Choose a version:
85% The original file has 62438 bytes (61.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 53062 bytes (51.8k, 85%).

After GZIP compression these minified files vary in size:
Boot
  16534 bytes (16.1k)
CDN
cdnjs
  15953 bytes (15.6k)
CDN
gzip -6 (default)
  15804 bytes (15.4k)
local copy
gzip -9
  15740 bytes (15.4k)
local copy
libdeflate -12
  15203 bytes (14.8k)
local copy
7zip -mx=9 -tgzip
  15174 bytes (14.8k)
local copy
kzip -s0 -rn -b4
  15167 bytes (14.8k)
local copy
pigz -11 -n
  15136 bytes (14.8k)
local copy
Zopfli
  15114 bytes (14.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.1.1.min.js

You will automatically get the smallest Socket.IO 2.1.1 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 --mls4 --bsr7 --lazy --ohh

(found June 26, 2018)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 4  --mls4
block splitting recursion 7  --bsr7
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/2.1.1/socket.io.min.js --location | md5sum
d3ef4d4b9d94a1263f9c7469abc3d9ba  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.1.1.min.zopfli.js.gz | md5sum
d3ef4d4b9d94a1263f9c7469abc3d9ba  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.1.1/socket.io.min.js --location | sha1sum
429551574f0236446952f4b88e99dcea2ac6530a  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.1.1.min.zopfli.js.gz | sha1sum
429551574f0236446952f4b88e99dcea2ac6530a  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 16534 bytes d3ef4d4b9d94a1263f9c7469abc3d9ba (invalid)
cdnjs 15953 bytes d3ef4d4b9d94a1263f9c7469abc3d9ba (invalid)

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

Other Versions

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

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
15114 bytes -1 byte zopfli --i1000000 --mls4 --bsr7 --lazy --ohh June 26, 2018 @ 20:51
15115 bytes -1 byte zopfli --i100000 --mls4 --bsr7 --lazy --ohh June 26, 2018 @ 17:20
15116 bytes -1 byte zopfli --i100000 --mls2 --bsr9 --lazy --ohh June 26, 2018 @ 12:10
15117 bytes -2 bytes zopfli --i10000 --mls2 --bsr9 --lazy --ohh June 26, 2018 @ 11:45
15119 bytes -1 byte zopfli --i1000 --mls16 --bsr25 --lazy --ohh June 26, 2018 @ 11:21
15120 bytes -1 byte zopfli --i1000 --mls4 --bsr16 --lazy --ohh June 26, 2018 @ 11:21
15121 bytes -2 bytes zopfli --i1000 --mls256 --bsr12 --lazy --ohh June 26, 2018 @ 11:20
15123 bytes -1 byte zopfli --i1000 --mls256 --bsr16 --lazy --ohh June 26, 2018 @ 11:20
15124 bytes -1 byte zopfli --i1000 --mls256 --bsr23 --lazy --ohh June 26, 2018 @ 11:20
15125 bytes zopfli --i100 --mls32 --bsr20 --lazy --ohh June 26, 2018 @ 11:14

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

Most recent activity on June 26, 2018 @ 20:51.

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
15129 15129 15128 15129 15128 15131 15132 15134 15132 15133 15131 15134 15132 15133 15135
15122 15120 15121 15122 15122 15132 15127 15129 15129 15125 15139 15143 15132 15130 15127
15128 15129 15129 15120 15126 15125 15120 15129 15128 15127 15140 15130 15133 15128 15128
15125 15114 15130 15126 15124 15134 15123 15123 15123 15125 15129 15130 15131 15131 15131
15131 15129 15132 15130 15131 15125 15132 15129 15122 15123 15126 15124 15131 15125 15127
15116 15116 15131 15122 15130 15125 15130 15121 15123 15127 15126 15131 15127 15130 15126
15125 15128 15131 15129 15129 15131 15131 15122 15131 15126 15139 15131 15131 15130 15129
15129 15130 15124 15120 15130 15125 15125 15128 15127 15130 15139 15124 15140 15127 15126
15125 15119 15131 15132 15123 15124 15127 15119 15122 15122 15120 15140 15131 15132 15124
15131 15122 15130 15127 15124 15128 15130 15120 15126 15126 15124 15130 15130 15130 15125
15122 15116 15131 15128 15117 15128 15130 15121 15123 15124 15140 15124 15129 15127 15124
15124 15125 15129 15127 15124 15124 15123 15122 15124 15125 15139 15132 15128 15133 15129
15127 15117 15130 15127 15126 15127 15130 15121 15123 15126 15139 15131 15133 15129 15126
15128 15117 15130 15131 15116 15128 15131 15128 15127 15123 15140 15124 15127 15127 15125
15122 15122 15130 15122 15125 15129 15130 15122 15123 15125 15142 15131 15130 15131 15130
15127 15119 15119 15126 15119 15129 15130 15123 15133 15126 15139 15131 15129 15132 15125
15127 15120 15130 15128 15116 15125 15130 15121 15123 15126 15140 15125 15131 15130 15125
15128 15126 15124 15124 15129 15125 15130 15123 15133 15128 15140 15140 15130 15131 15125
15120 15120 15131 15123 15121 15130 15130 15131 15131 15131 15129 15130 15128 15130 15124
15123 15132 15126 15131 15131 15128 15125 15122 15122 15126 15129 15129 15127 15128 15127
15122 15118 15130 15116 15116 15125 15126 15122 15123 15127 15129 15120 15132 15132 15125
15122 15123 15130 15130 15117 15125 15127 15121 15125 15123 15127 15128 15129 15131 15128
15124 15122 15129 15126 15125 15127 15126 15121 15124 15129 15130 15130 15131 15131 15124

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 15125 bytes 100%
1,000 15119 bytes -6 bytes 100%
10,000 15117 bytes -2 bytes 100%
100,000 15115 bytes -2 bytes 3.48%
1,000,000 15114 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
15186 bytes +72 bytes (+0.48%) +19 bytes
15182 bytes +68 bytes (+0.45%) +15 bytes
15179 bytes +65 bytes (+0.43%) +12 bytes
15206 bytes +92 bytes (+0.61%) +39 bytes
15167 bytes +53 bytes (+0.35%)
15182 bytes +68 bytes (+0.45%) +15 bytes
15217 bytes +103 bytes (+0.68%) +50 bytes
15253 bytes +139 bytes (+0.92%) +86 bytes
15281 bytes +167 bytes (+1.10%) +114 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12633 bytes -2481 bytes (-16.42%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12816 bytes -2298 bytes (-15.20%)
Brotli (Wikipedia) brotli brotli -q 11 13950 bytes -1164 bytes (-7.70%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13994 bytes -1120 bytes (-7.41%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14414 bytes -700 bytes (-4.63%)
LZMA2 (Wikipedia) xz xz -9 14676 bytes -438 bytes (-2.90%)
ZSTD (Wikipedia) zstd zstd -19 14944 bytes -170 bytes (-1.12%)

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 2018.
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