Choose a version:
86% The original file has 62766 bytes (61.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 53861 bytes (52.6k, 86%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  16005 bytes (15.6k)
local copy
Boot
  15945 bytes (15.6k)
CDN
cdnjs
  15945 bytes (15.6k)
CDN
gzip -9
  15918 bytes (15.5k)
local copy
libdeflate -12
  15371 bytes (15.0k)
local copy
7zip -mx=9 -tgzip
  15349 bytes (15.0k)
local copy
zultra
  15348 bytes (15.0k)
local copy
kzip -s0 -rn -b3
  15309 bytes (15.0k)
local copy
pigz -11 -n
  15299 bytes (14.9k)
local copy
Zopfli
  15282 bytes (14.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.3.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 2.3.1 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 --mls8 --bsr15 --lazy --ohh

(found October 7, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 8  --mls8
block splitting recursion 15  --bsr15
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/2.3.1/socket.io.min.js --location | md5sum
dbe3192202619421fc93504978086449  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.3.1.min.zopfli.js.gz | md5sum
dbe3192202619421fc93504978086449  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.3.1/socket.io.min.js --location | sha1sum
2eb79a19cc326ae4edc61b3369b7b6f18f0c8ee9  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.3.1.min.zopfli.js.gz | sha1sum
2eb79a19cc326ae4edc61b3369b7b6f18f0c8ee9  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15945 bytes dbe3192202619421fc93504978086449 (invalid)
cdnjs 15945 bytes dbe3192202619421fc93504978086449 (invalid)

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

Other Versions

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

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
15282 bytes -1 byte zopfli --i1000000 --mls8 --bsr15 --lazy --ohh October 7, 2020 @ 15:14
15283 bytes -1 byte zopfli --i100000 --mls8 --bsr15 --lazy --ohh October 6, 2020 @ 17:55
15284 bytes -1 byte zopfli --i10000 --mls8 --bsr15 --lazy --ohh October 6, 2020 @ 17:33
15285 bytes -1 byte zopfli --i1000 --mls8 --bsr15 --lazy --ohh October 6, 2020 @ 16:54
15286 bytes -3 bytes zopfli --i1000 --mls8 --bsr25 --lazy --ohh October 6, 2020 @ 16:54
15289 bytes -2 bytes zopfli --i1000 --mls2 --bsr15 --lazy --ohh October 6, 2020 @ 16:54
15291 bytes zopfli --i100 --mls1024 --bsr20 --lazy --ohh October 6, 2020 @ 16:34

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

Most recent activity on October 7, 2020 @ 21:42.

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
15301 15302 15300 15299 15298 15299 15300 15298 15299 15296 15295 15296 15312 15310 15298
15289 15290 15289 15289 15292 15289 15295 15293 15294 15293 15298 15297 15295 15297 15291
15295 15291 15292 15291 15290 15291 15288 15292 15292 15286 15303 15294 15291 15293 15291
15290 15289 15288 15288 15296 15289 15289 15291 15291 15293 15291 15293 15293 15292 15295
15291 15291 15293 15291 15288 15292 15290 15292 15294 15292 15295 15294 15295 15293 15292
15293 15294 15287 15289 15289 15289 15292 15296 15294 15289 15294 15289 15294 15295 15288
15292 15289 15289 15294 15299 15296 15293 15292 15293 15290 15292 15297 15301 15292 15291
15294 15292 15292 15295 15291 15293 15292 15291 15294 15295 15297 15296 15291 15289 15290
15295 15294 15291 15297 15288 15291 15292 15292 15293 15293 15294 15291 15295 15302 15294
15289 15290 15293 15291 15291 15290 15284 15292 15293 15290 15294 15297 15294 15291 15293
15292 15292 15290 15282 15289 15292 15291 15294 15293 15293 15296 15292 15295 15295 15293
15287 15289 15282 15291 15292 15290 15292 15295 15290 15292 15293 15294 15293 15303 15291
15291 15292 15296 15291 15292 15290 15294 15292 15294 15295 15296 15294 15294 15293 15293
15293 15293 15293 15292 15289 15290 15294 15292 15291 15295 15295 15292 15291 15298 15292
15291 15292 15292 15291 15291 15292 15294 15292 15292 15293 15298 15295 15292 15292 15292
15293 15293 15289 15293 15289 15291 15292 15296 15293 15292 15295 15290 15294 15286 15289
15291 15290 15289 15291 15289 15290 15293 15292 15293 15287 15293 15297 15294 15295 15291
15295 15295 15292 15289 15290 15290 15293 15292 15297 15293 15295 15294 15291 15288 15292
15290 15289 15292 15292 15290 15292 15292 15292 15293 15293 15297 15296 15291 15292 15292
15294 15294 15292 15297 15293 15292 15292 15292 15293 15291 15295 15290 15294 15295 15303
15295 15294 15282 15291 15292 15291 15292 15296 15291 15290 15297 15293 15291 15292 15295
15290 15292 15294 15287 15287 15290 15291 15292 15293 15293 15298 15289 15292 15293 15292
15292 15294 15292 15296 15295 15292 15293 15294 15292 15291 15292 15293 15293 15294 15292

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 15291 bytes 100%
1,000 15285 bytes -6 bytes 100%
10,000 15284 bytes -1 byte 100%
100,000 15283 bytes -1 byte 1.16%
1,000,000 15282 bytes -1 byte 0.87%
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
15346 bytes +64 bytes (+0.42%) +37 bytes
15347 bytes +65 bytes (+0.43%) +38 bytes
15318 bytes +36 bytes (+0.24%) +9 bytes
15309 bytes +27 bytes (+0.18%)
15328 bytes +46 bytes (+0.30%) +19 bytes
15358 bytes +76 bytes (+0.50%) +49 bytes
15386 bytes +104 bytes (+0.68%) +77 bytes
15410 bytes +128 bytes (+0.84%) +101 bytes
15429 bytes +147 bytes (+0.96%) +120 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 12780 bytes -2502 bytes (-16.37%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12942 bytes -2340 bytes (-15.31%)
Brotli (Wikipedia) brotli brotli -q 11 14114 bytes -1168 bytes (-7.64%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14133 bytes -1149 bytes (-7.52%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14601 bytes -681 bytes (-4.46%)
LZMA2 (Wikipedia) xz xz -9 14836 bytes -446 bytes (-2.92%)
Zstandard (Wikipedia) zstd zstd -19 15021 bytes -261 bytes (-1.71%)

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 2020.
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.