Choose a version:
53% The original file has 174605 bytes (170.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 91999 bytes (89.8k, 53%).

After GZIP compression these minified files vary in size:
Boot
  23880 bytes (23.3k)
CDN
gzip -6 (default)
  23805 bytes (23.2k)
local copy
gzip -9
  23692 bytes (23.1k)
local copy
libdeflate -12
  22896 bytes (22.4k)
local copy
7zip -mx=9 -tgzip
  22837 bytes (22.3k)
local copy
kzip -s0 -rn -b0
  22811 bytes (22.3k)
local copy
pigz -11 -n
  22802 bytes (22.3k)
local copy
Zopfli
  22784 bytes (22.3k)
local copy
cdnjs
  20416 bytes (19.9k)
CDN
cdnhttps
  20239 bytes (19.8k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.3.3.min.js

You will automatically get the smallest Socket.IO 1.3.3 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 --mls16 --bsr8 --lazy --ohh

(found June 29, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 16  --mls16
block splitting recursion 8  --bsr8
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/1.3.3/socket.io.min.js --location | md5sum
18cb2726a4625b3548c3bcc23bbb3e69  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.3.min.zopfli.js.gz | md5sum
18cb2726a4625b3548c3bcc23bbb3e69  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.3.3/socket.io.min.js --location | sha1sum
4187a3f8db883ab2607bb03bab24e01c138bd4ce  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.3.min.zopfli.js.gz | sha1sum
4187a3f8db883ab2607bb03bab24e01c138bd4ce  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 23880 bytes 29793458fc86a29af60f8bbff8aba907 < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< var inherit=_dereq_("component-inherit");module.exports=JS [...]
< if((extra&64512)==56320){output.push(((value&1023)<<10)+(e [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> }).call(this,"undefined"!=typeof self?self:"undefined"!=ty [...]
> }if(o=i.charCodeAt(P),101==o||69==o){for(o=i.charCodeAt(++ [...]
(invalid)
cdnjs 20416 bytes 29793458fc86a29af60f8bbff8aba907 < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< var inherit=_dereq_("component-inherit");module.exports=JS [...]
< if((extra&64512)==56320){output.push(((value&1023)<<10)+(e [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> }).call(this,"undefined"!=typeof self?self:"undefined"!=ty [...]
> }if(o=i.charCodeAt(P),101==o||69==o){for(o=i.charCodeAt(++ [...]
(invalid)
cdnhttps 20239 bytes 29793458fc86a29af60f8bbff8aba907 < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< var inherit=_dereq_("component-inherit");module.exports=JS [...]
< if((extra&64512)==56320){output.push(((value&1023)<<10)+(e [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> }).call(this,"undefined"!=typeof self?self:"undefined"!=ty [...]
> }if(o=i.charCodeAt(P),101==o||69==o){for(o=i.charCodeAt(++ [...]
(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
22784 bytes -1 byte zopfli --i100000 --mls16 --bsr8 --lazy --ohh June 29, 2016 @ 16:50
22785 bytes -1 byte zopfli --i10000 --mls16 --bsr8 --lazy --ohh June 29, 2016 @ 15:27
22786 bytes -5 bytes zopfli --i1000 --mls16 --bsr8 --lazy --ohh June 29, 2016 @ 15:24
22791 bytes zopfli --i100 --mls16 --bsr8 --lazy --ohh June 29, 2016 @ 14:06

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 @ 02:51.

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 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
22791 22790 22801 22799 22790 22791 22792 22791 22792 22795 22806 22804 22805 22805 22804
22814 22815 22816 22818 22797 22797 22792 22791 22790 22792 22804 22805 22802 22804 22803
22788 22789 22789 22789 22789 22788 22787 22787 22789 22789 22804 22804 22805 22804 22807
22788 22787 22788 22788 22787 22788 22787 22787 22788 22789 22790 22805 22803 22804 22805
22789 22789 22787 22784 22788 22787 22787 22790 22789 22789 22790 22804 22804 22805 22805
22787 22791 22789 22787 22791 22788 22787 22790 22789 22789 22803 22804 22803 22805 22804
22789 22788 22801 22797 22796 22794 22787 22787 22787 22789 22804 22804 22804 22804 22804
22788 22796 22796 22797 22795 22795 22795 22796 22788 22788 22789 22803 22805 22805 22804
22786 22787 22795 22794 22788 22789 22787 22787 22787 22789 22790 22790 22805 22804 22804
22788 22787 22787 22792 22787 22789 22786 22787 22789 22790 22804 22790 22803 22804 22802
22788 22787 22788 22787 22789 22789 22786 22787 22786 22792 22789 22802 22805 22803 22807
22787 22787 22788 22787 22793 22790 22787 22787 22788 22789 22793 22804 22803 22804 22804
22787 22787 22795 22797 22796 22795 22796 22796 22788 22791 22790 22790 22804 22803 22807
22787 22787 22788 22795 22788 22789 22787 22787 22787 22788 22792 22804 22803 22804 22807
22787 22787 22789 22787 22787 22789 22789 22788 22788 22791 22805 22805 22803 22805 22807
22788 22788 22789 22788 22792 22790 22788 22788 22788 22791 22790 22790 22805 22805 22805
22789 22795 22794 22794 22797 22798 22787 22788 22790 22789 22805 22802 22805 22805 22804
22789 22787 22796 22795 22787 22790 22787 22787 22788 22789 22790 22804 22803 22805 22805
22787 22787 22798 22801 22796 22792 22786 22787 22786 22795 22804 22804 22805 22805 22805
22788 22787 22788 22788 22788 22795 22786 22787 22789 22788 22805 22803 22805 22805 22806
22790 22788 22789 22789 22788 22786 22787 22787 22786 22791 22805 22790 22805 22804 22804
22787 22787 22794 22795 22795 22795 22786 22787 22788 22789 22790 22790 22805 22804 22804
22786 22788 22787 22788 22789 22789 22788 22786 22789 22790 22793 22804 22803 22805 22804

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 22791 bytes 100%
1,000 22786 bytes -5 bytes 100%
10,000 22785 bytes -1 byte 100%
100,000 22784 bytes -1 byte 0.29%
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
22811 bytes +27 bytes (+0.12%)
22829 bytes +45 bytes (+0.20%) +18 bytes
22814 bytes +30 bytes (+0.13%) +3 bytes
22856 bytes +72 bytes (+0.32%) +45 bytes
22885 bytes +101 bytes (+0.44%) +74 bytes
22918 bytes +134 bytes (+0.59%) +107 bytes
22953 bytes +169 bytes (+0.74%) +142 bytes
22984 bytes +200 bytes (+0.88%) +173 bytes
22982 bytes +198 bytes (+0.87%) +171 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 18388 bytes -4396 bytes (-19.29%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 19403 bytes -3381 bytes (-14.84%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 20382 bytes -2402 bytes (-10.54%)
Brotli (Wikipedia) brotli brotli -q 11 21481 bytes -1303 bytes (-5.72%)
LZMA2 (Wikipedia) xz xz -9 22228 bytes -556 bytes (-2.44%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 22470 bytes -314 bytes (-1.38%)
ZSTD (Wikipedia) zstd zstd -19 22560 bytes -224 bytes (-0.98%)

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