Choose a version:
64% The original file has 211903 bytes (206.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 136285 bytes (133.1k, 64%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  33087 bytes (32.3k)
local copy
gzip -9
  32744 bytes (32.0k)
local copy
7zip -mx=9 -tgzip
  31307 bytes (30.6k)
local copy
libdeflate -12
  31284 bytes (30.6k)
local copy
kzip -s0 -rn -b1
  31230 bytes (30.5k)
local copy
pigz -11 -n
  31204 bytes (30.5k)
local copy
Zopfli
  31136 bytes (30.4k)
local copy
Boot
  26210 bytes (25.6k)
CDN
cdnjs
  22297 bytes (21.8k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.7.0.min.js

You will automatically get the smallest Socket.IO 1.7.0 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 --i1000000 --mb8 --mls64 --bsr19 --lazy --ohh

(found September 8, 2017)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 19  --bsr19
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/1.7.0/socket.io.min.js --location | md5sum
27f1739c56e9bbf64738e2dbe7a6e062  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.7.0.min.zopfli.js.gz | md5sum
27f1739c56e9bbf64738e2dbe7a6e062  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.7.0/socket.io.min.js --location | sha1sum
fdc34c087db6694c6105b60232cc7e2269341ec8  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.7.0.min.zopfli.js.gz | sha1sum
fdc34c087db6694c6105b60232cc7e2269341ec8  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 26210 bytes 8c95b98a636a39df32c396d665ef6908 < (function webpackUniversalModuleDefinition(root, factory) [...]
< if(typeof exports === 'object' && typeof module === 'obje [...]
< module.exports = factory();
< else if(typeof define === 'function' && define.amd)
< define([], factory);
< else if(typeof exports === 'object')
< exports["io"] = factory();
< else
< root["io"] = factory();
< })(this, function() {
[...]
(invalid)
cdnjs 22297 bytes 8c95b98a636a39df32c396d665ef6908 < (function webpackUniversalModuleDefinition(root, factory) [...]
< if(typeof exports === 'object' && typeof module === 'obje [...]
< module.exports = factory();
< else if(typeof define === 'function' && define.amd)
< define([], factory);
< else if(typeof exports === 'object')
< exports["io"] = factory();
< else
< root["io"] = factory();
< })(this, function() {
[...]
(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
31136 bytes -5 bytes zopfli --i1000000 --mls64 --bsr19 --lazy --ohh September 8, 2017 @ 16:59
31141 bytes -1 byte zopfli --i10000 --mls64 --bsr19 --lazy --ohh September 7, 2017 @ 18:40
31142 bytes -2 bytes zopfli --i10000 --mls4096 --bsr25 --lazy --ohh September 7, 2017 @ 02:13
31144 bytes -1 byte zopfli --i1000 --mls4096 --bsr9 --lazy --ohh September 6, 2017 @ 21:06
31145 bytes -2 bytes zopfli --i1000 --mls8192 --bsr23 --lazy --ohh September 6, 2017 @ 14:00
31147 bytes -4 bytes zopfli --i1000 --mls2048 --bsr30 --lazy --ohh September 6, 2017 @ 12:51
31151 bytes zopfli --i100 --mls8192 --bsr16 --lazy --ohh September 6, 2017 @ 11:40

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 @ 16:59.

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
31154 31154 31153 31155 31155 31154 31146 31154 31154 31152 31152 31153 31152 31154 31154
31155 31145 31155 31154 31153 31145 31143 31144 31146 31142 31145 31142 31143 31145 31154
31146 31145 31147 31147 31145 31145 31143 31145 31146 31145 31145 31154 31146 31142 31149
31144 31145 31151 31144 31146 31145 31145 31145 31147 31145 31149 31142 31145 31142 31151
31144 31144 31145 31145 31145 31143 31145 31143 31144 31144 31147 31147 31145 31144 31150
31152 31143 31145 31144 31146 31145 31144 31142 31142 31143 31143 31142 31142 31151 31150
31144 31144 31144 31144 31144 31146 31145 31142 31146 31145 31145 31143 31142 31145 31144
31143 31145 31144 31144 31144 31146 31145 31142 31146 31145 31142 31143 31142 31142 31151
31146 31146 31145 31145 31153 31145 31145 31146 31144 31145 31145 31146 31142 31142 31152
31146 31143 31152 31145 31143 31145 31143 31142 31145 31146 31146 31142 31142 31145 31150
31152 31145 31151 31144 31152 31145 31145 31142 31146 31145 31147 31142 31143 31145 31151
31144 31145 31151 31142 31143 31145 31145 31142 31145 31145 31146 31143 31142 31145 31151
31146 31145 31144 31142 31146 31144 31143 31144 31146 31142 31142 31143 31142 31142 31148
31144 31151 31144 31145 31145 31145 31145 31145 31145 31145 31142 31142 31142 31142 31151
31144 31144 31144 31144 31151 31146 31145 31143 31145 31145 31142 31144 31142 31142 31151
31145 31144 31144 31144 31152 31136 31143 31145 31145 31145 31146 31142 31145 31145 31150
31144 31144 31153 31144 31151 31144 31142 31144 31146 31145 31145 31144 31142 31143 31151
31145 31143 31145 31144 31145 31145 31142 31144 31146 31142 31145 31142 31145 31145 31151
31145 31146 31144 31142 31145 31145 31142 31144 31146 31143 31143 31143 31142 31143 31150
31146 31146 31154 31145 31151 31145 31142 31144 31144 31145 31146 31143 31142 31143 31151
31145 31146 31145 31144 31153 31142 31143 31145 31144 31145 31146 31142 31146 31142 31151
31152 31145 31151 31144 31152 31145 31145 31144 31146 31142 31145 31143 31142 31142 31151
31144 31144 31144 31144 31145 31145 31145 31142 31146 31145 31145 31146 31145 31142 31151

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 31151 bytes 100%
1,000 31144 bytes -7 bytes 100%
10,000 31141 bytes -3 bytes 100%
100,000 31141 bytes 1.45%
1,000,000 31136 bytes -5 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
31235 bytes +99 bytes (+0.32%) +5 bytes
31230 bytes +94 bytes (+0.30%)
31275 bytes +139 bytes (+0.45%) +45 bytes
31315 bytes +179 bytes (+0.57%) +85 bytes
31275 bytes +139 bytes (+0.45%) +45 bytes
31314 bytes +178 bytes (+0.57%) +84 bytes
31324 bytes +188 bytes (+0.60%) +94 bytes
31339 bytes +203 bytes (+0.65%) +109 bytes
31370 bytes +234 bytes (+0.75%) +140 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 22794 bytes -8342 bytes (-26.79%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 23654 bytes -7482 bytes (-24.03%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 26016 bytes -5120 bytes (-16.44%)
Brotli (Wikipedia) brotli brotli -q 11 26980 bytes -4156 bytes (-13.35%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 27363 bytes -3773 bytes (-12.12%)
LZMA2 (Wikipedia) xz xz -9 28348 bytes -2788 bytes (-8.95%)
ZSTD (Wikipedia) zstd zstd -19 28998 bytes -2138 bytes (-6.87%)

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