Choose a version:
53% The original file has 184375 bytes (180.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 97182 bytes (94.9k, 53%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  25307 bytes (24.7k)
local copy
gzip -9
  25217 bytes (24.6k)
local copy
Boot
  25046 bytes (24.5k)
CDN
libdeflate -12
  24347 bytes (23.8k)
local copy
kzip -s0 -rn -b1
  24287 bytes (23.7k)
local copy
7zip -mx=9 -tgzip
  24286 bytes (23.7k)
local copy
pigz -11 -n
  24273 bytes (23.7k)
local copy
Zopfli
  24250 bytes (23.7k)
local copy
cdnjs
  21479 bytes (21.0k)
CDN
cdnhttps
  21250 bytes (20.8k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.4.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.4.2 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 --mls2048 --bsr30 --lazy --ohh

(found June 30, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 30  --bsr30
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/1.4.2/socket.io.min.js --location | md5sum
e02b1773fad2bd6ff801661d458dcd53  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.2.min.zopfli.js.gz | md5sum
e02b1773fad2bd6ff801661d458dcd53  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.4.2/socket.io.min.js --location | sha1sum
dc3af739bd3b040fac3c8c78030d0fae756a0550  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.2.min.zopfli.js.gz | sha1sum
dc3af739bd3b040fac3c8c78030d0fae756a0550  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 25046 bytes be4c474fc5fd31b49346f10366104bc6 < (function(f){if(typeof exports==="object"&&typeof module!= [...]
< this.once("close",onclose);this.once("upgrading",onupgrade [...]
< for(var i=0;i<lenStr.length;i++){lengthAry[i]=parseInt(len [...]
< if(typeof define=="function"&&typeof define.amd=="object"& [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> this.script=e;var r="undefined"!=typeof navigator&&/gecko/ [...]
> }function s(){this.reconstructor=null}function a(t){var e= [...]
(invalid)
cdnjs 21479 bytes be4c474fc5fd31b49346f10366104bc6 < (function(f){if(typeof exports==="object"&&typeof module!= [...]
< this.once("close",onclose);this.once("upgrading",onupgrade [...]
< for(var i=0;i<lenStr.length;i++){lengthAry[i]=parseInt(len [...]
< if(typeof define=="function"&&typeof define.amd=="object"& [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> this.script=e;var r="undefined"!=typeof navigator&&/gecko/ [...]
> }function s(){this.reconstructor=null}function a(t){var e= [...]
(invalid)
cdnhttps 21250 bytes be4c474fc5fd31b49346f10366104bc6 < (function(f){if(typeof exports==="object"&&typeof module!= [...]
< this.once("close",onclose);this.once("upgrading",onupgrade [...]
< for(var i=0;i<lenStr.length;i++){lengthAry[i]=parseInt(len [...]
< if(typeof define=="function"&&typeof define.amd=="object"& [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> this.script=e;var r="undefined"!=typeof navigator&&/gecko/ [...]
> }function s(){this.reconstructor=null}function a(t){var e= [...]
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:

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
24250 bytes -1 byte zopfli --i100000 --mls2048 --bsr30 --lazy --ohh June 30, 2016 @ 11:14
24251 bytes -1 byte zopfli --i10000 --mls2048 --bsr30 --lazy --ohh June 30, 2016 @ 09:46
24252 bytes -2 bytes zopfli --i1000 --mls2048 --bsr30 --lazy --ohh June 29, 2016 @ 17:23
24254 bytes -1 byte zopfli --i1000 --mls2048 --bsr21 --lazy --ohh June 29, 2016 @ 15:42
24255 bytes zopfli --i100 --mls2048 --bsr23 --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 November 22, 2016 @ 16:41.

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
24259 24261 24261 24261 24262 24267 24260 24262 24266 24265 24264 24265 24264 24264 24270
24266 24263 24265 24264 24266 24254 24272 24273 24273 24274 24261 24262 24259 24269 24262
24258 24259 24259 24257 24258 24257 24256 24258 24258 24258 24261 24259 24258 24266 24257
24264 24267 24261 24261 24259 24259 24261 24257 24260 24263 24260 24258 24256 24269 24259
24266 24267 24261 24271 24267 24259 24268 24273 24262 24269 24268 24269 24258 24266 24257
24257 24257 24259 24262 24252 24258 24258 24262 24261 24262 24256 24262 24259 24267 24259
24264 24253 24260 24257 24255 24258 24256 24257 24260 24262 24252 24263 24256 24266 24259
24265 24263 24263 24262 24264 24262 24269 24271 24271 24272 24271 24263 24260 24266 24258
24266 24257 24257 24260 24261 24250 24256 24257 24258 24260 24258 24261 24255 24269 24258
24262 24259 24257 24258 24255 24257 24256 24260 24259 24260 24252 24267 24259 24258 24259
24265 24264 24262 24264 24260 24261 24269 24270 24262 24262 24260 24269 24253 24267 24257
24258 24260 24258 24260 24257 24258 24258 24260 24264 24257 24258 24258 24255 24268 24259
24258 24258 24261 24261 24261 24258 24257 24258 24259 24257 24257 24263 24256 24256 24260
24266 24257 24262 24262 24255 24257 24256 24258 24259 24258 24258 24259 24258 24256 24257
24257 24258 24259 24263 24258 24255 24269 24258 24259 24262 24251 24259 24257 24257 24256
24256 24256 24258 24258 24253 24261 24259 24258 24260 24257 24252 24259 24257 24268 24257
24258 24252 24258 24258 24252 24256 24258 24258 24258 24255 24250 24265 24254 24267 24257
24256 24257 24257 24258 24254 24257 24255 24263 24260 24263 24253 24263 24263 24257 24257
24263 24262 24262 24261 24255 24260 24256 24263 24259 24258 24252 24256 24254 24268 24258
24267 24253 24262 24261 24263 24264 24259 24262 24257 24259 24251 24267 24256 24256 24258
24264 24258 24257 24259 24257 24264 24256 24260 24260 24260 24258 24263 24257 24268 24258
24257 24257 24258 24258 24253 24260 24257 24261 24258 24258 24250 24263 24257 24256 24257
24263 24259 24261 24259 24253 24258 24270 24259 24259 24258 24253 24264 24256 24269 24258

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 24255 bytes 100%
1,000 24252 bytes -3 bytes 100%
10,000 24250 bytes -2 bytes 100%
100,000 24250 bytes 0.87%
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
24290 bytes +40 bytes (+0.16%) +3 bytes
24287 bytes +37 bytes (+0.15%)
24289 bytes +39 bytes (+0.16%) +2 bytes
24331 bytes +81 bytes (+0.33%) +44 bytes
24307 bytes +57 bytes (+0.24%) +20 bytes
24327 bytes +77 bytes (+0.32%) +40 bytes
24359 bytes +109 bytes (+0.45%) +72 bytes
24393 bytes +143 bytes (+0.59%) +106 bytes
24424 bytes +174 bytes (+0.72%) +137 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 19166 bytes -5084 bytes (-20.96%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 20328 bytes -3922 bytes (-16.17%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 22194 bytes -2056 bytes (-8.48%)
Brotli (Wikipedia) brotli brotli -q 11 22438 bytes -1812 bytes (-7.47%)
LZMA2 (Wikipedia) xz xz -9 23180 bytes -1070 bytes (-4.41%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 23483 bytes -767 bytes (-3.16%)
Zstandard (Wikipedia) zstd zstd -19 23549 bytes -701 bytes (-2.89%)

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