Choose a version:
86% The original file has 60730 bytes (59.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 52419 bytes (51.2k, 86%).

After GZIP compression these minified files vary in size:
Boot
  18502 bytes (18.1k)
CDN
cdnjs
  15682 bytes (15.3k)
CDN
gzip -6 (default)
  15524 bytes (15.2k)
local copy
gzip -9
  15460 bytes (15.1k)
local copy
libdeflate -12
  14939 bytes (14.6k)
local copy
7zip -mx=9 -tgzip
  14912 bytes (14.6k)
local copy
kzip -s0 -rn -b4
  14903 bytes (14.6k)
local copy
pigz -11 -n
  14870 bytes (14.5k)
local copy
Zopfli
  14849 bytes (14.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.0.4.min.js

You will automatically get the smallest Socket.IO 2.0.4 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 --mls128 --bsr18 --lazy --ohh

(found October 23, 2017)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 18  --bsr18
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/2.0.4/socket.io.min.js --location | md5sum
d36d8ec02255a85e55e9d951e977e402  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.0.4.min.zopfli.js.gz | md5sum
d36d8ec02255a85e55e9d951e977e402  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.0.4/socket.io.min.js --location | sha1sum
c6c00e3a418b10284a0728f305c8ac29604150fc  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.0.4.min.zopfli.js.gz | sha1sum
c6c00e3a418b10284a0728f305c8ac29604150fc  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 18502 bytes d36d8ec02255a85e55e9d951e977e402 November 14, 2017 @ 18:05
cdnjs 15682 bytes d36d8ec02255a85e55e9d951e977e402 (invalid)

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

Other Versions

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

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
14849 bytes -1 byte zopfli --i1000000 --mls128 --bsr18 --lazy --ohh October 23, 2017 @ 17:56
14850 bytes -1 byte zopfli --i10000 --mls128 --bsr18 --lazy --ohh October 23, 2017 @ 15:11
14851 bytes -2 bytes zopfli --i10000 --mls2 --bsr40 --lazy --ohh October 23, 2017 @ 15:00
14853 bytes -1 byte zopfli --i10000 --mls128 --bsr9 --lazy --ohh October 23, 2017 @ 14:54
14854 bytes -1 byte zopfli --i1000 --mls128 --bsr9 --lazy --ohh October 23, 2017 @ 14:41
14855 bytes -1 byte zopfli --i1000 --mls4 --bsr8 --lazy --ohh October 23, 2017 @ 14:35
14856 bytes zopfli --i100 --mls512 --bsr30 --lazy --ohh October 23, 2017 @ 14:29

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

Most recent activity on November 14, 2017 @ 18:05.

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
14864 14863 14864 14862 14864 14863 14862 14862 14861 14877 14877 14876 14877 14870 14874
14865 14853 14864 14864 14863 14863 14864 14854 14863 14860 14874 14875 14878 14864 14860
14859 14858 14857 14858 14854 14859 14855 14861 14860 14871 14872 14876 14861 14857 14857
14855 14857 14854 14856 14854 14854 14855 14854 14862 14872 14873 14872 14863 14853 14859
14865 14854 14855 14853 14855 14862 14855 14860 14861 14871 14872 14873 14876 14853 14868
14854 14860 14859 14853 14855 14854 14853 14855 14871 14862 14874 14876 14864 14858 14857
14856 14858 14855 14853 14856 14858 14854 14858 14857 14872 14858 14872 14876 14855 14859
14854 14863 14855 14854 14854 14863 14853 14857 14864 14872 14874 14873 14876 14861 14868
14852 14853 14854 14855 14854 14857 14851 14857 14857 14872 14873 14871 14863 14857 14859
14858 14857 14858 14862 14854 14857 14852 14854 14859 14864 14872 14873 14864 14857 14868
14854 14858 14854 14854 14855 14851 14853 14864 14858 14863 14872 14872 14861 14854 14868
14854 14857 14854 14854 14854 14854 14855 14854 14862 14872 14856 14873 14864 14854 14859
14859 14855 14857 14853 14854 14855 14854 14855 14860 14872 14873 14873 14875 14857 14868
14854 14854 14858 14860 14852 14862 14855 14857 14859 14863 14872 14872 14860 14856 14862
14852 14854 14855 14854 14852 14851 14849 14854 14861 14872 14870 14872 14862 14851 14868
14854 14857 14853 14853 14853 14858 14856 14859 14859 14872 14873 14872 14876 14856 14859
14854 14857 14853 14854 14853 14863 14851 14857 14859 14863 14870 14872 14877 14859 14858
14854 14857 14859 14854 14858 14853 14857 14854 14859 14871 14872 14871 14875 14853 14868
14855 14853 14855 14862 14853 14853 14851 14854 14862 14872 14872 14871 14877 14858 14868
14852 14857 14855 14854 14854 14861 14853 14858 14863 14871 14873 14873 14876 14854 14858
14855 14856 14856 14853 14855 14864 14853 14857 14862 14871 14857 14861 14860 14857 14859
14854 14855 14857 14853 14855 14851 14853 14857 14853 14872 14872 14872 14863 14854 14867
14851 14856 14858 14853 14855 14854 14856 14855 14866 14862 14873 14877 14877 14851 14866

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 14856 bytes 100%
1,000 14854 bytes -2 bytes 100%
10,000 14850 bytes -4 bytes 100%
100,000 14850 bytes 0.58%
1,000,000 14849 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
14913 bytes +64 bytes (+0.43%) +10 bytes
14914 bytes +65 bytes (+0.44%) +11 bytes
14919 bytes +70 bytes (+0.47%) +16 bytes
14937 bytes +88 bytes (+0.59%) +34 bytes
14903 bytes +54 bytes (+0.36%)
14919 bytes +70 bytes (+0.47%) +16 bytes
14951 bytes +102 bytes (+0.69%) +48 bytes
14991 bytes +142 bytes (+0.96%) +88 bytes
15018 bytes +169 bytes (+1.14%) +115 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 12433 bytes -2416 bytes (-16.27%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12616 bytes -2233 bytes (-15.04%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13696 bytes -1153 bytes (-7.76%)
Brotli (Wikipedia) brotli brotli -q 11 13746 bytes -1103 bytes (-7.43%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14198 bytes -651 bytes (-4.38%)
LZMA2 (Wikipedia) xz xz -9 14452 bytes -397 bytes (-2.67%)
ZSTD (Wikipedia) zstd zstd -19 14687 bytes -162 bytes (-1.09%)

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 2017.
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