Choose a version:
34% The original file has 211903 bytes (206.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 72905 bytes (71.2k, 34%).

After GZIP compression these minified files vary in size:
Boot
  26210 bytes (25.6k)
CDN, direct link
cdnjs
  22297 bytes (21.8k)
CDN, direct link
gzip -6 (default)
  22102 bytes (21.6k)
local copy
cdnhttps
  22054 bytes (21.5k)
CDN, direct link
gzip -9
  22010 bytes (21.5k)
local copy
7zip -mx=9 -tgzip
  21286 bytes (20.8k)
local copy
libdeflate -12
  21285 bytes (20.8k)
local copy
kzip -s0 -rn -b3
  21198 bytes (20.7k)
local copy
pigz -11 -n
  21195 bytes (20.7k)
local copy
Zopfli
  21171 bytes (20.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.6.0.min.js

You will automatically get the smallest Socket.IO 1.6.0 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 --mls8 --bsr16 --lazy --ohh

(found November 21, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 8  --mls8
block splitting recursion 16  --bsr16
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/1.6.0/socket.io.min.js --location | md5sum
8c95b98a636a39df32c396d665ef6908  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.6.0.min.zopfli.js.gz | md5sum
8c95b98a636a39df32c396d665ef6908  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.6.0/socket.io.min.js --location | sha1sum
2bee6ee40e62c6de09c56504e6a398efd566e82a  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.6.0.min.zopfli.js.gz | sha1sum
2bee6ee40e62c6de09c56504e6a398efd566e82a  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 26210 bytes 8c95b98a636a39df32c396d665ef6908 (invalid)
cdnjs 22297 bytes 8c95b98a636a39df32c396d665ef6908 (invalid)
cdnhttps 22054 bytes 8c95b98a636a39df32c396d665ef6908 November 22, 2016 @ 16:41

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

Other Versions

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

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
21171 bytes -2 bytes zopfli --i1000000 --mls8 --bsr16 --lazy --ohh November 21, 2016 @ 21:40
21173 bytes -1 byte zopfli --i100000 --mls8 --bsr16 --lazy --ohh November 21, 2016 @ 17:04
21174 bytes -3 bytes zopfli --i10000 --mls8 --bsr16 --lazy --ohh November 21, 2016 @ 16:05
21177 bytes zopfli --i100 --mls8 --bsr16 --lazy --ohh November 21, 2016 @ 16:01

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

Most recent activity on January 25, 2017 @ 15:29.

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:
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
21185 21191 21190 21184 21180 21191 21187 21183 21184 21177 21176 21180 21183 21183 21187
21188 21186 21186 21186 21181 21182 21178 21176 21179 21177 21179 21182 21180 21179 21178
21178 21175 21177 21182 21183 21175 21174 21174 21176 21175 21179 21179 21177 21179 21184
21178 21175 21179 21182 21181 21178 21175 21174 21174 21178 21172 21181 21178 21182 21180
21177 21182 21183 21180 21181 21178 21179 21177 21177 21179 21179 21182 21184 21176 21197
21180 21186 21186 21180 21179 21179 21179 21178 21179 21179 21177 21180 21178 21175 21178
21178 21179 21178 21177 21174 21180 21178 21174 21175 21175 21172 21181 21183 21182 21181
21179 21179 21188 21180 21179 21173 21178 21177 21179 21178 21176 21181 21178 21184 21181
21180 21180 21179 21182 21180 21180 21175 21178 21174 21178 21174 21179 21182 21182 21178
21177 21180 21177 21180 21180 21181 21174 21174 21178 21178 21175 21183 21183 21183 21183
21179 21180 21175 21178 21180 21175 21178 21186 21178 21176 21172 21180 21184 21182 21183
21178 21177 21179 21180 21178 21178 21176 21174 21174 21177 21174 21182 21180 21182 21179
21179 21173 21171 21178 21179 21175 21174 21174 21180 21174 21174 21183 21179 21182 21182
21183 21182 21188 21180 21179 21181 21172 21176 21178 21175 21172 21179 21178 21185 21179
21179 21174 21183 21180 21179 21173 21174 21176 21175 21174 21174 21182 21181 21175 21180
21179 21174 21179 21179 21180 21182 21178 21180 21179 21178 21175 21178 21181 21182 21179
21177 21175 21179 21178 21179 21173 21174 21178 21179 21181 21172 21181 21178 21182 21180
21180 21177 21174 21182 21179 21178 21178 21178 21178 21178 21180 21182 21178 21183 21179
21176 21175 21178 21180 21179 21179 21174 21178 21179 21174 21176 21177 21183 21182 21184
21180 21174 21180 21180 21180 21180 21174 21179 21175 21178 21177 21180 21184 21185 21179
21177 21174 21179 21180 21182 21180 21178 21179 21174 21180 21172 21180 21182 21182 21179
21184 21176 21179 21180 21180 21176 21181 21179 21174 21180 21176 21182 21183 21183 21178
21181 21174 21174 21180 21181 21177 21178 21177 21181 21181 21177 21182 21178 21183 21182

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 21177 bytes 100%
1,000 21177 bytes 100%
10,000 21174 bytes -3 bytes 100%
100,000 21173 bytes -1 byte 100%
1,000,000 21171 bytes -2 bytes 3.48%
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
21263 bytes +92 bytes (+0.43%) +65 bytes
21263 bytes +92 bytes (+0.43%) +65 bytes
21203 bytes +32 bytes (+0.15%) +5 bytes
21198 bytes +27 bytes (+0.13%)
21205 bytes +34 bytes (+0.16%) +7 bytes
21227 bytes +56 bytes (+0.26%) +29 bytes
21276 bytes +105 bytes (+0.50%) +78 bytes
21295 bytes +124 bytes (+0.59%) +97 bytes
21328 bytes +157 bytes (+0.74%) +130 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 17610 bytes -3561 bytes (-16.82%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 17904 bytes -3267 bytes (-15.43%)
Brotli (Wikipedia) brotli brotli -q 11 19407 bytes -1764 bytes (-8.33%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 19439 bytes -1732 bytes (-8.18%)
LZMA2 (Wikipedia) xz xz -9 20216 bytes -955 bytes (-4.51%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 20603 bytes -568 bytes (-2.68%)
ZSTD (Wikipedia) zstd zstd -19 20693 bytes -478 bytes (-2.26%)

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