Choose a version:
35% The original file has 185291 bytes (180.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 64273 bytes (62.8k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15235 bytes (14.9k)
local copy
gzip -9
  15161 bytes (14.8k)
local copy
Boot
  15154 bytes (14.8k)
CDN
cdnjs
  15154 bytes (14.8k)
CDN
7zip -mx=9 -tgzip
  14728 bytes (14.4k)
local copy
libdeflate -12
  14703 bytes (14.4k)
local copy
zultra
  14676 bytes (14.3k)
local copy
pigz -11 -n
  14627 bytes (14.3k)
local copy
kzip -s0 -rn -b2
  14624 bytes (14.3k)
local copy
Zopfli
  14616 bytes (14.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.1.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.1.1 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

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 --mls2048 --bsr16 --lazy --ohh

(found May 19, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 16  --bsr16
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/4.1.1/socket.io.min.js --location | md5sum
9721737ad577f3038e380afc75c6620a  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.1.1.min.zopfli.js.gz | md5sum
9721737ad577f3038e380afc75c6620a  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.1.1/socket.io.min.js --location | sha1sum
110894d92491452c31d691552857c5dbda77a11c  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.1.1.min.zopfli.js.gz | sha1sum
110894d92491452c31d691552857c5dbda77a11c  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15154 bytes 9721737ad577f3038e380afc75c6620a June 8, 2021 @ 00:17
cdnjs 15154 bytes 9721737ad577f3038e380afc75c6620a June 8, 2021 @ 00:17

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

Other Versions

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

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
14616 bytes -1 byte zopfli --i1000000 --mls2048 --bsr16 --lazy --ohh May 19, 2021 @ 22:22
14617 bytes -1 byte zopfli --i100000 --mls2048 --bsr4 --lazy --ohh May 19, 2021 @ 14:59
14618 bytes -1 byte zopfli --i10000 --mls2048 --bsr4 --lazy --ohh May 19, 2021 @ 11:23
14619 bytes -1 byte zopfli --i10000 --mls512 --bsr9 --lazy --ohh May 19, 2021 @ 10:55
14620 bytes -1 byte zopfli --i1000 --mls512 --bsr9 --lazy --ohh May 19, 2021 @ 10:49
14621 bytes -1 byte zopfli --i1000 --mls128 --bsr14 --lazy --ohh May 19, 2021 @ 10:49
14622 bytes zopfli --i100 --mls256 --bsr18 --lazy --ohh May 19, 2021 @ 10:38

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

Most recent activity on June 8, 2021 @ 00:17.

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
14622 14622 14622 14622 14621 14621 14622 14622 14622 14622 14622 14623 14622 14621 14621
14618 14618 14620 14620 14619 14619 14619 14621 14619 14619 14616 14616 14620 14639 14638
14621 14621 14621 14619 14621 14619 14620 14619 14619 14620 14619 14620 14639 14637 14623
14620 14620 14621 14620 14619 14621 14619 14619 14619 14619 14619 14619 14622 14635 14623
14620 14620 14621 14621 14620 14621 14619 14619 14619 14618 14619 14619 14622 14622 14624
14620 14620 14621 14619 14619 14621 14619 14619 14619 14620 14620 14619 14641 14622 14619
14619 14619 14620 14620 14621 14619 14619 14619 14619 14620 14619 14616 14641 14638 14619
14620 14620 14621 14620 14621 14620 14619 14618 14619 14618 14619 14619 14638 14621 14640
14620 14620 14623 14621 14619 14619 14620 14619 14620 14619 14621 14620 14640 14635 14620
14620 14620 14621 14620 14617 14620 14620 14619 14620 14618 14616 14619 14638 14639 14639
14620 14620 14621 14621 14621 14620 14619 14619 14619 14619 14619 14619 14620 14639 14639
14620 14620 14621 14621 14621 14621 14619 14619 14619 14616 14619 14619 14620 14635 14621
14620 14620 14621 14621 14619 14620 14619 14619 14619 14620 14616 14620 14621 14639 14623
14620 14620 14621 14620 14621 14620 14619 14618 14619 14620 14619 14620 14619 14635 14619
14620 14620 14620 14618 14620 14621 14619 14620 14620 14619 14620 14618 14622 14641 14620
14619 14619 14622 14619 14622 14620 14620 14619 14619 14620 14619 14619 14620 14641 14623
14620 14620 14622 14619 14621 14619 14620 14618 14619 14618 14621 14619 14622 14639 14632
14621 14621 14621 14620 14620 14619 14619 14619 14620 14620 14620 14619 14620 14635 14620
14620 14620 14621 14620 14620 14620 14620 14618 14620 14620 14619 14619 14619 14639 14639
14620 14620 14620 14620 14619 14621 14621 14619 14619 14618 14619 14620 14622 14640 14640
14621 14621 14622 14619 14621 14620 14620 14619 14620 14618 14619 14616 14622 14640 14640
14621 14621 14621 14621 14621 14619 14619 14619 14619 14619 14621 14619 14636 14623 14623
14620 14620 14620 14619 14620 14621 14620 14619 14621 14619 14619 14619 14637 14640 14640

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 14622 bytes 100%
1,000 14620 bytes -2 bytes 100%
10,000 14618 bytes -2 bytes 100%
100,000 14617 bytes -1 byte 6.38%
1,000,000 14616 bytes -1 byte 2.32%
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
14664 bytes +48 bytes (+0.33%) +40 bytes
14664 bytes +48 bytes (+0.33%) +40 bytes
14624 bytes +8 bytes (+0.05%)
14647 bytes +31 bytes (+0.21%) +23 bytes
14683 bytes +67 bytes (+0.46%) +59 bytes
14694 bytes +78 bytes (+0.53%) +70 bytes
14710 bytes +94 bytes (+0.64%) +86 bytes
14763 bytes +147 bytes (+1.01%) +139 bytes
14790 bytes +174 bytes (+1.19%) +166 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 12257 bytes -2359 bytes (-16.14%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12536 bytes -2080 bytes (-14.23%)
Brotli (Wikipedia) brotli brotli -q 11 13231 bytes -1385 bytes (-9.48%)
LZMA2 (Wikipedia) xz xz -9 14016 bytes -600 bytes (-4.11%)
Zstandard (Wikipedia) zstd zstd -19 14135 bytes -481 bytes (-3.29%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14266 bytes -350 bytes (-2.39%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15729 bytes +1113 bytes (+7.61%)

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):


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 2021.
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.