Choose a version:
53% The original file has 184588 bytes (180.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 97657 bytes (95.4k, 53%).

After GZIP compression these minified files vary in size:
Boot
  30573 bytes (29.9k)
CDN
cdnjs
  25672 bytes (25.1k)
CDN
cdnhttps
  25491 bytes (24.9k)
CDN
gzip -6 (default)
  25458 bytes (24.9k)
local copy
gzip -9
  25343 bytes (24.7k)
local copy
libdeflate -12
  24458 bytes (23.9k)
local copy
7zip -mx=9 -tgzip
  24451 bytes (23.9k)
local copy
kzip -s0 -rn -b2
  24402 bytes (23.8k)
local copy
pigz -11 -n
  24398 bytes (23.8k)
local copy
Zopfli
  24378 bytes (23.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.4.4.min.js

You will automatically get the smallest Socket.IO 1.4.4 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 --mls1024 --bsr15 --lazy --ohh

(found July 1, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 15  --bsr15
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/1.4.4/socket.io.min.js --location | md5sum
154157e30bbf780a4564b84dcde03254  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.4.min.zopfli.js.gz | md5sum
154157e30bbf780a4564b84dcde03254  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.4.4/socket.io.min.js --location | sha1sum
89bdaa254c5fd0c22b9c4ba6e9dfc54d99da99b2  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.4.min.zopfli.js.gz | sha1sum
89bdaa254c5fd0c22b9c4ba6e9dfc54d99da99b2  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 30573 bytes f602f8eb1dfe8e6c3daff2229093a7ab < base64+=chars[(bytes[i+1]&15)<<2|bytes[i+2]>>6];base64+=ch [...]
< });if(this.autoConnect){onConnecting()}}function onConnect [...]
< if(element===undef){delete source[property]}else{source[pr [...]
> base64+=chars[(bytes[i+1]&15)<<2|bytes[i+2]>>6];base64+=ch [...]
> socket.id=self.engine.id});if(this.autoConnect){onConnecti [...]
> };var update=function(source,property,callback){var elemen [...]
(invalid)
cdnjs 25672 bytes f602f8eb1dfe8e6c3daff2229093a7ab < base64+=chars[(bytes[i+1]&15)<<2|bytes[i+2]>>6];base64+=ch [...]
< });if(this.autoConnect){onConnecting()}}function onConnect [...]
< if(element===undef){delete source[property]}else{source[pr [...]
> base64+=chars[(bytes[i+1]&15)<<2|bytes[i+2]>>6];base64+=ch [...]
> socket.id=self.engine.id});if(this.autoConnect){onConnecti [...]
> };var update=function(source,property,callback){var elemen [...]
(invalid)
cdnhttps 25491 bytes f602f8eb1dfe8e6c3daff2229093a7ab < base64+=chars[(bytes[i+1]&15)<<2|bytes[i+2]>>6];base64+=ch [...]
< });if(this.autoConnect){onConnecting()}}function onConnect [...]
< if(element===undef){delete source[property]}else{source[pr [...]
> base64+=chars[(bytes[i+1]&15)<<2|bytes[i+2]>>6];base64+=ch [...]
> socket.id=self.engine.id});if(this.autoConnect){onConnecti [...]
> };var update=function(source,property,callback){var elemen [...]
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
24378 bytes -1 byte zopfli --i100000 --mls1024 --bsr15 --lazy --ohh July 1, 2016 @ 10:39
24379 bytes -4 bytes zopfli --i10000 --mls1024 --bsr15 --lazy --ohh June 29, 2016 @ 17:31
24383 bytes -2 bytes zopfli --i1000 --mls1024 --bsr15 --lazy --ohh June 29, 2016 @ 16:01
24385 bytes -3 bytes zopfli --i1000 --mls128 --bsr11 --lazy --ohh June 29, 2016 @ 15:34
24388 bytes zopfli --i100 --mls128 --bsr11 --lazy --ohh June 29, 2016 @ 13:21

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
24392 24393 24392 24393 24400 24400 24391 24392 24393 24393 24393 24395 24392 24392 24391
24390 24393 24392 24391 24389 24388 24386 24385 24386 24392 24388 24391 24387 24389 24390
24396 24391 24392 24384 24386 24388 24395 24391 24401 24392 24392 24392 24387 24390 24389
24396 24394 24383 24383 24396 24384 24387 24380 24394 24390 24391 24391 24387 24390 24395
24390 24381 24390 24383 24385 24385 24385 24392 24381 24380 24387 24393 24394 24390 24390
24394 24396 24393 24398 24395 24384 24388 24392 24385 24381 24390 24391 24388 24390 24390
24389 24392 24387 24391 24385 24388 24385 24385 24387 24391 24392 24388 24387 24390 24388
24387 24385 24390 24387 24388 24383 24384 24385 24388 24387 24392 24391 24387 24390 24390
24394 24378 24390 24383 24387 24384 24384 24381 24383 24391 24382 24391 24383 24389 24388
24386 24388 24391 24393 24395 24394 24387 24380 24395 24391 24392 24391 24387 24390 24388
24394 24391 24393 24394 24384 24384 24391 24388 24383 24391 24391 24392 24388 24390 24390
24395 24392 24397 24389 24386 24383 24388 24390 24391 24378 24387 24391 24387 24390 24390
24395 24395 24395 24393 24384 24390 24388 24386 24382 24381 24381 24391 24393 24390 24390
24389 24395 24395 24387 24384 24389 24387 24386 24384 24392 24387 24391 24387 24390 24390
24394 24393 24391 24382 24397 24386 24388 24392 24380 24391 24382 24391 24395 24389 24388
24394 24394 24395 24393 24395 24386 24392 24392 24384 24391 24382 24391 24394 24390 24388
24394 24392 24394 24396 24399 24393 24390 24383 24384 24381 24392 24393 24393 24390 24390
24394 24395 24395 24383 24398 24385 24391 24391 24384 24380 24384 24392 24388 24390 24389
24392 24395 24396 24387 24383 24384 24384 24392 24384 24380 24382 24391 24394 24390 24388
24394 24393 24397 24388 24384 24394 24387 24388 24381 24391 24382 24390 24387 24390 24388
24396 24395 24395 24382 24397 24388 24385 24381 24380 24380 24392 24391 24387 24390 24387
24395 24394 24395 24389 24394 24385 24390 24392 24384 24391 24388 24391 24387 24390 24390
24393 24393 24394 24395 24395 24383 24382 24388 24383 24379 24388 24392 24388 24389 24387

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 24388 bytes 100%
1,000 24381 bytes -7 bytes 100%
10,000 24379 bytes -2 bytes 100%
100,000 24378 bytes -1 byte 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
24418 bytes +40 bytes (+0.16%) +16 bytes
24419 bytes +41 bytes (+0.17%) +17 bytes
24402 bytes +24 bytes (+0.10%)
24435 bytes +57 bytes (+0.23%) +33 bytes
24458 bytes +80 bytes (+0.33%) +56 bytes
24496 bytes +118 bytes (+0.48%) +94 bytes
24522 bytes +144 bytes (+0.59%) +120 bytes
24559 bytes +181 bytes (+0.74%) +157 bytes
24592 bytes +214 bytes (+0.88%) +190 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 19205 bytes -5173 bytes (-21.22%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 20393 bytes -3985 bytes (-16.35%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 22090 bytes -2288 bytes (-9.39%)
Brotli (Wikipedia) brotli brotli -q 11 22527 bytes -1851 bytes (-7.59%)
LZMA2 (Wikipedia) xz xz -9 23288 bytes -1090 bytes (-4.47%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 23579 bytes -799 bytes (-3.28%)
ZSTD (Wikipedia) zstd zstd -19 23689 bytes -689 bytes (-2.83%)

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