Choose a version:
52% The original file has 169678 bytes (165.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 88884 bytes (86.8k, 52%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  23251 bytes (22.7k)
local copy
Boot
  23165 bytes (22.6k)
CDN
gzip -9
  23164 bytes (22.6k)
local copy
libdeflate -12
  22392 bytes (21.9k)
local copy
7zip -mx=9 -tgzip
  22312 bytes (21.8k)
local copy
kzip -s0 -rn -b2
  22281 bytes (21.8k)
local copy
pigz -11 -n
  22270 bytes (21.7k)
local copy
Zopfli
  22248 bytes (21.7k)
local copy
cdnjs
  19895 bytes (19.4k)
CDN
cdnhttps
  19735 bytes (19.3k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.3.0.min.js

You will automatically get the smallest Socket.IO 1.3.0 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 --i100000 --mb8 --mls64 --bsr18 --lazy --ohh

(found June 29, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 64  --mls64
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/1.3.0/socket.io.min.js --location | md5sum
6583c4887c0335fad997a8470aeaaa43  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.0.min.zopfli.js.gz | md5sum
6583c4887c0335fad997a8470aeaaa43  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.3.0/socket.io.min.js --location | sha1sum
717e97ef9397eb2f87654883800fa8a7fa56fc3f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.0.min.zopfli.js.gz | sha1sum
717e97ef9397eb2f87654883800fa8a7fa56fc3f  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 23165 bytes 997598ed6d5ecdb6c727e8dabf492138 < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.form=null;this.iframe=null}Polling.prototype.doClose. [...]
< for(var key in utf8){hasOwnProperty.call(utf8,key)&&(freeE [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> a("polling got data %s",t);var n=function(t){return"openin [...]
(invalid)
cdnjs 19895 bytes 997598ed6d5ecdb6c727e8dabf492138 < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.form=null;this.iframe=null}Polling.prototype.doClose. [...]
< for(var key in utf8){hasOwnProperty.call(utf8,key)&&(freeE [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> a("polling got data %s",t);var n=function(t){return"openin [...]
(invalid)
cdnhttps 19735 bytes 997598ed6d5ecdb6c727e8dabf492138 < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.form=null;this.iframe=null}Polling.prototype.doClose. [...]
< for(var key in utf8){hasOwnProperty.call(utf8,key)&&(freeE [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> a("polling got data %s",t);var n=function(t){return"openin [...]
(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
22248 bytes -1 byte zopfli --i100000 --mls64 --bsr18 --lazy --ohh June 29, 2016 @ 17:40
22249 bytes -3 bytes zopfli --i10000 --mls64 --bsr18 --lazy --ohh June 29, 2016 @ 16:08
22252 bytes -5 bytes zopfli --i1000 --mls128 --bsr20 --lazy --ohh June 29, 2016 @ 15:26
22257 bytes zopfli --i100 --mls64 --bsr18 --lazy --ohh June 29, 2016 @ 13:57

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 @ 07:29.

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:
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
22260 22260 22261 22261 22261 22260 22261 22261 22260 22275 22279 22278 22279 22275 22276
22280 22279 22287 22292 22258 22259 22263 22259 22261 22259 22279 22277 22274 22276 22277
22263 22263 22273 22280 22264 22259 22259 22257 22260 22260 22278 22278 22278 22279 22277
22262 22259 22259 22263 22260 22257 22258 22257 22258 22258 22278 22277 22278 22276 22280
22262 22286 22291 22259 22257 22263 22260 22258 22262 22280 22278 22278 22276 22278 22277
22280 22261 22258 22258 22263 22264 22261 22258 22258 22258 22277 22275 22277 22277 22277
22258 22258 22272 22261 22257 22258 22257 22259 22258 22258 22278 22275 22278 22279 22277
22273 22272 22272 22273 22257 22259 22258 22257 22262 22257 22277 22276 22276 22279 22277
22275 22273 22273 22274 22270 22273 22262 22293 22257 22258 22278 22277 22276 22277 22278
22257 22257 22259 22261 22257 22261 22257 22260 22258 22257 22278 22275 22276 22277 22277
22274 22264 22259 22261 22261 22258 22258 22258 22258 22259 22277 22277 22277 22277 22278
22260 22260 22258 22264 22257 22257 22257 22257 22257 22258 22278 22277 22278 22277 22279
22282 22280 22273 22274 22258 22258 22259 22263 22262 22258 22276 22275 22275 22277 22277
22266 22259 22259 22261 22257 22263 22257 22258 22258 22258 22278 22277 22276 22277 22278
22266 22262 22259 22261 22260 22248 22250 22261 22263 22258 22277 22275 22276 22278 22281
22279 22280 22272 22273 22263 22258 22259 22257 22258 22257 22275 22275 22278 22278 22279
22259 22259 22272 22281 22258 22259 22250 22257 22263 22258 22275 22278 22278 22277 22280
22266 22262 22259 22281 22270 22274 22257 22259 22258 22257 22275 22277 22278 22278 22278
22267 22264 22259 22265 22269 22264 22265 22258 22259 22257 22277 22277 22277 22278 22277
22259 22258 22260 22263 22257 22259 22258 22257 22257 22257 22278 22275 22278 22277 22277
22262 22280 22273 22272 22257 22275 22259 22292 22262 22260 22278 22277 22277 22279 22279
22265 22262 22259 22260 22257 22265 22258 22258 22258 22276 22275 22277 22276 22279 22277
22263 22260 22272 22281 22259 22259 22265 22258 22258 22258 22278 22278 22276 22276 22277

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 22257 bytes 100%
1,000 22252 bytes -5 bytes 100%
10,000 22249 bytes -3 bytes 100%
100,000 22248 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
22282 bytes +34 bytes (+0.15%) +1 byte
22295 bytes +47 bytes (+0.21%) +14 bytes
22281 bytes +33 bytes (+0.15%)
22330 bytes +82 bytes (+0.37%) +49 bytes
22355 bytes +107 bytes (+0.48%) +74 bytes
22390 bytes +142 bytes (+0.64%) +109 bytes
22426 bytes +178 bytes (+0.80%) +145 bytes
22453 bytes +205 bytes (+0.92%) +172 bytes
22458 bytes +210 bytes (+0.94%) +177 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 18022 bytes -4226 bytes (-18.99%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 18985 bytes -3263 bytes (-14.67%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 19793 bytes -2455 bytes (-11.03%)
Brotli (Wikipedia) brotli brotli -q 11 21004 bytes -1244 bytes (-5.59%)
LZMA2 (Wikipedia) xz xz -9 21768 bytes -480 bytes (-2.16%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 21922 bytes -326 bytes (-1.47%)
ZSTD (Wikipedia) zstd zstd -19 22079 bytes -169 bytes (-0.76%)

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