Choose a version:
52% The original file has 167393 bytes (163.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 87533 bytes (85.5k, 52%).

After GZIP compression these minified files vary in size:
Boot
  27546 bytes (26.9k)
CDN
cdnjs
  23088 bytes (22.5k)
CDN
gzip -6 (default)
  22891 bytes (22.4k)
local copy
cdnhttps
  22868 bytes (22.3k)
CDN
gzip -9
  22805 bytes (22.3k)
local copy
libdeflate -12
  22023 bytes (21.5k)
local copy
7zip -mx=9 -tgzip
  21978 bytes (21.5k)
local copy
kzip -s0 -rn -b0
  21930 bytes (21.4k)
local copy
pigz -11 -n
  21926 bytes (21.4k)
local copy
Zopfli
  21903 bytes (21.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.2.1.min.js

You will automatically get the smallest Socket.IO 1.2.1 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 --i100000 --mb8 --mls1024 --bsr11 --lazy --ohh

(found June 30, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 11  --bsr11
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/1.2.1/socket.io.min.js --location | md5sum
98bcc2b8e872bc028f38ffd287203621  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.2.1.min.zopfli.js.gz | md5sum
98bcc2b8e872bc028f38ffd287203621  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.2.1/socket.io.min.js --location | sha1sum
e3ce580382b5648e905714f7ac6823dd559ad310  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.2.1.min.zopfli.js.gz | sha1sum
e3ce580382b5648e905714f7ac6823dd559ad310  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 27546 bytes 98bcc2b8e872bc028f38ffd287203621 (invalid)
cdnjs 23088 bytes 98bcc2b8e872bc028f38ffd287203621 (invalid)
cdnhttps 22868 bytes 98bcc2b8e872bc028f38ffd287203621 (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
21903 bytes -1 byte zopfli --i100000 --mls1024 --bsr11 --lazy --ohh June 30, 2016 @ 14:00
21904 bytes -3 bytes zopfli --i10000 --mls1024 --bsr11 --lazy --ohh June 30, 2016 @ 13:17
21907 bytes -4 bytes zopfli --i1000 --mls1024 --bsr11 --lazy --ohh June 29, 2016 @ 15:23
21911 bytes -2 bytes zopfli --i1000 --mls32 --bsr6 --lazy --ohh June 29, 2016 @ 15:23
21913 bytes zopfli --i100 --mls1024 --bsr11 --lazy --ohh June 29, 2016 @ 13:43

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 @ 00:58.

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
21940 21926 21941 21942 21921 21922 21940 21942 21917 21934 21943 21930 21929 21928 21936
21941 21939 21946 21927 21920 21916 21913 21913 21912 21911 21929 21928 21927 21931 21931
21911 21912 21912 21912 21909 21912 21912 21913 21914 21912 21935 21932 21929 21927 21932
21918 21928 21920 21919 21909 21920 21920 21920 21915 21940 21930 21928 21928 21926 21931
21942 21919 21912 21913 21914 21913 21921 21910 21910 21910 21912 21928 21927 21931 21927
21920 21913 21920 21909 21913 21921 21912 21913 21910 21912 21911 21928 21926 21927 21932
21912 21927 21912 21911 21912 21912 21912 21916 21916 21908 21928 21911 21928 21927 21931
21918 21927 21920 21920 21918 21912 21920 21912 21917 21903 21912 21928 21931 21931 21931
21928 21928 21918 21920 21919 21920 21920 21920 21911 21943 21941 21930 21931 21928 21927
21919 21929 21920 21920 21920 21912 21917 21911 21911 21943 21940 21930 21928 21932 21931
21912 21913 21912 21927 21908 21914 21912 21911 21910 21909 21913 21928 21930 21931 21927
21912 21913 21920 21914 21907 21912 21912 21910 21910 21909 21926 21928 21928 21927 21931
21929 21928 21921 21931 21912 21920 21913 21913 21910 21913 21927 21931 21928 21931 21930
21921 21910 21921 21920 21920 21911 21920 21913 21910 21915 21912 21928 21928 21931 21927
21929 21928 21940 21918 21919 21919 21920 21919 21912 21913 21911 21927 21931 21931 21927
21912 21912 21919 21920 21911 21913 21912 21910 21911 21909 21930 21911 21931 21929 21928
21919 21928 21921 21912 21912 21912 21911 21910 21916 21914 21910 21911 21928 21931 21929
21919 21928 21919 21922 21920 21920 21921 21910 21917 21909 21911 21927 21911 21927 21927
21919 21928 21922 21920 21913 21920 21911 21920 21917 21943 21942 21928 21911 21931 21931
21919 21913 21920 21923 21908 21919 21913 21911 21911 21913 21930 21931 21927 21931 21927
21929 21927 21923 21920 21920 21920 21920 21921 21910 21941 21943 21928 21911 21927 21928
21927 21912 21912 21920 21912 21919 21919 21920 21910 21943 21941 21927 21928 21927 21931
21912 21928 21920 21920 21919 21920 21912 21910 21910 21913 21942 21911 21931 21931 21932

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 21913 bytes 100%
1,000 21907 bytes -6 bytes 100%
10,000 21904 bytes -3 bytes 100%
100,000 21903 bytes -1 byte 0.29%
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
21930 bytes +27 bytes (+0.12%)
21947 bytes +44 bytes (+0.20%) +17 bytes
21936 bytes +33 bytes (+0.15%) +6 bytes
21982 bytes +79 bytes (+0.36%) +52 bytes
22010 bytes +107 bytes (+0.49%) +80 bytes
22040 bytes +137 bytes (+0.63%) +110 bytes
22074 bytes +171 bytes (+0.78%) +144 bytes
22100 bytes +197 bytes (+0.90%) +170 bytes
22105 bytes +202 bytes (+0.92%) +175 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 17758 bytes -4145 bytes (-18.92%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 18651 bytes -3252 bytes (-14.85%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 19365 bytes -2538 bytes (-11.59%)
Brotli (Wikipedia) brotli brotli -q 11 20729 bytes -1174 bytes (-5.36%)
LZMA2 (Wikipedia) xz xz -9 21432 bytes -471 bytes (-2.15%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 21566 bytes -337 bytes (-1.54%)
ZSTD (Wikipedia) zstd zstd -19 21740 bytes -163 bytes (-0.74%)

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