Choose a version:
53% The original file has 173966 bytes (169.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 91538 bytes (89.4k, 53%).

After GZIP compression these minified files vary in size:
Boot
  23745 bytes (23.2k)
CDN
gzip -6 (default)
  23688 bytes (23.1k)
local copy
gzip -9
  23583 bytes (23.0k)
local copy
libdeflate -12
  22772 bytes (22.2k)
local copy
7zip -mx=9 -tgzip
  22708 bytes (22.2k)
local copy
kzip -s0 -rn -b0
  22673 bytes (22.1k)
local copy
pigz -11 -n
  22668 bytes (22.1k)
local copy
Zopfli
  22651 bytes (22.1k)
local copy
cdnjs
  20323 bytes (19.8k)
CDN
cdnhttps
  20146 bytes (19.7k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.3.5.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.3.5 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 --mls512 --bsr7 --lazy --ohh

(found July 4, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 512  --mls512
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/1.3.5/socket.io.min.js --location | md5sum
38dc67c66183f4f3dc4af8eb5bb31300  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.5.min.zopfli.js.gz | md5sum
38dc67c66183f4f3dc4af8eb5bb31300  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.3.5/socket.io.min.js --location | sha1sum
95e44cbfd9951ea658db0856a43ddf5ed0796748  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.5.min.zopfli.js.gz | sha1sum
95e44cbfd9951ea658db0856a43ddf5ed0796748  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 23745 bytes 6553172c6dc3b4fa88b3d76b4eb148fc < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< output+=stringFromCharCode(value>>>10&1023|55296);value=56 [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> if("null"==i.slice(P,P+4))return P+=4,null;q()}return"$"}, [...]
(invalid)
cdnjs 20323 bytes 6553172c6dc3b4fa88b3d76b4eb148fc < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< output+=stringFromCharCode(value>>>10&1023|55296);value=56 [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> if("null"==i.slice(P,P+4))return P+=4,null;q()}return"$"}, [...]
(invalid)
cdnhttps 20146 bytes 6553172c6dc3b4fa88b3d76b4eb148fc < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< output+=stringFromCharCode(value>>>10&1023|55296);value=56 [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> if("null"==i.slice(P,P+4))return P+=4,null;q()}return"$"}, [...]
(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
22651 bytes -1 byte zopfli --i100000 --mls512 --bsr7 --lazy --ohh July 4, 2016 @ 12:18
22652 bytes -1 byte zopfli --i1000 --mls2048 --bsr40 --lazy --ohh June 29, 2016 @ 17:27
22653 bytes -1 byte zopfli --i1000 --mls1024 --bsr17 --lazy --ohh June 29, 2016 @ 17:14
22654 bytes -1 byte zopfli --i1000 --mls128 --bsr23 --lazy --ohh June 29, 2016 @ 15:59
22655 bytes zopfli --i100 --mls2048 --bsr25 --lazy --ohh June 29, 2016 @ 13:25

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

Most recent activity on July 4, 2016 @ 13:24.

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
22658 22658 22658 22657 22657 22666 22657 22657 22661 22661 22669 22670 22670 22669 22668
22681 22682 22682 22682 22683 22663 22663 22662 22655 22655 22668 22668 22668 22668 22668
22654 22654 22655 22654 22654 22655 22654 22653 22653 22655 22653 22668 22669 22669 22669
22657 22659 22655 22655 22656 22654 22653 22654 22651 22653 22670 22667 22667 22669 22669
22654 22653 22654 22655 22653 22655 22654 22653 22656 22653 22669 22670 22669 22669 22669
22659 22660 22655 22654 22662 22665 22661 22653 22654 22655 22653 22667 22669 22668 22669
22658 22654 22660 22655 22660 22662 22660 22660 22654 22657 22659 22671 22669 22669 22668
22656 22658 22653 22654 22654 22662 22661 22660 22654 22653 22670 22670 22669 22668 22668
22653 22671 22661 22661 22654 22664 22653 22654 22654 22653 22670 22669 22667 22668 22669
22653 22653 22662 22654 22654 22655 22654 22653 22655 22657 22659 22668 22668 22668 22667
22659 22653 22654 22654 22656 22653 22653 22653 22654 22657 22655 22667 22669 22668 22673
22654 22655 22653 22655 22652 22655 22658 22655 22658 22653 22660 22668 22669 22669 22669
22658 22658 22660 22655 22661 22662 22660 22660 22651 22654 22656 22668 22668 22669 22668
22659 22653 22653 22654 22656 22656 22653 22654 22653 22652 22670 22669 22668 22668 22667
22659 22654 22653 22654 22654 22656 22654 22655 22654 22657 22670 22667 22668 22669 22669
22659 22657 22662 22654 22655 22655 22654 22654 22655 22653 22655 22668 22669 22669 22669
22667 22662 22661 22662 22660 22661 22661 22654 22654 22653 22654 22668 22669 22669 22669
22656 22656 22661 22661 22655 22655 22653 22654 22653 22653 22653 22669 22669 22668 22672
22657 22653 22654 22653 22658 22657 22653 22653 22654 22654 22670 22667 22669 22669 22669
22656 22654 22654 22655 22654 22654 22653 22654 22653 22653 22670 22667 22669 22668 22669
22658 22656 22663 22661 22656 22662 22653 22654 22653 22654 22653 22667 22669 22668 22669
22658 22659 22661 22654 22658 22657 22653 22653 22658 22655 22653 22670 22669 22669 22669
22659 22655 22654 22654 22652 22654 22654 22654 22657 22654 22652 22669 22669 22669 22669

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 22655 bytes 100%
1,000 22652 bytes -3 bytes 100%
10,000 22652 bytes 100%
100,000 22651 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
22673 bytes +22 bytes (+0.10%)
22693 bytes +42 bytes (+0.19%) +20 bytes
22674 bytes +23 bytes (+0.10%) +1 byte
22720 bytes +69 bytes (+0.30%) +47 bytes
22748 bytes +97 bytes (+0.43%) +75 bytes
22778 bytes +127 bytes (+0.56%) +105 bytes
22811 bytes +160 bytes (+0.71%) +138 bytes
22839 bytes +188 bytes (+0.83%) +166 bytes
22842 bytes +191 bytes (+0.84%) +169 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
ZPAQ (Wikipedia) zpaq zpaq -method 69 18317 bytes -4334 bytes (-19.13%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 19325 bytes -3326 bytes (-14.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 20276 bytes -2375 bytes (-10.49%)
Brotli (Wikipedia) brotli brotli -q 11 21370 bytes -1281 bytes (-5.66%)
LZMA2 (Wikipedia) xz xz -9 22112 bytes -539 bytes (-2.38%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 22317 bytes -334 bytes (-1.47%)
Zstandard (Wikipedia) zstd zstd -19 22444 bytes -207 bytes (-0.91%)

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