 Zopfli version instead of the best available CDN (2.83% smaller than unpkg, 4488 vs. 4615 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 --mls512 --bsr10 --lazy --ohh

(found February 12, 2019)

Description	Value	Parameter
iterations	100000	--i100000
maximum blocks	8	--mb8
maximum length score	512	--mls512
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/1.4.0/underscore-min.js --location | md5sum
51b45e9de2a9359e0d17e94e671e829a -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.0.min.zopfli.js.gz | md5sum
51b45e9de2a9359e0d17e94e671e829a -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.4.0/underscore-min.js --location | sha1sum
7fdcf03767901f9aad28dcd11e97bcb96f51c4b9 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.0.min.zopfli.js.gz | sha1sum
7fdcf03767901f9aad28dcd11e97bcb96f51c4b9 -

These CDNs send you the original file:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	5140 bytes	51b45e9de2a9359e0d17e94e671e829a	(invalid)
cdnjs	4635 bytes	51b45e9de2a9359e0d17e94e671e829a	(invalid)
unpkg	4615 bytes	51b45e9de2a9359e0d17e94e671e829a	July 11, 2016 @ 16:49

And some CDNs send you a different file:

CDN	Size (compressed)	MD5 (uncompressed)	Comment / Diff	Timestamp
Baidu	4637 bytes	50117e8f4bc2072dff5dab939bd3269e	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
4488 bytes	-1 byte	zopfli --i100000 --mls512 --bsr10 --lazy --ohh	February 12, 2019 @ 22:39
4489 bytes	-1 byte	zopfli --i10000 --mls512 --bsr9 --lazy --ohh	September 8, 2015 @ 14:47
4490 bytes	-1 byte	zopfli --i1000 --mls512 --bsr9 --lazy --ohh	September 8, 2015 @ 10:52
4491 bytes	–	zopfli --i100 --mls8192 --bsr4 --lazy --ohh	September 8, 2015 @ 10:33

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 @ 12:54.

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	4497	4497	4491	4490	4491	4490	4498	4491	4490	4491	4497	4493	4491	4492	4492
4	4495	4495	4496	4495	4494	4495	4493	4491	4494	4492	4493	4490	4488	4491	4491
6	4494	4494	4495	4493	4494	4491	4491	4491	4493	4491	4492	4490	4489	4491	4491
7	4490	4490	4494	4493	4493	4493	4493	4490	4491	4491	4493	4491	4497	4491	4491
8	4491	4491	4491	4489	4491	4495	4495	4490	4489	4492	4491	4488	4491	4492	4492
9	4495	4495	4495	4495	4493	4491	4494	4488	4489	4489	4491	4490	4490	4491	4491
10	4495	4495	4490	4489	4491	4490	4493	4490	4488	4491	4493	4490	4490	4490	4490
11	4490	4490	4499	4491	4491	4493	4493	4490	4490	4489	4493	4490	4488	4490	4490
12	4497	4497	4490	4491	4491	4491	4491	4490	4491	4491	4492	4489	4491	4491	4491
13	4497	4497	4489	4491	4490	4491	4493	4491	4491	4492	4491	4489	4489	4489	4489
14	4499	4499	4490	4489	4494	4493	4493	4488	4491	4492	4491	4490	4488	4491	4491
15	4493	4493	4491	4494	4491	4492	4493	4488	4489	4489	4492	4490	4492	4491	4491
16	4493	4493	4493	4489	4491	4490	4494	4496	4491	4491	4488	4496	4491	4491	4491
17	4491	4491	4495	4491	4491	4495	4491	4490	4491	4491	4490	4491	4491	4490	4490
18	4491	4491	4490	4491	4491	4491	4491	4488	4489	4490	4488	4490	4492	4492	4492
19	4495	4495	4493	4493	4494	4489	4493	4490	4491	4491	4493	4489	4492	4492	4492
20	4493	4493	4493	4494	4494	4490	4493	4490	4489	4491	4493	4489	4492	4491	4491
21	4490	4490	4491	4494	4494	4491	4493	4490	4491	4491	4491	4490	4490	4491	4491
22	4490	4490	4491	4489	4494	4490	4491	4490	4491	4491	4493	4490	4492	4491	4491
23	4490	4490	4491	4491	4491	4491	4493	4492	4491	4491	4488	4490	4492	4490	4490
25	4494	4494	4494	4492	4490	4490	4492	4488	4491	4491	4488	4489	4492	4490	4490
30	4493	4493	4490	4491	4494	4490	4493	4490	4492	4492	4491	4489	4491	4491	4491
40	4495	4495	4494	4494	4490	4489	4492	4488	4492	4491	4492	4489	4492	4491	4491

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	4491 bytes	–	100%
1,000	4490 bytes	-1 byte	100%
10,000	4489 bytes	-1 byte	100%
100,000	4488 bytes	-1 byte	12.17%
1,000,000	4488 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)	4499 bytes	+11 bytes (+0.25%)	–
1	4499 bytes	+11 bytes (+0.25%)	–
2	4518 bytes	+30 bytes (+0.67%)	+19 bytes
3	4518 bytes	+30 bytes (+0.67%)	+19 bytes
4	4550 bytes	+62 bytes (+1.38%)	+51 bytes
5	4585 bytes	+97 bytes (+2.16%)	+86 bytes
6	4621 bytes	+133 bytes (+2.96%)	+122 bytes
7	4650 bytes	+162 bytes (+3.61%)	+151 bytes
8	4681 bytes	+193 bytes (+4.30%)	+182 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	4005 bytes	-483 bytes (-10.76%)
Brotli (Wikipedia)	brotli	brotli -q 11	4173 bytes	-315 bytes (-7.02%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	4405 bytes	-83 bytes (-1.85%)
Zstandard (Wikipedia)	zstd	zstd -19	4512 bytes	+24 bytes (+0.53%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	4526 bytes	+38 bytes (+0.85%)
LZMA2 (Wikipedia)	xz	xz -9	4580 bytes	+92 bytes (+2.05%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	4857 bytes	+369 bytes (+8.22%)

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.