Socket.IO 4.5.3 - as small as it gets (near-perfect GZIP compression)

Choose a version:

The original file has 118682 bytes (115.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 44004 bytes (43.0k, 37%).

After GZIP compression these minified files vary in size:

gzip -6 (default)	13092 bytes (12.8k)	local copy
cdnjs	13060 bytes (12.8k)	CDN
gzip -9	13035 bytes (12.7k)	local copy
kzip -s0 -rn -b2	12622 bytes (12.3k)	local copy
libdeflate -12	12599 bytes (12.3k)	local copy
7zip -mx=9 -tgzip	12596 bytes (12.3k)	local copy
zultra	12584 bytes (12.3k)	local copy
pigz -11 -n	12580 bytes (12.3k)	local copy
Zopfli	12561 bytes (12.3k)	local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.5.3.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.5.3 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

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 --mls8192 --bsr7 --lazy --ohh

(found January 5, 2023)

Description	Value	Parameter
iterations	100000	--i100000
maximum blocks	8	--mb8
maximum length score	8192	--mls8192
block splitting recursion	7	--bsr7
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/socketio/socket.io-client/4.5.3/socket.io.min.js --location | md5sum
a54f5222914cb0b3138dd3daf13cb4d6 -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.3.min.zopfli.js.gz | md5sum
a54f5222914cb0b3138dd3daf13cb4d6 -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.5.3/socket.io.min.js --location | sha1sum
ba59fb413d25ae1b27813e5c0cd0b337e8d3ff09 -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.3.min.zopfli.js.gz | sha1sum
ba59fb413d25ae1b27813e5c0cd0b337e8d3ff09 -

All listed CDNs deliver identical contents:

CDN	Size (compressed)	MD5 (uncompressed)	Timestamp
cdnjs	13060 bytes	a54f5222914cb0b3138dd3daf13cb4d6	(invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available Socket.IO versions at minime.stephan-brumme.com:

4.5.4, 4.5.3, 4.5.2, 4.5.1, 4.5.0, 4.4.1, 4.4.0, 4.3.2, 4.3.1, 4.3.0, 4.2.0, 4.1.3, 4.1.2, 4.1.1, 4.1.0, 4.0.2, 4.0.1, 4.0.0,
3.1.2, 3.1.1, 3.1.0, 3.0.5, 3.0.4, 3.0.3, 3.0.2, 3.0.1, 3.0.0,
2.4.0, 2.3.1, 2.3.0, 2.2.0, 2.1.1, 2.1.0, 2.0.4, 2.0.3, 2.0.2, 2.0.1, 2.0.0,
1.7.4, 1.7.2, 1.7.1, 1.7.0, 1.6.0, 1.5.0, 1.4.5, 1.4.4, 1.4.3, 1.4.2, 1.4.1, 1.4.0, 1.3.7, 1.3.6, 1.3.5, 1.3.4, 1.3.3, 1.3.2, 1.3.1, 1.3.0, 1.2.1, 1.2.0, 1.1.0, 1.0.6, 1.0.5, 1.0.4, 1.0.3, 1.0.2, 1.0.1, 1.0.0

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, ThreeJS, UnderscoreJS and Vue.

Changelog

Best Zopfli parameters so far:

Size	Improvement	Parameters	Found
12561 bytes	-1 byte	zopfli --i100000 --mls8192 --bsr7 --lazy --ohh	January 5, 2023 @ 14:54
12562 bytes	-1 byte	zopfli --i100000 --mls8192 --bsr10 --lazy --ohh	January 4, 2023 @ 11:10
12563 bytes	-1 byte	zopfli --i1000 --mls8192 --bsr10 --lazy --ohh	January 4, 2023 @ 03:49
12564 bytes	-3 bytes	zopfli --i1000 --mls8192 --bsr30 --lazy --ohh	January 3, 2023 @ 18:41
12567 bytes	-2 bytes	zopfli --i1000 --mls16384 --bsr15 --lazy --ohh	January 3, 2023 @ 18:19
12569 bytes	–	zopfli --i100 --mls4096 --bsr4 --lazy --ohh	January 3, 2023 @ 16:37

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on January 5, 2023 @ 21:24.

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	12569	12570	12569	12568	12567	12568	12567	12566	12565	12569	12570	12567	12565	12570	12567
4	12571	12572	12567	12572	12570	12571	12571	12572	12572	12568	12566	12562	12570	12567	12574
6	12567	12570	12569	12568	12569	12569	12569	12570	12569	12572	12570	12568	12564	12570	12569
7	12569	12572	12570	12567	12570	12567	12568	12576	12570	12567	12566	12565	12561	12565	12565
8	12569	12568	12569	12568	12567	12568	12567	12566	12565	12569	12566	12562	12564	12575	12567
9	12567	12570	12570	12567	12570	12568	12567	12569	12569	12570	12566	12563	12570	12570	12569
10	12570	12569	12565	12571	12568	12566	12565	12565	12569	12566	12566	12565	12562	12566	12565
11	12572	12569	12566	12566	12569	12569	12567	12569	12565	12568	12566	12562	12562	12572	12569
12	12570	12570	12569	12568	12569	12573	12572	12568	12566	12567	12566	12565	12573	12564	12572
13	12571	12574	12568	12571	12569	12567	12574	12571	12569	12566	12567	12565	12573	12572	12573
14	12572	12567	12570	12568	12568	12568	12574	12569	12568	12571	12566	12564	12571	12570	12572
15	12570	12573	12571	12570	12570	12569	12571	12575	12569	12567	12566	12565	12562	12565	12565
16	12569	12568	12566	12567	12571	12567	12571	12565	12568	12567	12566	12562	12569	12573	12573
17	12567	12570	12565	12570	12569	12569	12565	12565	12569	12567	12566	12564	12570	12569	12565
18	12570	12571	12566	12570	12569	12566	12567	12568	12568	12566	12566	12565	12570	12570	12570
19	12571	12567	12565	12566	12566	12570	12565	12569	12568	12569	12566	12565	12562	12571	12574
20	12568	12567	12570	12569	12568	12567	12566	12568	12570	12570	12567	12565	12562	12571	12570
21	12569	12569	12565	12565	12569	12569	12565	12569	12566	12567	12566	12570	12562	12569	12572
22	12570	12570	12570	12567	12570	12566	12568	12565	12570	12566	12566	12565	12569	12564	12569
23	12572	12570	12565	12570	12568	12572	12571	12568	12565	12568	12566	12564	12570	12573	12573
25	12570	12570	12569	12570	12567	12570	12571	12568	12570	12571	12566	12565	12564	12565	12573
30	12568	12569	12565	12568	12569	12573	12565	12568	12570	12567	12566	12565	12564	12570	12573
40	12569	12567	12572	12573	12569	12574	12567	12569	12569	12567	12566	12565	12562	12565	12565

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	12569 bytes	–	100%
1,000	12563 bytes	-6 bytes	100%
10,000	12563 bytes	–	100%
100,000	12561 bytes	-2 bytes	3.77%
1,000,000	12561 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)	12626 bytes	+65 bytes (+0.52%)	+4 bytes
1	12623 bytes	+62 bytes (+0.49%)	+1 byte
2	12622 bytes	+61 bytes (+0.49%)	–
3	12663 bytes	+102 bytes (+0.81%)	+41 bytes
4	12652 bytes	+91 bytes (+0.72%)	+30 bytes
5	12680 bytes	+119 bytes (+0.95%)	+58 bytes
6	12712 bytes	+151 bytes (+1.20%)	+90 bytes
7	12731 bytes	+170 bytes (+1.35%)	+109 bytes
8	12765 bytes	+204 bytes (+1.62%)	+143 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	10695 bytes	-1866 bytes (-14.86%)
ZPAQ (Wikipedia)	zpaq	zpaq -method 69	11092 bytes	-1469 bytes (-11.69%)
PPMd (Wikipedia)	7zip	7za a -mx=9 -m0=ppmd	11637 bytes	-924 bytes (-7.36%)
Brotli (Wikipedia)	brotli	brotli -q 11	11754 bytes	-807 bytes (-6.42%)
Burrows-Wheeler transform (Wikipedia)	bzip2	bzip2 -9	12273 bytes	-288 bytes (-2.29%)
LZMA2 (Wikipedia)	xz	xz -9	12484 bytes	-77 bytes (-0.61%)
Zstandard (Wikipedia)	zstd	zstd -19	12519 bytes	-42 bytes (-0.33%)

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.