Choose a version:
35% The original file has 185009 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 64305 bytes (62.8k, 35%).

After GZIP compression these minified files vary in size:
cdnjs
  15430 bytes (15.1k)
CDN
gzip -6 (default)
  15264 bytes (14.9k)
local copy
gzip -9
  15186 bytes (14.8k)
local copy
Boot
  15177 bytes (14.8k)
CDN
7zip -mx=9 -tgzip
  14735 bytes (14.4k)
local copy
libdeflate -12
  14718 bytes (14.4k)
local copy
zultra
  14697 bytes (14.4k)
local copy
pigz -11 -n
  14652 bytes (14.3k)
local copy
kzip -s0 -rn -b2
  14645 bytes (14.3k)
local copy
Zopfli
  14636 bytes (14.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.0.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.0.1 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 --mls32 --bsr14 --lazy --ohh

(found April 9, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 32  --mls32
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/socketio/socket.io-client/4.0.1/socket.io.min.js --location | md5sum
a785805085250550c3abbe834e47d0d1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.0.1.min.zopfli.js.gz | md5sum
a785805085250550c3abbe834e47d0d1  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.0.1/socket.io.min.js --location | sha1sum
8680f273cf2a1bf60f51f07c7206052fcc16d6ab  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.0.1.min.zopfli.js.gz | sha1sum
8680f273cf2a1bf60f51f07c7206052fcc16d6ab  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 15430 bytes a785805085250550c3abbe834e47d0d1 (invalid)
Boot 15177 bytes a785805085250550c3abbe834e47d0d1 (invalid)

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
14636 bytes -1 byte zopfli --i1000000 --mls32 --bsr14 --lazy --ohh April 9, 2021 @ 15:45
14637 bytes -1 byte zopfli --i1000 --mls4096 --bsr25 --lazy --ohh April 6, 2021 @ 17:34
14638 bytes -4 bytes zopfli --i1000 --mls8 --bsr12 --lazy --ohh April 6, 2021 @ 17:24
14642 bytes zopfli --i100 --mls16 --bsr10 --lazy --ohh April 6, 2021 @ 16:29

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

Most recent activity on April 10, 2021 @ 03:20.

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 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
14642 14642 14643 14641 14642 14641 14641 14641 14641 14641 14642 14644 14642 14641 14641
14639 14639 14638 14638 14638 14638 14642 14640 14640 14639 14640 14637 14640 14654 14660
14640 14638 14640 14638 14638 14638 14638 14637 14638 14638 14640 14641 14656 14640 14656
14639 14639 14638 14638 14639 14638 14638 14638 14640 14638 14638 14640 14640 14640 14656
14638 14639 14638 14638 14639 14638 14638 14637 14638 14641 14638 14637 14641 14641 14659
14638 14638 14638 14638 14638 14638 14638 14638 14640 14640 14641 14637 14658 14638 14638
14639 14639 14639 14638 14636 14638 14639 14637 14640 14640 14640 14650 14660 14656 14640
14639 14639 14638 14638 14641 14639 14638 14638 14638 14641 14638 14638 14640 14641 14656
14640 14638 14638 14638 14639 14638 14638 14637 14638 14638 14641 14639 14661 14656 14656
14638 14638 14638 14638 14639 14638 14638 14638 14638 14638 14638 14637 14641 14656 14656
14639 14638 14638 14638 14636 14639 14638 14638 14638 14638 14638 14637 14656 14641 14657
14640 14638 14639 14638 14638 14638 14638 14638 14640 14638 14638 14637 14640 14656 14656
14640 14639 14640 14638 14639 14639 14640 14638 14638 14638 14638 14639 14656 14639 14656
14638 14638 14639 14638 14641 14638 14638 14638 14640 14640 14638 14637 14640 14656 14656
14640 14639 14639 14638 14640 14638 14638 14638 14638 14638 14640 14639 14641 14654 14655
14639 14639 14638 14638 14639 14638 14638 14637 14638 14638 14638 14637 14640 14639 14654
14640 14639 14639 14640 14639 14638 14638 14637 14638 14639 14638 14638 14640 14640 14657
14641 14638 14639 14640 14638 14639 14638 14638 14638 14638 14640 14638 14640 14639 14656
14638 14638 14639 14638 14639 14639 14638 14638 14638 14641 14640 14638 14656 14640 14656
14638 14638 14639 14638 14641 14638 14640 14638 14638 14638 14640 14637 14641 14656 14656
14639 14639 14639 14638 14639 14638 14638 14638 14638 14639 14638 14637 14656 14656 14656
14638 14638 14638 14638 14639 14638 14638 14638 14638 14638 14638 14637 14640 14657 14657
14639 14639 14639 14638 14641 14638 14638 14638 14639 14638 14638 14640 14656 14656 14656

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 14642 bytes 100%
1,000 14637 bytes -5 bytes 100%
10,000 14637 bytes 100%
100,000 14637 bytes 5.51%
1,000,000 14636 bytes -1 byte 5.51%
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
14679 bytes +43 bytes (+0.29%) +34 bytes
14679 bytes +43 bytes (+0.29%) +34 bytes
14645 bytes +9 bytes (+0.06%)
14667 bytes +31 bytes (+0.21%) +22 bytes
14702 bytes +66 bytes (+0.45%) +57 bytes
14711 bytes +75 bytes (+0.51%) +66 bytes
14730 bytes +94 bytes (+0.64%) +85 bytes
14782 bytes +146 bytes (+1.00%) +137 bytes
14804 bytes +168 bytes (+1.15%) +159 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 12267 bytes -2369 bytes (-16.19%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12541 bytes -2095 bytes (-14.31%)
Brotli (Wikipedia) brotli brotli -q 11 13231 bytes -1405 bytes (-9.60%)
LZMA2 (Wikipedia) xz xz -9 14028 bytes -608 bytes (-4.15%)
Zstandard (Wikipedia) zstd zstd -19 14152 bytes -484 bytes (-3.31%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14253 bytes -383 bytes (-2.62%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15733 bytes +1097 bytes (+7.50%)

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.