Choose a version:
64% The original file has 210221 bytes (205.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 134784 bytes (131.6k, 64%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  32776 bytes (32.0k)
local copy
gzip -9
  32444 bytes (31.7k)
local copy
7zip -mx=9 -tgzip
  31013 bytes (30.3k)
local copy
libdeflate -12
  30964 bytes (30.2k)
local copy
kzip -s0 -rn -b1
  30947 bytes (30.2k)
local copy
pigz -11 -n
  30907 bytes (30.2k)
local copy
Zopfli
  30851 bytes (30.1k)
local copy
Boot
  26138 bytes (25.5k)
CDN
cdnjs
  22277 bytes (21.8k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.7.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.7.2 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 --mls4 --bsr4 --lazy --ohh

(found September 8, 2017)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 4  --mls4
block splitting recursion 4  --bsr4
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/1.7.2/socket.io.min.js --location | md5sum
29d9112bf992893fb69c22b48b35d536  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.7.2.min.zopfli.js.gz | md5sum
29d9112bf992893fb69c22b48b35d536  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.7.2/socket.io.min.js --location | sha1sum
3b57e6b17bc0e381d06a875215e3be8865f1d79a  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.7.2.min.zopfli.js.gz | sha1sum
3b57e6b17bc0e381d06a875215e3be8865f1d79a  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 26138 bytes c69d56555022b5e4fbc64882e2cf6c51 < (function webpackUniversalModuleDefinition(root, factory) [...]
< if(typeof exports === 'object' && typeof module === 'obje [...]
< module.exports = factory(require("JSON"));
< else if(typeof define === 'function' && define.amd)
< define(["JSON"], factory);
< else if(typeof exports === 'object')
< exports["io"] = factory(require("JSON"));
< else
< root["io"] = factory(root["JSON"]);
< })(this, function(__WEBPACK_EXTERNAL_MODULE_5__) {
[...]
(invalid)
cdnjs 22277 bytes c69d56555022b5e4fbc64882e2cf6c51 < (function webpackUniversalModuleDefinition(root, factory) [...]
< if(typeof exports === 'object' && typeof module === 'obje [...]
< module.exports = factory(require("JSON"));
< else if(typeof define === 'function' && define.amd)
< define(["JSON"], factory);
< else if(typeof exports === 'object')
< exports["io"] = factory(require("JSON"));
< else
< root["io"] = factory(root["JSON"]);
< })(this, function(__WEBPACK_EXTERNAL_MODULE_5__) {
[...]
(invalid)

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
30851 bytes -2 bytes zopfli --i1000000 --mls4 --bsr4 --lazy --ohh September 8, 2017 @ 16:55
30853 bytes -1 byte zopfli --i100000 --mls4 --bsr4 --lazy --ohh September 7, 2017 @ 12:09
30854 bytes -3 bytes zopfli --i10000 --mls8192 --bsr14 --lazy --ohh September 7, 2017 @ 00:06
30857 bytes -1 byte zopfli --i10000 --mls4 --bsr6 --lazy --ohh September 6, 2017 @ 23:47
30858 bytes -3 bytes zopfli --i1000 --mls8192 --bsr10 --lazy --ohh September 6, 2017 @ 13:04
30861 bytes -2 bytes zopfli --i1000 --mls4096 --bsr11 --lazy --ohh September 6, 2017 @ 12:42
30863 bytes zopfli --i100 --mls4096 --bsr11 --lazy --ohh September 6, 2017 @ 11:50

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

Most recent activity on September 8, 2017 @ 16:55.

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
30861 30863 30862 30865 30864 30866 30865 30867 30866 30867 30867 30867 30865 30866 30868
30859 30851 30858 30856 30856 30858 30857 30856 30859 30858 30860 30859 30868 30857 30857
30859 30857 30857 30858 30856 30856 30857 30859 30858 30858 30858 30857 30858 30857 30856
30857 30858 30859 30864 30864 30858 30858 30857 30857 30855 30858 30857 30856 30858 30864
30857 30860 30858 30857 30856 30859 30860 30859 30857 30855 30859 30857 30858 30860 30872
30858 30858 30856 30860 30857 30858 30860 30857 30858 30858 30858 30857 30857 30857 30865
30857 30858 30857 30865 30856 30857 30856 30859 30859 30858 30859 30859 30855 30857 30865
30858 30858 30858 30857 30856 30858 30858 30858 30857 30855 30857 30856 30866 30857 30865
30858 30858 30858 30857 30858 30858 30860 30858 30857 30858 30858 30857 30858 30857 30865
30857 30857 30857 30857 30857 30858 30857 30857 30858 30858 30859 30857 30859 30856 30864
30857 30858 30857 30858 30858 30860 30858 30858 30857 30858 30858 30857 30854 30857 30864
30857 30858 30859 30864 30863 30860 30858 30858 30856 30858 30858 30858 30858 30857 30864
30860 30860 30859 30857 30856 30858 30857 30859 30857 30860 30858 30857 30858 30857 30864
30857 30857 30858 30857 30858 30858 30858 30858 30858 30858 30859 30857 30858 30856 30865
30857 30858 30858 30857 30856 30858 30858 30858 30858 30858 30859 30857 30857 30857 30865
30857 30857 30856 30857 30858 30856 30860 30858 30858 30858 30859 30855 30858 30859 30864
30857 30858 30858 30858 30857 30858 30857 30858 30857 30858 30857 30856 30859 30858 30865
30858 30859 30859 30858 30859 30857 30858 30859 30857 30858 30857 30855 30858 30858 30856
30858 30857 30856 30858 30858 30864 30858 30857 30858 30858 30857 30858 30858 30857 30864
30857 30858 30857 30856 30858 30858 30860 30858 30856 30859 30857 30857 30857 30858 30866
30858 30857 30858 30864 30856 30858 30858 30857 30857 30858 30859 30854 30858 30854 30866
30858 30858 30858 30858 30857 30860 30858 30864 30856 30858 30858 30857 30865 30854 30864
30855 30858 30857 30857 30856 30858 30858 30858 30858 30858 30857 30858 30858 30858 30864

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 30863 bytes 100%
1,000 30858 bytes -5 bytes 100%
10,000 30854 bytes -4 bytes 100%
100,000 30853 bytes -1 byte 2.32%
1,000,000 30851 bytes -2 bytes 0.29%
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
30948 bytes +97 bytes (+0.31%) +1 byte
30947 bytes +96 bytes (+0.31%)
30988 bytes +137 bytes (+0.44%) +41 bytes
31020 bytes +169 bytes (+0.55%) +73 bytes
31061 bytes +210 bytes (+0.68%) +114 bytes
31029 bytes +178 bytes (+0.58%) +82 bytes
31055 bytes +204 bytes (+0.66%) +108 bytes
31030 bytes +179 bytes (+0.58%) +83 bytes
31082 bytes +231 bytes (+0.75%) +135 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 22781 bytes -8070 bytes (-26.16%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 23654 bytes -7197 bytes (-23.33%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 25938 bytes -4913 bytes (-15.92%)
Brotli (Wikipedia) brotli brotli -q 11 26969 bytes -3882 bytes (-12.58%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 27217 bytes -3634 bytes (-11.78%)
LZMA2 (Wikipedia) xz xz -9 28380 bytes -2471 bytes (-8.01%)
Zstandard (Wikipedia) zstd zstd -19 29004 bytes -1847 bytes (-5.99%)

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