UnderscoreJS 1.8.1 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 51359 bytes (50.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 16143 bytes (15.8k, 31%).

After GZIP compression these minified files vary in size:

Boot	6389 bytes (6.2k)	CDN
jsdelivr	6389 bytes (6.2k)	CDN
cdnjs	5690 bytes (5.6k)	CDN
gzip -6 (default)	5687 bytes (5.6k)	local copy
gzip -9	5673 bytes (5.5k)	local copy
unpkg	5667 bytes (5.5k)	CDN
Yandex	5649 bytes (5.5k)	CDN
7zip -mx=9 -tgzip	5505 bytes (5.4k)	local copy
zultra	5505 bytes (5.4k)	local copy
libdeflate -12	5502 bytes (5.4k)	local copy
pigz -11 -n	5498 bytes (5.4k)	local copy
kzip -s0 -rn -b0	5495 bytes (5.4k)	local copy
Zopfli	5484 bytes (5.4k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.8.1.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.8.1 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 165 bytes by using my UnderscoreJS 1.8.1 Zopfli version instead of the best available CDN (3.01% smaller than Yandex, 5484 vs. 5649 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 --mls4 --bsr14 --lazy --ohh

(found October 21, 2015)

Description	Value	Parameter
iterations	100000	--i100000
maximum blocks	8	--mb8
maximum length score	4	--mls4
block splitting recursion	14	--bsr14
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/1.8.1/underscore-min.js --location | md5sum
6959908db2ddae758885b6c2cb2f07a5 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.8.1.min.zopfli.js.gz | md5sum
6959908db2ddae758885b6c2cb2f07a5 -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.8.1/underscore-min.js --location | sha1sum
3debb1117d56240c44bbeaac8bae20e0a6bcb27c -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.8.1.min.zopfli.js.gz | sha1sum
3debb1117d56240c44bbeaac8bae20e0a6bcb27c -

All listed CDNs deliver identical contents:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	6389 bytes	6959908db2ddae758885b6c2cb2f07a5	(invalid)
jsdelivr	6389 bytes	6959908db2ddae758885b6c2cb2f07a5	(invalid)
cdnjs	5690 bytes	6959908db2ddae758885b6c2cb2f07a5	(invalid)
unpkg	5667 bytes	6959908db2ddae758885b6c2cb2f07a5	July 11, 2016 @ 16:49
Yandex	5649 bytes	6959908db2ddae758885b6c2cb2f07a5	December 22, 2015 @ 17:09

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
5484 bytes	-1 byte	zopfli --i100000 --mls4 --bsr14 --lazy --ohh	October 21, 2015 @ 13:34
5485 bytes	-2 bytes	zopfli --i10000 --mls4 --bsr11 --lazy --ohh	September 22, 2015 @ 07:16
5487 bytes	-1 byte	zopfli --i10000 --mls32 --bsr12 --lazy --ohh	September 19, 2015 @ 02:24
5488 bytes	-1 byte	zopfli --i100000 --mls8192 --bsr8 --lazy --ohh	September 8, 2015 @ 21:52
5489 bytes	-1 byte	zopfli --i10000 --mls8192 --bsr16 --lazy --ohh	September 8, 2015 @ 11:22
5490 bytes	-1 byte	zopfli --i1000 --mls8192 --bsr4 --lazy --ohh	September 8, 2015 @ 10:39
5491 bytes	-1 byte	zopfli --i1000 --mls512 --bsr8 --lazy --ohh	September 8, 2015 @ 10:39
5492 bytes	–	zopfli --i100 --mls8192 --bsr8 --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 @ 12:49.

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	5500	5493	5501	5500	5501	5500	5501	5493	5493	5493	5493	5492	5491	5492	5492
4	5493	5493	5493	5492	5493	5502	5493	5493	5493	5493	5493	5490	5488	5493	5493
6	5487	5503	5493	5493	5493	5497	5493	5493	5493	5493	5493	5489	5490	5491	5491
7	5503	5504	5501	5501	5500	5504	5493	5493	5493	5493	5493	5491	5491	5492	5492
8	5500	5493	5501	5500	5500	5498	5492	5492	5489	5493	5493	5490	5488	5492	5492
9	5503	5499	5501	5494	5501	5498	5492	5493	5493	5493	5493	5494	5490	5493	5493
10	5502	5493	5501	5500	5501	5492	5492	5493	5489	5493	5493	5490	5490	5492	5492
11	5502	5485	5499	5500	5499	5501	5492	5493	5493	5493	5493	5490	5490	5490	5490
12	5503	5503	5499	5500	5487	5497	5492	5492	5489	5493	5493	5490	5490	5492	5492
13	5502	5502	5501	5493	5501	5497	5493	5493	5493	5493	5493	5489	5489	5492	5492
14	5501	5484	5501	5500	5500	5500	5493	5492	5493	5493	5493	5489	5489	5492	5492
15	5501	5502	5499	5493	5493	5491	5493	5492	5493	5493	5493	5491	5491	5491	5491
16	5501	5502	5501	5493	5499	5491	5493	5493	5489	5493	5493	5490	5489	5486	5487
17	5501	5496	5499	5500	5500	5491	5493	5492	5493	5493	5493	5491	5491	5491	5491
18	5502	5505	5499	5493	5493	5497	5493	5492	5493	5493	5493	5491	5490	5490	5490
19	5501	5503	5501	5493	5500	5491	5493	5492	5491	5493	5493	5491	5491	5492	5492
20	5501	5502	5499	5500	5501	5501	5492	5492	5493	5493	5493	5491	5490	5487	5487
21	5502	5503	5501	5486	5493	5497	5493	5493	5491	5493	5493	5492	5491	5492	5492
22	5493	5493	5499	5500	5499	5497	5491	5493	5493	5493	5493	5489	5491	5491	5491
23	5502	5485	5499	5500	5501	5497	5492	5492	5493	5493	5493	5491	5491	5490	5490
25	5502	5503	5499	5486	5499	5491	5492	5492	5493	5493	5493	5491	5490	5490	5490
30	5502	5502	5500	5500	5501	5491	5493	5493	5493	5493	5493	5491	5490	5490	5490
40	5502	5502	5501	5501	5499	5498	5493	5493	5493	5493	5493	5491	5491	5491	5491

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	5490 bytes	–	100%
1,000	5487 bytes	-3 bytes	100%
10,000	5485 bytes	-2 bytes	100%
100,000	5484 bytes	-1 byte	17.68%
1,000,000	5484 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)	5495 bytes	+11 bytes (+0.20%)	–
1	5495 bytes	+11 bytes (+0.20%)	–
2	5519 bytes	+35 bytes (+0.64%)	+24 bytes
3	5544 bytes	+60 bytes (+1.09%)	+49 bytes
4	5575 bytes	+91 bytes (+1.66%)	+80 bytes
5	5585 bytes	+101 bytes (+1.84%)	+90 bytes
6	5622 bytes	+138 bytes (+2.52%)	+127 bytes
7	5654 bytes	+170 bytes (+3.10%)	+159 bytes
8	5690 bytes	+206 bytes (+3.76%)	+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	4832 bytes	-652 bytes (-11.89%)
Brotli (Wikipedia)	brotli	brotli -q 11	5119 bytes	-365 bytes (-6.66%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	5423 bytes	-61 bytes (-1.11%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	5453 bytes	-31 bytes (-0.57%)
Zstandard (Wikipedia)	zstd	zstd -19	5500 bytes	+16 bytes (+0.29%)
LZMA2 (Wikipedia)	xz	xz -9	5548 bytes	+64 bytes (+1.17%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	5627 bytes	+143 bytes (+2.61%)

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.