Choose a version:
52% The original file has 156380 bytes (152.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 81950 bytes (80.0k, 52%).

After GZIP compression these minified files vary in size:
Boot
  25651 bytes (25.0k)
CDN
cdnjs
  21452 bytes (20.9k)
CDN
gzip -6 (default)
  21258 bytes (20.8k)
local copy
cdnhttps
  21235 bytes (20.7k)
CDN
gzip -9
  21182 bytes (20.7k)
local copy
7zip -mx=9 -tgzip
  20427 bytes (19.9k)
local copy
libdeflate -12
  20419 bytes (19.9k)
local copy
kzip -s0 -rn -b0
  20368 bytes (19.9k)
local copy
pigz -11 -n
  20368 bytes (19.9k)
local copy
Zopfli
  20349 bytes (19.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.1.0.min.js

You will automatically get the smallest Socket.IO 1.1.0 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 --bsr23 --lazy --ohh

(found June 30, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 23  --bsr23
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/1.1.0/socket.io.min.js --location | md5sum
0cdf61402e8ecc6a190b9fb4019dc90b  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.1.0.min.zopfli.js.gz | md5sum
0cdf61402e8ecc6a190b9fb4019dc90b  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.1.0/socket.io.min.js --location | sha1sum
832d9eae89fbfc968bf5c34449ec6907ec91be0e  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.1.0.min.zopfli.js.gz | sha1sum
832d9eae89fbfc968bf5c34449ec6907ec91be0e  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 25651 bytes 0cdf61402e8ecc6a190b9fb4019dc90b (invalid)
cdnjs 21452 bytes 0cdf61402e8ecc6a190b9fb4019dc90b (invalid)
cdnhttps 21235 bytes 0cdf61402e8ecc6a190b9fb4019dc90b (invalid)

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
20349 bytes -1 byte zopfli --i100000 --mls128 --bsr23 --lazy --ohh June 30, 2016 @ 13:36
20350 bytes -1 byte zopfli --i1000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 18:48
20351 bytes -1 byte zopfli --i1000 --mls16 --bsr18 --lazy --ohh June 29, 2016 @ 15:23
20352 bytes -2 bytes zopfli --i1000 --mls128 --bsr8 --lazy --ohh June 29, 2016 @ 15:23
20354 bytes zopfli --i100 --mls16 --bsr16 --lazy --ohh June 29, 2016 @ 12:20

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on July 3, 2016 @ 03:32.

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
20361 20352 20365 20365 20354 20365 20351 20349 20355 20353 20368 20355 20355 20367 20370
20358 20361 20358 20362 20356 20357 20356 20357 20354 20376 20357 20361 20370 20367 20372
20358 20358 20357 20357 20360 20357 20362 20360 20360 20356 20372 20369 20358 20370 20370
20360 20361 20357 20351 20357 20358 20355 20361 20353 20356 20371 20354 20371 20367 20367
20360 20352 20357 20357 20356 20357 20351 20352 20356 20354 20372 20355 20352 20367 20352
20360 20350 20351 20351 20351 20355 20351 20351 20355 20356 20372 20351 20354 20367 20367
20357 20357 20357 20350 20361 20352 20352 20356 20349 20356 20371 20352 20354 20369 20371
20358 20357 20357 20360 20357 20358 20353 20358 20361 20355 20357 20355 20357 20367 20375
20357 20357 20357 20359 20361 20359 20361 20359 20361 20357 20357 20357 20357 20367 20352
20351 20352 20357 20352 20360 20358 20365 20360 20374 20376 20372 20357 20356 20367 20371
20355 20353 20357 20355 20361 20359 20355 20359 20361 20354 20358 20351 20357 20367 20352
20357 20357 20357 20352 20357 20349 20361 20355 20355 20357 20371 20358 20352 20367 20352
20358 20358 20357 20349 20362 20361 20357 20359 20353 20383 20374 20352 20351 20367 20371
20357 20358 20357 20361 20357 20355 20362 20351 20351 20354 20371 20355 20353 20367 20370
20355 20352 20355 20351 20355 20355 20354 20356 20354 20356 20371 20352 20357 20367 20353
20357 20357 20356 20351 20356 20354 20362 20361 20354 20356 20371 20355 20353 20367 20372
20358 20352 20357 20357 20357 20361 20361 20361 20356 20356 20373 20352 20360 20368 20351
20357 20357 20358 20360 20361 20360 20352 20358 20356 20355 20371 20354 20353 20367 20352
20355 20355 20356 20351 20356 20354 20361 20359 20349 20357 20371 20352 20355 20367 20370
20357 20356 20357 20352 20360 20354 20349 20359 20354 20356 20371 20355 20352 20367 20352
20357 20357 20357 20359 20355 20354 20359 20355 20362 20356 20371 20355 20352 20369 20352
20354 20354 20355 20358 20354 20355 20352 20355 20354 20356 20349 20352 20352 20369 20352
20357 20359 20357 20351 20351 20362 20351 20356 20351 20351 20351 20351 20351 20366 20354

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 20354 bytes 100%
1,000 20350 bytes -4 bytes 100%
10,000 20350 bytes 100%
100,000 20349 bytes -1 byte 2.61%
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
20368 bytes +19 bytes (+0.09%)
20397 bytes +48 bytes (+0.24%) +29 bytes
20370 bytes +21 bytes (+0.10%) +2 bytes
20419 bytes +70 bytes (+0.34%) +51 bytes
20445 bytes +96 bytes (+0.47%) +77 bytes
20479 bytes +130 bytes (+0.64%) +111 bytes
20515 bytes +166 bytes (+0.82%) +147 bytes
20520 bytes +171 bytes (+0.84%) +152 bytes
20550 bytes +201 bytes (+0.99%) +182 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 16605 bytes -3744 bytes (-18.40%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 17350 bytes -2999 bytes (-14.74%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 18059 bytes -2290 bytes (-11.25%)
Brotli (Wikipedia) brotli brotli -q 11 19247 bytes -1102 bytes (-5.42%)
LZMA2 (Wikipedia) xz xz -9 19936 bytes -413 bytes (-2.03%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19980 bytes -369 bytes (-1.81%)
ZSTD (Wikipedia) zstd zstd -19 20222 bytes -127 bytes (-0.62%)

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