Choose a version:
53% The original file has 184424 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 96841 bytes (94.6k, 53%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  25314 bytes (24.7k)
local copy
gzip -9
  25219 bytes (24.6k)
local copy
Boot
  25038 bytes (24.5k)
CDN
libdeflate -12
  24344 bytes (23.8k)
local copy
kzip -s0 -rn -b1
  24292 bytes (23.7k)
local copy
7zip -mx=9 -tgzip
  24288 bytes (23.7k)
local copy
pigz -11 -n
  24275 bytes (23.7k)
local copy
Zopfli
  24253 bytes (23.7k)
local copy
cdnjs
  21468 bytes (21.0k)
CDN
cdnhttps
  21233 bytes (20.7k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.4.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.4.0 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 --mls128 --bsr12 --lazy --ohh

(found June 30, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 12  --bsr12
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/1.4.0/socket.io.min.js --location | md5sum
11c86e7b77011778358018b61d0593fa  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.0.min.zopfli.js.gz | md5sum
11c86e7b77011778358018b61d0593fa  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.4.0/socket.io.min.js --location | sha1sum
d9f5ecfeb10923b45c2b5a4bb410f482e5a6b3d5  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.0.min.zopfli.js.gz | sha1sum
d9f5ecfeb10923b45c2b5a4bb410f482e5a6b3d5  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 25038 bytes 036477ee39a19da177df4c4ea8c1565b < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.readyState="open";Socket.priorWebsocketSuccess="webso [...]
< binaryType=null}var bufferTail=data;var buffers=[];var num [...]
< }else{instance=new WebSocket(uri)}return instance}if(WebSo [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> document.body.removeChild(t)},100)},o.prototype.doWrite=fu [...]
> var i=t.charAt(r);if(null==i||Number(i)!=i){--r;break}if(e [...]
(invalid)
cdnjs 21468 bytes 036477ee39a19da177df4c4ea8c1565b < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.readyState="open";Socket.priorWebsocketSuccess="webso [...]
< binaryType=null}var bufferTail=data;var buffers=[];var num [...]
< }else{instance=new WebSocket(uri)}return instance}if(WebSo [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> document.body.removeChild(t)},100)},o.prototype.doWrite=fu [...]
> var i=t.charAt(r);if(null==i||Number(i)!=i){--r;break}if(e [...]
(invalid)
cdnhttps 21233 bytes 036477ee39a19da177df4c4ea8c1565b < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.readyState="open";Socket.priorWebsocketSuccess="webso [...]
< binaryType=null}var bufferTail=data;var buffers=[];var num [...]
< }else{instance=new WebSocket(uri)}return instance}if(WebSo [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> document.body.removeChild(t)},100)},o.prototype.doWrite=fu [...]
> var i=t.charAt(r);if(null==i||Number(i)!=i){--r;break}if(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
24253 bytes -1 byte zopfli --i100000 --mls128 --bsr12 --lazy --ohh June 30, 2016 @ 14:45
24254 bytes -2 bytes zopfli --i10000 --mls128 --bsr12 --lazy --ohh June 30, 2016 @ 12:05
24256 bytes -2 bytes zopfli --i10000 --mls2048 --bsr20 --lazy --ohh June 29, 2016 @ 16:12
24258 bytes -3 bytes zopfli --i1000 --mls2048 --bsr20 --lazy --ohh June 29, 2016 @ 15:39
24261 bytes zopfli --i100 --mls2048 --bsr20 --lazy --ohh June 29, 2016 @ 13:23

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
24270 24263 24270 24271 24272 24273 24264 24264 24273 24261 24272 24267 24265 24284 24271
24273 24272 24271 24275 24275 24273 24274 24273 24273 24273 24275 24272 24270 24267 24269
24261 24259 24265 24260 24268 24266 24262 24264 24261 24262 24262 24274 24270 24270 24268
24267 24266 24271 24267 24273 24274 24274 24271 24270 24274 24275 24269 24271 24267 24268
24267 24271 24271 24267 24269 24264 24276 24266 24272 24269 24270 24269 24270 24270 24267
24265 24260 24263 24262 24263 24262 24262 24264 24273 24259 24262 24263 24268 24267 24269
24265 24262 24269 24262 24257 24266 24263 24266 24259 24259 24257 24267 24270 24267 24267
24268 24266 24270 24266 24273 24274 24274 24274 24272 24273 24273 24264 24270 24271 24267
24270 24260 24269 24266 24261 24265 24253 24265 24258 24264 24259 24271 24268 24267 24267
24259 24260 24262 24262 24263 24266 24257 24268 24263 24262 24264 24268 24268 24267 24268
24265 24270 24269 24265 24273 24274 24273 24272 24268 24267 24274 24270 24270 24267 24267
24262 24260 24263 24262 24264 24262 24266 24263 24267 24261 24265 24268 24270 24267 24267
24266 24266 24269 24262 24273 24274 24274 24259 24267 24266 24277 24271 24268 24268 24267
24265 24267 24269 24267 24274 24274 24273 24266 24267 24273 24275 24271 24270 24267 24267
24265 24265 24268 24266 24258 24274 24274 24266 24271 24273 24274 24267 24272 24267 24267
24273 24259 24265 24262 24263 24264 24263 24266 24276 24262 24263 24270 24272 24267 24268
24266 24260 24261 24262 24263 24261 24263 24266 24257 24260 24256 24269 24270 24268 24268
24266 24261 24262 24261 24265 24258 24259 24264 24257 24261 24256 24271 24270 24268 24268
24265 24266 24269 24266 24273 24260 24261 24265 24266 24263 24263 24271 24270 24270 24267
24266 24267 24269 24270 24273 24274 24273 24272 24273 24276 24275 24273 24270 24267 24268
24268 24259 24268 24265 24264 24265 24261 24264 24256 24263 24263 24268 24271 24267 24267
24265 24266 24269 24268 24264 24274 24265 24267 24273 24273 24275 24268 24271 24267 24267
24262 24260 24269 24265 24263 24264 24262 24266 24268 24266 24263 24270 24270 24267 24268

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 24261 bytes 100%
1,000 24257 bytes -4 bytes 100%
10,000 24254 bytes -3 bytes 100%
100,000 24253 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
24293 bytes +40 bytes (+0.16%) +1 byte
24292 bytes +39 bytes (+0.16%)
24295 bytes +42 bytes (+0.17%) +3 bytes
24334 bytes +81 bytes (+0.33%) +42 bytes
24302 bytes +49 bytes (+0.20%) +10 bytes
24327 bytes +74 bytes (+0.31%) +35 bytes
24363 bytes +110 bytes (+0.45%) +71 bytes
24397 bytes +144 bytes (+0.59%) +105 bytes
24428 bytes +175 bytes (+0.72%) +136 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 19155 bytes -5098 bytes (-21.02%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 20317 bytes -3936 bytes (-16.23%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 22239 bytes -2014 bytes (-8.30%)
Brotli (Wikipedia) brotli brotli -q 11 22445 bytes -1808 bytes (-7.45%)
LZMA2 (Wikipedia) xz xz -9 23176 bytes -1077 bytes (-4.44%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 23480 bytes -773 bytes (-3.19%)
Zstandard (Wikipedia) zstd zstd -19 23544 bytes -709 bytes (-2.92%)

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