Choose a version:
35% The original file has 185025 bytes (180.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 64323 bytes (62.8k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15270 bytes (14.9k)
local copy
gzip -9
  15190 bytes (14.8k)
local copy
Boot
  15182 bytes (14.8k)
CDN
cdnjs
  15182 bytes (14.8k)
CDN
7zip -mx=9 -tgzip
  14737 bytes (14.4k)
local copy
libdeflate -12
  14722 bytes (14.4k)
local copy
zultra
  14696 bytes (14.4k)
local copy
kzip -s0 -rn -b2
  14652 bytes (14.3k)
local copy
pigz -11 -n
  14652 bytes (14.3k)
local copy
Zopfli
  14642 bytes (14.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.0.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.0.2 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 --mls256 --bsr4 --lazy --ohh

(found May 10, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 4  --bsr4
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/4.0.2/socket.io.min.js --location | md5sum
335299463ba1675b83774c83caa71405  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.0.2.min.zopfli.js.gz | md5sum
335299463ba1675b83774c83caa71405  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.0.2/socket.io.min.js --location | sha1sum
6881d12fdf4bfc25a313b2792313c8d4f6f02519  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.0.2.min.zopfli.js.gz | sha1sum
6881d12fdf4bfc25a313b2792313c8d4f6f02519  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15182 bytes 335299463ba1675b83774c83caa71405 June 7, 2021 @ 19:00
cdnjs 15182 bytes 335299463ba1675b83774c83caa71405 June 7, 2021 @ 19:00

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
14642 bytes -1 byte zopfli --i100000 --mls256 --bsr4 --lazy --ohh May 10, 2021 @ 22:33
14643 bytes -1 byte zopfli --i1000 --mls32 --bsr7 --lazy --ohh May 10, 2021 @ 17:14
14644 bytes -3 bytes zopfli --i1000 --mls256 --bsr7 --lazy --ohh May 10, 2021 @ 16:52
14647 bytes zopfli --i100 --mls2048 --bsr15 --lazy --ohh May 10, 2021 @ 16:21

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

Most recent activity on June 7, 2021 @ 19:00.

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
14647 14647 14649 14647 14646 14649 14645 14645 14645 14647 14647 14649 14647 14646 14647
14643 14645 14643 14644 14645 14644 14645 14642 14644 14644 14644 14646 14645 14645 14665
14643 14644 14643 14644 14645 14642 14645 14644 14644 14644 14644 14644 14661 14645 14661
14645 14645 14643 14644 14643 14642 14644 14642 14644 14644 14644 14646 14645 14645 14661
14644 14645 14644 14644 14645 14642 14644 14643 14644 14646 14644 14645 14647 14646 14664
14643 14645 14645 14644 14644 14645 14644 14644 14643 14644 14644 14644 14645 14645 14644
14644 14645 14643 14644 14644 14642 14644 14642 14643 14644 14644 14645 14665 14656 14656
14644 14644 14644 14643 14644 14644 14644 14642 14644 14643 14644 14644 14645 14645 14661
14645 14645 14643 14644 14645 14642 14643 14643 14643 14643 14645 14645 14662 14663 14645
14643 14645 14643 14644 14644 14642 14644 14642 14644 14643 14644 14645 14645 14661 14645
14645 14645 14643 14644 14643 14645 14645 14642 14644 14644 14644 14644 14662 14646 14661
14644 14645 14645 14644 14644 14642 14646 14645 14644 14643 14644 14644 14645 14645 14661
14644 14644 14645 14644 14645 14644 14645 14644 14644 14643 14644 14645 14645 14645 14661
14645 14645 14645 14644 14644 14644 14644 14642 14644 14646 14644 14644 14645 14661 14644
14644 14645 14644 14644 14645 14644 14644 14644 14644 14644 14646 14646 14645 14645 14645
14645 14644 14643 14644 14645 14644 14644 14644 14644 14644 14643 14646 14645 14660 14660
14645 14644 14645 14644 14645 14644 14644 14643 14643 14644 14644 14643 14645 14646 14645
14645 14645 14643 14645 14644 14645 14644 14642 14644 14644 14644 14643 14645 14645 14661
14643 14645 14645 14644 14645 14644 14644 14644 14643 14644 14643 14643 14662 14645 14661
14643 14645 14643 14644 14644 14642 14644 14642 14644 14643 14644 14645 14645 14661 14661
14644 14644 14645 14644 14645 14642 14644 14642 14642 14645 14644 14645 14661 14661 14661
14643 14645 14643 14644 14645 14642 14644 14642 14642 14644 14644 14644 14645 14662 14662
14645 14645 14645 14644 14645 14644 14644 14644 14645 14644 14644 14646 14662 14662 14661

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 14647 bytes 100%
1,000 14643 bytes -4 bytes 100%
10,000 14643 bytes 100%
100,000 14642 bytes -1 byte 18.55%
1,000,000 14642 bytes 6.67%
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
14686 bytes +44 bytes (+0.30%) +34 bytes
14687 bytes +45 bytes (+0.31%) +35 bytes
14652 bytes +10 bytes (+0.07%)
14674 bytes +32 bytes (+0.22%) +22 bytes
14708 bytes +66 bytes (+0.45%) +56 bytes
14718 bytes +76 bytes (+0.52%) +66 bytes
14739 bytes +97 bytes (+0.66%) +87 bytes
14788 bytes +146 bytes (+1.00%) +136 bytes
14815 bytes +173 bytes (+1.18%) +163 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 12275 bytes -2367 bytes (-16.17%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12543 bytes -2099 bytes (-14.34%)
Brotli (Wikipedia) brotli brotli -q 11 13242 bytes -1400 bytes (-9.56%)
LZMA2 (Wikipedia) xz xz -9 14036 bytes -606 bytes (-4.14%)
Zstandard (Wikipedia) zstd zstd -19 14154 bytes -488 bytes (-3.33%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14261 bytes -381 bytes (-2.60%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15739 bytes +1097 bytes (+7.49%)

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.