Choose a version:
52% The original file has 155214 bytes (151.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 80786 bytes (78.9k, 52%).

After GZIP compression these minified files vary in size:
Boot
  25045 bytes (24.5k)
CDN
cdnjs
  20928 bytes (20.4k)
CDN
gzip -6 (default)
  20743 bytes (20.3k)
local copy
cdnhttps
  20720 bytes (20.2k)
CDN
gzip -9
  20666 bytes (20.2k)
local copy
7zip -mx=9 -tgzip
  19939 bytes (19.5k)
local copy
libdeflate -12
  19929 bytes (19.5k)
local copy
pigz -11 -n
  19891 bytes (19.4k)
local copy
kzip -s0 -rn -b0
  19878 bytes (19.4k)
local copy
Zopfli
  19862 bytes (19.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.0.6.min.js

You will automatically get the smallest Socket.IO 1.0.6 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 --mls256 --bsr13 --lazy --ohh

(found July 1, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 13  --bsr13
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/1.0.6/socket.io.min.js --location | md5sum
9e27d3999ef24d82c6ad274b9c0e4b0d  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.6.min.zopfli.js.gz | md5sum
9e27d3999ef24d82c6ad274b9c0e4b0d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.0.6/socket.io.min.js --location | sha1sum
f76737a8b7ca0a8a85fd35aa9f6f461e7447aead  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.6.min.zopfli.js.gz | sha1sum
f76737a8b7ca0a8a85fd35aa9f6f461e7447aead  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 25045 bytes 9e27d3999ef24d82c6ad274b9c0e4b0d (invalid)
cdnjs 20928 bytes 9e27d3999ef24d82c6ad274b9c0e4b0d (invalid)
cdnhttps 20720 bytes 9e27d3999ef24d82c6ad274b9c0e4b0d (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
19862 bytes -1 byte zopfli --i100000 --mls256 --bsr13 --lazy --ohh July 1, 2016 @ 14:48
19863 bytes -1 byte zopfli --i10000 --mls16 --bsr13 --lazy --ohh June 30, 2016 @ 08:17
19864 bytes -1 byte zopfli --i1000 --mls256 --bsr13 --lazy --ohh June 29, 2016 @ 17:15
19865 bytes -1 byte zopfli --i10000 --mls256 --bsr18 --lazy --ohh June 29, 2016 @ 15:48
19866 bytes -1 byte zopfli --i1000 --mls256 --bsr18 --lazy --ohh June 29, 2016 @ 15:29
19867 bytes zopfli --i100 --mls256 --bsr18 --lazy --ohh June 29, 2016 @ 13:14

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

Most recent activity on July 3, 2016 @ 03:00.

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 or 100,000.
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
19875 19876 19876 19874 19873 19867 19892 19866 19881 19880 19880 19880 19880 19879 19882
19874 19871 19873 19874 19869 19875 19886 19872 19886 19884 19867 19879 19880 19881 19881
19876 19868 19873 19869 19869 19872 19887 19869 19876 19885 19878 19879 19880 19884 19884
19868 19875 19876 19869 19868 19872 19888 19871 19865 19880 19879 19880 19878 19879 19879
19864 19865 19867 19874 19868 19870 19871 19872 19864 19879 19878 19878 19878 19879 19879
19866 19865 19865 19866 19866 19865 19865 19862 19866 19885 19878 19878 19880 19879 19880
19869 19870 19868 19868 19868 19866 19871 19870 19865 19879 19879 19878 19878 19879 19882
19868 19869 19869 19874 19872 19871 19872 19870 19864 19880 19878 19879 19879 19879 19880
19874 19869 19874 19870 19870 19876 19887 19874 19876 19885 19886 19879 19879 19879 19879
19872 19869 19868 19863 19865 19863 19865 19862 19864 19884 19878 19882 19883 19879 19880
19870 19871 19871 19871 19864 19866 19871 19865 19865 19879 19879 19878 19878 19879 19880
19868 19873 19874 19868 19868 19872 19872 19870 19865 19879 19879 19878 19878 19879 19880
19875 19875 19873 19875 19873 19869 19872 19871 19869 19878 19879 19878 19878 19879 19879
19867 19871 19867 19871 19874 19885 19879 19874 19864 19882 19879 19879 19878 19880 19883
19868 19868 19871 19864 19865 19865 19866 19865 19865 19883 19879 19879 19878 19879 19880
19867 19868 19868 19873 19869 19872 19884 19865 19864 19878 19878 19879 19878 19880 19879
19866 19866 19872 19875 19865 19874 19863 19864 19862 19880 19878 19878 19879 19879 19880
19874 19874 19874 19871 19868 19875 19872 19868 19864 19879 19879 19879 19878 19879 19880
19865 19866 19872 19871 19873 19875 19873 19873 19868 19883 19878 19878 19879 19880 19880
19874 19869 19874 19870 19868 19877 19871 19874 19866 19879 19878 19879 19879 19879 19879
19868 19868 19874 19870 19868 19871 19871 19874 19865 19885 19879 19878 19878 19879 19880
19868 19868 19865 19870 19864 19864 19865 19864 19865 19879 19879 19878 19879 19878 19879
19867 19867 19873 19865 19869 19864 19865 19864 19864 19879 19878 19879 19879 19879 19880

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 19867 bytes 100%
1,000 19864 bytes -3 bytes 100%
10,000 19863 bytes -1 byte 100%
100,000 19862 bytes -1 byte 1.74%
1,000,000
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
19878 bytes +16 bytes (+0.08%)
19904 bytes +42 bytes (+0.21%) +26 bytes
19879 bytes +17 bytes (+0.09%) +1 byte
19925 bytes +63 bytes (+0.32%) +47 bytes
19958 bytes +96 bytes (+0.48%) +80 bytes
19993 bytes +131 bytes (+0.66%) +115 bytes
19997 bytes +135 bytes (+0.68%) +119 bytes
20031 bytes +169 bytes (+0.85%) +153 bytes
20063 bytes +201 bytes (+1.01%) +185 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
ZPAQ (Wikipedia) zpaq zpaq -method 69 16242 bytes -3620 bytes (-18.23%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 16968 bytes -2894 bytes (-14.57%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 17621 bytes -2241 bytes (-11.28%)
Brotli (Wikipedia) brotli brotli -q 11 18788 bytes -1074 bytes (-5.41%)
LZMA2 (Wikipedia) xz xz -9 19476 bytes -386 bytes (-1.94%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19536 bytes -326 bytes (-1.64%)
ZSTD (Wikipedia) zstd zstd -19 19750 bytes -112 bytes (-0.56%)

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