UnderscoreJS 0.5.1 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 21601 bytes (21.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 7943 bytes (7.8k, 37%).

After GZIP compression these minified files vary in size:

Boot	2893 bytes (2.8k)	CDN
gzip -6 (default)	2625 bytes (2.6k)	local copy
gzip -9	2624 bytes (2.6k)	local copy
Baidu	2620 bytes (2.6k)	CDN
cdnjs	2608 bytes (2.5k)	CDN
libdeflate -12	2554 bytes (2.5k)	local copy
7zip -mx=9 -tgzip	2553 bytes (2.5k)	local copy
zultra	2552 bytes (2.5k)	local copy
pigz -11 -n	2550 bytes (2.5k)	local copy
kzip -s0 -rn -b1	2549 bytes (2.5k)	local copy
Zopfli	2546 bytes (2.5k)	local copy
Zopfli (defluff)	2545 bytes (2.5k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.1.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.5.1 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 62 bytes by using my UnderscoreJS 0.5.1 Zopfli version instead of the best available CDN (2.44% smaller than cdnjs, 2546 vs. 2608 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 --i1000000 --mb8 --mls4096 --bsr30 --lazy --ohh

(found March 3, 2020)

Description	Value	Parameter
iterations	1000000	--i1000000
maximum blocks	8	--mb8
maximum length score	4096	--mls4096
block splitting recursion	30	--bsr30
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

Even Smaller Files Thanks To Defluff

Zopfli's output can be further optimized by the defluff tool.
In this particular case, defluff saves 1 more byte (2545 bytes).

Download

Verify file integrity

After decompression, my uncompressed files are identical to the original ones:

MD5:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.1/underscore-min.js --location | md5sum
ad16532134e29bb985e5427af25745ee -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.1.min.zopfli.js.gz | md5sum
ad16532134e29bb985e5427af25745ee -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.1/underscore-min.js --location | sha1sum
f1cb62fb02a78ec5c32c20f50e1e523b2b18ede0 -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.1.min.zopfli.js.gz | sha1sum
f1cb62fb02a78ec5c32c20f50e1e523b2b18ede0 -

These CDNs send you the original file:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
Boot	2893 bytes	ad16532134e29bb985e5427af25745ee	(invalid)
cdnjs	2608 bytes	ad16532134e29bb985e5427af25745ee	(invalid)

And some CDNs send you a different file:

CDN	Size (compressed)	MD5 (uncompressed)	Comment / Diff	Timestamp
Baidu	2620 bytes	3f6e7804f166f4f45fe3be1531c3fdf6	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
2546 bytes	-1 byte	zopfli --i1000000 --mls4096 --bsr30 --lazy --ohh	March 3, 2020 @ 14:44
2547 bytes	-1 byte	zopfli --i100000 --mls4096 --bsr30 --lazy --ohh	February 10, 2019 @ 17:27
2548 bytes	-1 byte	zopfli --i1000 --mls64 --bsr2 --lazy --ohh	February 2, 2016 @ 20:58
2549 bytes	–	zopfli --i100 --mls16 --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 @ 12:47.

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	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
4	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
6	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
7	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
8	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
9	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
10	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
11	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
12	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
13	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
14	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
15	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
16	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
17	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
18	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
19	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
20	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
21	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
22	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
23	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
25	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547
30	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2546	2547	2547	2547
40	2548	2548	2548	2549	2548	2548	2548	2547	2548	2548	2549	2547	2547	2547	2547

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	2549 bytes	–	100%
1,000	2548 bytes	-1 byte	100%
10,000	2548 bytes	–	100%
100,000	2547 bytes	-1 byte	86.67%
1,000,000	2546 bytes	-1 byte	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)	2549 bytes	+3 bytes (+0.12%)	–
1	2549 bytes	+3 bytes (+0.12%)	–
2	2577 bytes	+31 bytes (+1.22%)	+28 bytes
3	2610 bytes	+64 bytes (+2.51%)	+61 bytes
4	2622 bytes	+76 bytes (+2.99%)	+73 bytes
5	2650 bytes	+104 bytes (+4.08%)	+101 bytes
6	2679 bytes	+133 bytes (+5.22%)	+130 bytes
7	2703 bytes	+157 bytes (+6.17%)	+154 bytes
8	2742 bytes	+196 bytes (+7.70%)	+193 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	2290 bytes	-256 bytes (-10.05%)
Brotli (Wikipedia)	brotli	brotli -q 11	2347 bytes	-199 bytes (-7.82%)
Zstandard (Wikipedia)	zstd	zstd -19	2562 bytes	+16 bytes (+0.63%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	2617 bytes	+71 bytes (+2.79%)
LZMA2 (Wikipedia)	xz	xz -9	2644 bytes	+98 bytes (+3.85%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	2793 bytes	+247 bytes (+9.70%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	3184 bytes	+638 bytes (+25.06%)

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.