Choose a version:
53% The original file has 183798 bytes (179.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 96693 bytes (94.4k, 53%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  25298 bytes (24.7k)
local copy
gzip -9
  25207 bytes (24.6k)
local copy
Boot
  25045 bytes (24.5k)
CDN
libdeflate -12
  24336 bytes (23.8k)
local copy
kzip -s0 -rn -b1
  24281 bytes (23.7k)
local copy
7zip -mx=9 -tgzip
  24277 bytes (23.7k)
local copy
pigz -11 -n
  24268 bytes (23.7k)
local copy
Zopfli
  24244 bytes (23.7k)
local copy
cdnjs
  21485 bytes (21.0k)
CDN
cdnhttps
  21247 bytes (20.7k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.4.1.min.js

You will automatically get the smallest Socket.IO 1.4.1 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 --bsr21 --lazy --ohh

(found July 1, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 21  --bsr21
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/1.4.1/socket.io.min.js --location | md5sum
0cb0cc24006d786154e7ab27c4993045  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.1.min.zopfli.js.gz | md5sum
0cb0cc24006d786154e7ab27c4993045  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.4.1/socket.io.min.js --location | sha1sum
faedab4666c42cab275a674a84b15c0a48e7ef96  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.1.min.zopfli.js.gz | sha1sum
faedab4666c42cab275a674a84b15c0a48e7ef96  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 25045 bytes 95b0bc321cb310e084ca5c35a291f26c < (function(f){if(typeof exports==="object"&&typeof module!= [...]
< this.once("close",onclose);this.once("upgrading",onupgrade [...]
< }})}map(packets,encodeOne,function(err,results){return cal [...]
< },{}],47:[function(_dereq_,module,exports){"use strict";va [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> var t=document.createElement("iframe");document.body.appen [...]
> if(e.nsp+=i,r==t.length)break}else e.nsp="/";var s=t.charA [...]
(invalid)
cdnjs 21485 bytes 95b0bc321cb310e084ca5c35a291f26c < (function(f){if(typeof exports==="object"&&typeof module!= [...]
< this.once("close",onclose);this.once("upgrading",onupgrade [...]
< }})}map(packets,encodeOne,function(err,results){return cal [...]
< },{}],47:[function(_dereq_,module,exports){"use strict";va [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> var t=document.createElement("iframe");document.body.appen [...]
> if(e.nsp+=i,r==t.length)break}else e.nsp="/";var s=t.charA [...]
(invalid)
cdnhttps 21247 bytes 95b0bc321cb310e084ca5c35a291f26c < (function(f){if(typeof exports==="object"&&typeof module!= [...]
< this.once("close",onclose);this.once("upgrading",onupgrade [...]
< }})}map(packets,encodeOne,function(err,results){return cal [...]
< },{}],47:[function(_dereq_,module,exports){"use strict";va [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> var t=document.createElement("iframe");document.body.appen [...]
> if(e.nsp+=i,r==t.length)break}else e.nsp="/";var s=t.charA [...]
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:

2.0.4, 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
24244 bytes -1 byte zopfli --i100000 --mls64 --bsr21 --lazy --ohh July 1, 2016 @ 11:54
24245 bytes -2 bytes zopfli --i10000 --mls64 --bsr10 --lazy --ohh June 29, 2016 @ 15:32
24247 bytes -3 bytes zopfli --i1000 --mls64 --bsr10 --lazy --ohh June 29, 2016 @ 15:24
24250 bytes zopfli --i100 --mls64 --bsr10 --lazy --ohh June 29, 2016 @ 13:32

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
24250 24251 24253 24253 24253 24257 24257 24258 24254 24252 24258 24259 24257 24255 24257
24260 24257 24258 24261 24260 24267 24261 24259 24261 24263 24263 24257 24252 24256 24257
24251 24252 24252 24251 24247 24256 24253 24249 24252 24249 24251 24254 24254 24255 24255
24259 24259 24262 24258 24260 24259 24267 24261 24258 24253 24255 24256 24257 24258 24255
24260 24262 24260 24260 24257 24257 24258 24266 24261 24261 24257 24257 24253 24255 24255
24250 24252 24250 24247 24247 24253 24255 24252 24257 24250 24252 24258 24253 24256 24256
24259 24261 24259 24256 24249 24244 24257 24250 24251 24262 24249 24258 24250 24255 24255
24258 24261 24261 24262 24260 24260 24258 24260 24262 24258 24262 24257 24251 24255 24255
24259 24259 24259 24260 24251 24256 24257 24248 24250 24257 24261 24257 24250 24258 24255
24251 24251 24253 24250 24251 24255 24255 24252 24251 24247 24260 24257 24252 24254 24255
24259 24261 24260 24260 24260 24261 24258 24258 24257 24258 24256 24256 24253 24255 24255
24250 24250 24252 24251 24247 24255 24252 24255 24250 24248 24250 24249 24253 24258 24255
24259 24263 24261 24259 24260 24260 24266 24267 24258 24252 24260 24256 24251 24255 24255
24261 24262 24261 24260 24260 24259 24267 24251 24260 24258 24261 24253 24251 24255 24255
24257 24259 24259 24251 24260 24261 24267 24248 24262 24248 24259 24258 24251 24256 24256
24260 24260 24252 24258 24248 24246 24251 24248 24258 24251 24261 24258 24259 24259 24256
24259 24249 24251 24249 24246 24255 24244 24250 24250 24250 24249 24257 24252 24258 24256
24251 24261 24253 24249 24246 24244 24244 24255 24257 24250 24260 24253 24251 24256 24255
24259 24260 24260 24259 24260 24260 24266 24250 24257 24248 24259 24257 24249 24255 24255
24260 24263 24261 24260 24259 24261 24267 24261 24260 24258 24262 24260 24250 24256 24255
24256 24260 24260 24259 24257 24253 24258 24254 24256 24248 24260 24256 24259 24258 24255
24250 24262 24261 24259 24260 24260 24251 24252 24258 24249 24260 24256 24251 24255 24255
24257 24261 24261 24260 24261 24259 24258 24255 24256 24251 24255 24256 24257 24258 24256

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 24250 bytes 100%
1,000 24247 bytes -3 bytes 100%
10,000 24245 bytes -2 bytes 100%
100,000 24244 bytes -1 byte 1.16%
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
24282 bytes +38 bytes (+0.16%) +1 byte
24281 bytes +37 bytes (+0.15%)
24286 bytes +42 bytes (+0.17%) +5 bytes
24320 bytes +76 bytes (+0.31%) +39 bytes
24298 bytes +54 bytes (+0.22%) +17 bytes
24317 bytes +73 bytes (+0.30%) +36 bytes
24347 bytes +103 bytes (+0.42%) +66 bytes
24387 bytes +143 bytes (+0.59%) +106 bytes
24414 bytes +170 bytes (+0.70%) +133 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 19158 bytes -5086 bytes (-20.98%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 20322 bytes -3922 bytes (-16.18%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 22199 bytes -2045 bytes (-8.44%)
Brotli (Wikipedia) brotli brotli -q 11 22432 bytes -1812 bytes (-7.47%)
LZMA2 (Wikipedia) xz xz -9 23172 bytes -1072 bytes (-4.42%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 23485 bytes -759 bytes (-3.13%)
ZSTD (Wikipedia) zstd zstd -19 23541 bytes -703 bytes (-2.90%)

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