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 136467 bytes (133.3k, 64%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  33146 bytes (32.4k)
local copy
gzip -9
  32811 bytes (32.0k)
local copy
7zip -mx=9 -tgzip
  31357 bytes (30.6k)
local copy
libdeflate -12
  31346 bytes (30.6k)
local copy
kzip -s0 -rn -b0
  31275 bytes (30.5k)
local copy
pigz -11 -n
  31256 bytes (30.5k)
local copy
Zopfli
  31190 bytes (30.5k)
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-1.7.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.7.1 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 --bsr7 --lazy --ohh

(found September 8, 2017)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 7  --bsr7
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/1.7.1/socket.io.min.js --location | md5sum
17f2479d908689a03381710c09495c65  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.7.1.min.zopfli.js.gz | md5sum
17f2479d908689a03381710c09495c65  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.7.1/socket.io.min.js --location | sha1sum
98374695681c03f8ec512e1435a41de9bf8c2b52  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.7.1.min.zopfli.js.gz | sha1sum
98374695681c03f8ec512e1435a41de9bf8c2b52  -

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(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 22297 bytes 8c95b98a636a39df32c396d665ef6908 < (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
31190 bytes -3 bytes zopfli --i1000000 --mls64 --bsr7 --lazy --ohh September 8, 2017 @ 18:40
31193 bytes -6 bytes zopfli --i100000 --mls64 --bsr7 --lazy --ohh September 7, 2017 @ 08:54
31199 bytes -1 byte zopfli --i1000 --mls4 --bsr4 --lazy --ohh September 6, 2017 @ 14:58
31200 bytes -2 bytes zopfli --i1000 --mls2048 --bsr7 --lazy --ohh September 6, 2017 @ 13:18
31202 bytes -3 bytes zopfli --i1000 --mls32 --bsr18 --lazy --ohh September 6, 2017 @ 12:43
31205 bytes zopfli --i100 --mls16384 --bsr16 --lazy --ohh September 6, 2017 @ 11:37

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 @ 18:40.

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
31209 31209 31209 31211 31208 31209 31209 31209 31209 31209 31208 31208 31208 31209 31209
31200 31198 31199 31199 31201 31201 31201 31212 31200 31200 31200 31199 31198 31201 31207
31200 31200 31211 31200 31200 31201 31201 31200 31200 31200 31200 31207 31201 31198 31200
31202 31200 31201 31201 31201 31190 31201 31200 31198 31200 31199 31203 31199 31201 31206
31200 31201 31200 31200 31200 31201 31201 31196 31200 31200 31201 31208 31199 31200 31208
31200 31201 31200 31200 31201 31200 31201 31200 31198 31200 31201 31200 31197 31198 31204
31200 31200 31199 31200 31201 31201 31198 31200 31196 31199 31201 31200 31197 31201 31206
31201 31199 31199 31200 31201 31201 31207 31194 31198 31200 31201 31207 31200 31206 31206
31200 31201 31208 31199 31207 31199 31207 31201 31200 31200 31201 31200 31200 31202 31208
31200 31201 31201 31198 31200 31200 31201 31201 31200 31200 31201 31200 31201 31200 31208
31200 31200 31200 31200 31207 31200 31202 31200 31198 31200 31199 31200 31200 31200 31208
31201 31201 31201 31201 31200 31195 31201 31200 31199 31200 31199 31200 31200 31198 31208
31200 31200 31199 31202 31201 31201 31201 31199 31200 31201 31199 31199 31200 31199 31206
31200 31200 31199 31201 31201 31201 31198 31200 31198 31199 31201 31200 31200 31200 31206
31200 31200 31201 31201 31201 31200 31207 31200 31198 31199 31199 31201 31200 31198 31206
31200 31200 31200 31200 31201 31198 31198 31200 31200 31200 31201 31200 31197 31198 31207
31200 31200 31200 31201 31200 31201 31200 31200 31200 31200 31200 31201 31199 31200 31208
31201 31200 31200 31201 31200 31201 31201 31200 31199 31201 31199 31200 31197 31200 31206
31200 31200 31201 31201 31200 31200 31201 31198 31200 31200 31200 31201 31200 31198 31206
31201 31200 31201 31200 31201 31195 31200 31200 31199 31200 31200 31198 31201 31200 31206
31200 31200 31200 31201 31201 31200 31201 31200 31200 31200 31200 31199 31201 31195 31206
31202 31201 31199 31200 31200 31202 31200 31201 31200 31200 31199 31200 31199 31195 31206
31200 31200 31199 31200 31201 31201 31201 31200 31198 31200 31199 31198 31200 31200 31206

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 31205 bytes 100%
1,000 31199 bytes -6 bytes 100%
10,000 31194 bytes -5 bytes 100%
100,000 31193 bytes -1 byte 2.32%
1,000,000 31190 bytes -3 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
31275 bytes +85 bytes (+0.27%)
31286 bytes +96 bytes (+0.31%) +11 bytes
31311 bytes +121 bytes (+0.39%) +36 bytes
31368 bytes +178 bytes (+0.57%) +93 bytes
31325 bytes +135 bytes (+0.43%) +50 bytes
31362 bytes +172 bytes (+0.55%) +87 bytes
31397 bytes +207 bytes (+0.66%) +122 bytes
31381 bytes +191 bytes (+0.61%) +106 bytes
31419 bytes +229 bytes (+0.73%) +144 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 22831 bytes -8359 bytes (-26.80%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 23909 bytes -7281 bytes (-23.34%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 26066 bytes -5124 bytes (-16.43%)
Brotli (Wikipedia) brotli brotli -q 11 27000 bytes -4190 bytes (-13.43%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 27293 bytes -3897 bytes (-12.49%)
LZMA2 (Wikipedia) xz xz -9 28404 bytes -2786 bytes (-8.93%)
Zstandard (Wikipedia) zstd zstd -19 29038 bytes -2152 bytes (-6.90%)

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