Choose a version:
53% The original file has 184656 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 97690 bytes (95.4k, 53%).

After GZIP compression these minified files vary in size:
Boot
  30585 bytes (29.9k)
CDN
cdnjs
  25680 bytes (25.1k)
CDN
cdnhttps
  25498 bytes (24.9k)
CDN
gzip -6 (default)
  25467 bytes (24.9k)
local copy
gzip -9
  25352 bytes (24.8k)
local copy
libdeflate -12
  24460 bytes (23.9k)
local copy
7zip -mx=9 -tgzip
  24455 bytes (23.9k)
local copy
kzip -s0 -rn -b2
  24410 bytes (23.8k)
local copy
pigz -11 -n
  24405 bytes (23.8k)
local copy
Zopfli
  24381 bytes (23.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.4.5.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.4.5 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 --i1000000 --mb8 --mls1024 --bsr16 --lazy --ohh

(found June 29, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 16  --bsr16
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/1.4.5/socket.io.min.js --location | md5sum
60d95a466f90b4146189b2d49cbc48c8  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.5.min.zopfli.js.gz | md5sum
60d95a466f90b4146189b2d49cbc48c8  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.4.5/socket.io.min.js --location | sha1sum
137f37530457707477f252a8965a5872c1fc409e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.4.5.min.zopfli.js.gz | sha1sum
137f37530457707477f252a8965a5872c1fc409e  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 30585 bytes 2fd19ba922f18342f2f7b934b79b8e6f < socket.on("connect",function(){socket.id=self.engine.id}); [...]
< }return results}abort()}return value};var update=function( [...]
> socket.on("connect",function(){socket.id=self.engine.id}); [...]
> }abort()}return value};var update=function(source,property [...]
(invalid)
cdnjs 25680 bytes 2fd19ba922f18342f2f7b934b79b8e6f < socket.on("connect",function(){socket.id=self.engine.id}); [...]
< }return results}abort()}return value};var update=function( [...]
> socket.on("connect",function(){socket.id=self.engine.id}); [...]
> }abort()}return value};var update=function(source,property [...]
(invalid)
cdnhttps 25498 bytes 2fd19ba922f18342f2f7b934b79b8e6f < socket.on("connect",function(){socket.id=self.engine.id}); [...]
< }return results}abort()}return value};var update=function( [...]
> socket.on("connect",function(){socket.id=self.engine.id}); [...]
> }abort()}return value};var update=function(source,property [...]
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
24381 bytes -2 bytes zopfli --i1000000 --mls1024 --bsr16 --lazy --ohh June 29, 2016 @ 22:41
24383 bytes -1 byte zopfli --i100000 --mls1024 --bsr16 --lazy --ohh June 29, 2016 @ 16:45
24384 bytes -1 byte zopfli --i10000 --mls256 --bsr13 --lazy --ohh June 29, 2016 @ 15:32
24385 bytes -3 bytes zopfli --i10000 --mls1024 --bsr9 --lazy --ohh June 29, 2016 @ 15:29
24388 bytes -1 byte zopfli --i1000 --mls256 --bsr13 --lazy --ohh June 29, 2016 @ 15:28
24389 bytes -2 bytes zopfli --i1000 --mls512 --bsr13 --lazy --ohh June 29, 2016 @ 15:22
24391 bytes -1 byte zopfli --i1000 --mls512 --bsr16 --lazy --ohh June 29, 2016 @ 15:22
24392 bytes -3 bytes zopfli --i1000 --mls128 --bsr11 --lazy --ohh June 29, 2016 @ 14:19
24395 bytes zopfli --i100 --mls128 --bsr12 --lazy --ohh June 29, 2016 @ 13:18

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, 100,000 or 1,000,000.
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
24399 24401 24400 24401 24407 24404 24398 24399 24400 24400 24400 24402 24400 24400 24400
24397 24399 24399 24412 24398 24396 24393 24396 24398 24399 24396 24398 24395 24395 24398
24403 24400 24403 24391 24391 24393 24402 24396 24395 24399 24399 24399 24395 24398 24397
24401 24403 24391 24389 24405 24392 24398 24384 24404 24399 24399 24399 24395 24397 24403
24396 24390 24397 24390 24406 24393 24396 24401 24386 24399 24399 24400 24395 24398 24398
24403 24402 24403 24410 24404 24390 24395 24399 24388 24385 24398 24399 24396 24398 24397
24398 24399 24394 24398 24393 24396 24392 24392 24394 24395 24399 24396 24395 24398 24396
24394 24396 24396 24395 24395 24390 24391 24389 24393 24400 24390 24398 24396 24397 24396
24401 24400 24401 24390 24406 24390 24391 24385 24387 24386 24396 24399 24390 24396 24395
24391 24398 24397 24395 24405 24400 24394 24384 24385 24399 24400 24398 24395 24398 24396
24400 24400 24403 24400 24402 24392 24398 24396 24387 24399 24399 24400 24398 24398 24397
24401 24402 24403 24400 24399 24390 24395 24395 24392 24399 24395 24399 24395 24398 24397
24410 24403 24403 24391 24391 24397 24396 24393 24390 24381 24395 24399 24401 24398 24398
24401 24399 24403 24394 24392 24396 24395 24393 24391 24399 24395 24399 24395 24398 24398
24403 24400 24397 24390 24405 24390 24395 24400 24386 24399 24395 24398 24402 24397 24396
24401 24403 24400 24401 24404 24398 24399 24400 24385 24399 24396 24398 24403 24398 24398
24401 24402 24401 24405 24404 24401 24399 24400 24391 24386 24399 24398 24396 24398 24398
24401 24401 24400 24389 24405 24391 24398 24395 24386 24385 24396 24399 24396 24398 24395
24402 24401 24401 24394 24391 24390 24391 24400 24386 24386 24396 24396 24395 24397 24396
24401 24402 24401 24391 24392 24401 24388 24382 24385 24382 24396 24398 24396 24398 24396
24401 24400 24400 24404 24404 24395 24393 24382 24387 24385 24399 24398 24395 24398 24395
24401 24403 24403 24387 24401 24389 24394 24399 24403 24399 24396 24399 24395 24398 24398
24399 24399 24399 24403 24404 24391 24395 24384 24385 24385 24395 24400 24395 24397 24395

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 24395 bytes 100%
1,000 24386 bytes -9 bytes 100%
10,000 24384 bytes -2 bytes 100%
100,000 24383 bytes -1 byte 2.03%
1,000,000 24381 bytes -2 bytes 1.16%
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
24421 bytes +40 bytes (+0.16%) +11 bytes
24423 bytes +42 bytes (+0.17%) +13 bytes
24410 bytes +29 bytes (+0.12%)
24442 bytes +61 bytes (+0.25%) +32 bytes
24465 bytes +84 bytes (+0.34%) +55 bytes
24504 bytes +123 bytes (+0.50%) +94 bytes
24530 bytes +149 bytes (+0.61%) +120 bytes
24568 bytes +187 bytes (+0.77%) +158 bytes
24599 bytes +218 bytes (+0.89%) +189 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 19205 bytes -5176 bytes (-21.23%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 20400 bytes -3981 bytes (-16.33%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 22098 bytes -2283 bytes (-9.36%)
Brotli (Wikipedia) brotli brotli -q 11 22565 bytes -1816 bytes (-7.45%)
LZMA2 (Wikipedia) xz xz -9 23292 bytes -1089 bytes (-4.47%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 23487 bytes -894 bytes (-3.67%)
Zstandard (Wikipedia) zstd zstd -19 23693 bytes -688 bytes (-2.82%)

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