Choose a version:
28% The original file has 220814 bytes (215.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 61480 bytes (60.0k, 28%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  14850 bytes (14.5k)
local copy
Boot
  14766 bytes (14.4k)
CDN
cdnjs
  14766 bytes (14.4k)
CDN
gzip -9
  14762 bytes (14.4k)
local copy
7zip -mx=9 -tgzip
  14335 bytes (14.0k)
local copy
libdeflate -12
  14321 bytes (14.0k)
local copy
zultra
  14284 bytes (13.9k)
local copy
kzip -s0 -rn -b2
  14261 bytes (13.9k)
local copy
pigz -11 -n
  14255 bytes (13.9k)
local copy
Zopfli
  14238 bytes (13.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.0.5.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.0.5 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 --mls32 --bsr30 --lazy --ohh

(found January 20, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 30  --bsr30
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/3.0.5/socket.io.min.js --location | md5sum
1c5155e1df040a67dc28deb0d56816b2  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.5.min.zopfli.js.gz | md5sum
1c5155e1df040a67dc28deb0d56816b2  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.0.5/socket.io.min.js --location | sha1sum
bd671aea5450536d9b75a44af493400107936b31  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.5.min.zopfli.js.gz | sha1sum
bd671aea5450536d9b75a44af493400107936b31  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14766 bytes 1c5155e1df040a67dc28deb0d56816b2 (invalid)
cdnjs 14766 bytes 1c5155e1df040a67dc28deb0d56816b2 (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
14238 bytes -1 byte zopfli --i100000 --mls32 --bsr30 --lazy --ohh January 20, 2021 @ 12:54
14239 bytes -1 byte zopfli --i10000 --mls1024 --bsr18 --lazy --ohh January 15, 2021 @ 17:55
14240 bytes -1 byte zopfli --i1000 --mls8 --bsr16 --lazy --ohh January 15, 2021 @ 12:54
14241 bytes -1 byte zopfli --i1000 --mls32 --bsr30 --lazy --ohh January 15, 2021 @ 12:52
14242 bytes -4 bytes zopfli --i1000 --mls32 --bsr13 --lazy --ohh January 15, 2021 @ 12:50
14246 bytes zopfli --i100 --mls1024 --bsr8 --lazy --ohh January 15, 2021 @ 11:00

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

Most recent activity on January 28, 2021 @ 13:36.

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
14244 14244 14246 14245 14245 14246 14246 14247 14248 14249 14248 14248 14249 14247 14245
14241 14241 14240 14244 14239 14243 14240 14243 14239 14243 14243 14243 14240 14242 14239
14243 14243 14238 14243 14238 14243 14238 14243 14240 14240 14240 14239 14243 14240 14239
14243 14243 14238 14243 14241 14240 14238 14243 14239 14240 14240 14239 14239 14240 14239
14241 14241 14238 14240 14240 14243 14238 14243 14243 14240 14243 14239 14240 14242 14239
14243 14243 14238 14243 14238 14238 14238 14243 14240 14243 14240 14243 14239 14243 14243
14243 14243 14240 14241 14240 14242 14238 14243 14240 14240 14243 14240 14239 14240 14239
14238 14238 14240 14240 14240 14242 14240 14242 14243 14240 14243 14241 14240 14240 14245
14243 14243 14243 14243 14238 14238 14238 14243 14240 14240 14240 14244 14239 14240 14239
14238 14238 14240 14243 14240 14238 14240 14244 14239 14240 14240 14239 14243 14240 14239
14241 14241 14238 14240 14238 14243 14238 14243 14239 14240 14240 14240 14239 14240 14239
14243 14243 14238 14240 14238 14242 14238 14243 14239 14240 14240 14239 14240 14240 14239
14244 14241 14238 14240 14238 14242 14238 14243 14240 14240 14242 14240 14240 14240 14240
14243 14243 14240 14241 14241 14242 14240 14243 14240 14239 14242 14243 14240 14245 14241
14239 14239 14241 14240 14240 14242 14238 14243 14240 14239 14243 14239 14242 14240 14240
14239 14239 14240 14239 14240 14243 14238 14243 14239 14239 14240 14243 14240 14240 14238
14239 14239 14238 14243 14238 14243 14238 14243 14243 14239 14240 14240 14239 14240 14240
14240 14240 14240 14243 14240 14242 14238 14243 14243 14238 14243 14241 14240 14240 14240
14243 14243 14240 14241 14240 14243 14240 14243 14240 14238 14242 14243 14243 14243 14238
14243 14243 14240 14243 14238 14242 14240 14243 14239 14240 14243 14243 14239 14240 14239
14243 14243 14240 14240 14240 14242 14238 14243 14239 14240 14242 14243 14239 14240 14240
14241 14241 14238 14243 14238 14242 14238 14243 14239 14240 14242 14239 14239 14240 14239
14241 14241 14240 14243 14238 14242 14240 14243 14238 14240 14242 14240 14239 14240 14240

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 14246 bytes 100%
1,000 14240 bytes -6 bytes 100%
10,000 14239 bytes -1 byte 100%
100,000 14238 bytes -1 byte 27.54%
1,000,000 14238 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
14291 bytes +53 bytes (+0.37%) +30 bytes
14287 bytes +49 bytes (+0.34%) +26 bytes
14261 bytes +23 bytes (+0.16%)
14298 bytes +60 bytes (+0.42%) +37 bytes
14307 bytes +69 bytes (+0.48%) +46 bytes
14334 bytes +96 bytes (+0.67%) +73 bytes
14355 bytes +117 bytes (+0.82%) +94 bytes
14383 bytes +145 bytes (+1.02%) +122 bytes
14412 bytes +174 bytes (+1.22%) +151 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 11967 bytes -2271 bytes (-15.95%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12319 bytes -1919 bytes (-13.48%)
Brotli (Wikipedia) brotli brotli -q 11 12924 bytes -1314 bytes (-9.23%)
LZMA2 (Wikipedia) xz xz -9 13720 bytes -518 bytes (-3.64%)
Zstandard (Wikipedia) zstd zstd -19 13810 bytes -428 bytes (-3.01%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13917 bytes -321 bytes (-2.25%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15005 bytes +767 bytes (+5.39%)

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.