Choose a version:
78% The original file has 68686 bytes (67.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 53469 bytes (52.2k, 78%).

After GZIP compression these minified files vary in size:
Boot
  16005 bytes (15.6k)
CDN
cdnjs
  16005 bytes (15.6k)
CDN
gzip -6 (default)
  15852 bytes (15.5k)
local copy
gzip -9
  15778 bytes (15.4k)
local copy
libdeflate -12
  15248 bytes (14.9k)
local copy
7zip -mx=9 -tgzip
  15218 bytes (14.9k)
local copy
kzip -s0 -rn -b4
  15201 bytes (14.8k)
local copy
pigz -11 -n
  15177 bytes (14.8k)
local copy
Zopfli
  15157 bytes (14.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.3.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 2.3.0 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 --mls16 --bsr6 --lazy --ohh

(found November 13, 2019)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 16  --mls16
block splitting recursion 6  --bsr6
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/2.3.0/socket.io.min.js --location | md5sum
2c9f35da90f859c226d9f45d9002d08a  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.3.0.min.zopfli.js.gz | md5sum
2c9f35da90f859c226d9f45d9002d08a  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.3.0/socket.io.min.js --location | sha1sum
26fdfdd5366a190cc59e2d88684eb3f5764b33a3  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.3.0.min.zopfli.js.gz | sha1sum
26fdfdd5366a190cc59e2d88684eb3f5764b33a3  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 16005 bytes 2c9f35da90f859c226d9f45d9002d08a (invalid)
cdnjs 16005 bytes 2c9f35da90f859c226d9f45d9002d08a (invalid)

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

Other Versions

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

2.3.0, 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
15157 bytes -1 byte zopfli --i1000000 --mls16 --bsr6 --lazy --ohh November 13, 2019 @ 15:40
15158 bytes -1 byte zopfli --i100000 --mls16 --bsr6 --lazy --ohh November 13, 2019 @ 12:12
15159 bytes -1 byte zopfli --i10000 --mls16 --bsr6 --lazy --ohh November 13, 2019 @ 11:40
15160 bytes -1 byte zopfli --i10000 --mls8 --bsr40 --lazy --ohh November 13, 2019 @ 11:37
15161 bytes -2 bytes zopfli --i1000 --mls8 --bsr40 --lazy --ohh November 13, 2019 @ 11:34
15163 bytes -1 byte zopfli --i1000 --mls16 --bsr6 --lazy --ohh November 13, 2019 @ 11:32
15164 bytes -2 bytes zopfli --i1000 --mls128 --bsr40 --lazy --ohh November 13, 2019 @ 11:32
15166 bytes zopfli --i100 --mls128 --bsr40 --lazy --ohh November 13, 2019 @ 11:31

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

Most recent activity on December 5, 2019 @ 18:56.

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
15170 15170 15172 15171 15173 15173 15174 15172 15171 15171 15179 15179 15172 15175 15175
15166 15166 15169 15167 15167 15172 15167 15171 15167 15169 15170 15180 15175 15183 15173
15166 15167 15169 15157 15165 15169 15171 15169 15171 15168 15171 15166 15172 15169 15172
15168 15171 15169 15170 15168 15171 15169 15167 15166 15170 15170 15171 15171 15166 15173
15168 15168 15171 15169 15169 15168 15167 15168 15172 15169 15170 15166 15181 15171 15169
15168 15168 15168 15167 15164 15169 15164 15168 15168 15170 15171 15171 15171 15171 15171
15168 15167 15168 15165 15164 15168 15163 15167 15166 15168 15169 15179 15173 15179 15170
15167 15166 15168 15170 15168 15170 15170 15169 15171 15168 15177 15179 15170 15172 15172
15169 15167 15166 15166 15164 15167 15166 15169 15168 15167 15166 15171 15172 15171 15173
15168 15169 15168 15168 15171 15169 15164 15168 15164 15167 15173 15171 15172 15171 15168
15169 15167 15168 15167 15164 15170 15164 15167 15168 15167 15178 15165 15172 15168 15173
15168 15171 15174 15170 15164 15169 15164 15168 15167 15167 15177 15172 15169 15173 15171
15168 15169 15173 15168 15169 15172 15166 15172 15167 15167 15169 15171 15172 15171 15169
15167 15168 15172 15168 15165 15167 15166 15167 15164 15167 15171 15180 15169 15172 15172
15168 15168 15170 15169 15165 15167 15165 15168 15167 15166 15179 15171 15171 15172 15173
15169 15170 15169 15168 15164 15170 15165 15168 15166 15166 15171 15170 15172 15167 15170
15169 15169 15168 15169 15163 15165 15166 15170 15167 15168 15171 15179 15171 15171 15166
15169 15169 15169 15171 15169 15170 15165 15169 15167 15168 15170 15171 15169 15171 15165
15169 15167 15171 15169 15165 15171 15160 15166 15168 15166 15177 15180 15171 15171 15174
15167 15167 15168 15170 15165 15166 15165 15168 15168 15167 15172 15179 15172 15180 15172
15166 15169 15171 15163 15168 15170 15165 15167 15173 15168 15170 15170 15172 15172 15172
15169 15169 15169 15167 15164 15167 15169 15166 15168 15168 15170 15179 15172 15170 15173
15166 15166 15159 15167 15164 15166 15160 15168 15172 15168 15171 15168 15170 15171 15172

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 15166 bytes 100%
1,000 15161 bytes -5 bytes 100%
10,000 15159 bytes -2 bytes 100%
100,000 15158 bytes -1 byte 1.16%
1,000,000 15157 bytes -1 byte 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
15221 bytes +64 bytes (+0.42%) +20 bytes
15220 bytes +63 bytes (+0.42%) +19 bytes
15223 bytes +66 bytes (+0.44%) +22 bytes
15239 bytes +82 bytes (+0.54%) +38 bytes
15201 bytes +44 bytes (+0.29%)
15217 bytes +60 bytes (+0.40%) +16 bytes
15248 bytes +91 bytes (+0.60%) +47 bytes
15271 bytes +114 bytes (+0.75%) +70 bytes
15294 bytes +137 bytes (+0.90%) +93 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 - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12669 bytes -2488 bytes (-16.41%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12841 bytes -2316 bytes (-15.28%)
Brotli (Wikipedia) brotli brotli -q 11 13990 bytes -1167 bytes (-7.70%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14047 bytes -1110 bytes (-7.32%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14446 bytes -711 bytes (-4.69%)
LZMA2 (Wikipedia) xz xz -9 14712 bytes -445 bytes (-2.94%)
Zstandard (Wikipedia) zstd zstd -19 14893 bytes -264 bytes (-1.74%)

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 2019.
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.