Choose a version:
86% The original file has 61067 bytes (59.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 52804 bytes (51.6k, 86%).

After GZIP compression these minified files vary in size:
Boot
  18751 bytes (18.3k)
CDN
cdnjs
  15900 bytes (15.5k)
CDN
gzip -6 (default)
  15742 bytes (15.4k)
local copy
gzip -9
  15675 bytes (15.3k)
local copy
libdeflate -12
  15148 bytes (14.8k)
local copy
7zip -mx=9 -tgzip
  15128 bytes (14.8k)
local copy
kzip -s0 -rn -b4
  15114 bytes (14.8k)
local copy
pigz -11 -n
  15077 bytes (14.7k)
local copy
Zopfli
  15059 bytes (14.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.0.0.min.js

You will automatically get the smallest Socket.IO 2.0.0 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 --mls16384 --bsr16 --lazy --ohh

(found September 7, 2017)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 16384  --mls16384
block splitting recursion 16  --bsr16
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/2.0.0/socket.io.min.js --location | md5sum
b662b0a146686b726716c16f62eb5efe  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.0.0.min.zopfli.js.gz | md5sum
b662b0a146686b726716c16f62eb5efe  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.0.0/socket.io.min.js --location | sha1sum
a8c010b564aafbea8c79b4e83be9d3bc7d3a09d0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.0.0.min.zopfli.js.gz | sha1sum
a8c010b564aafbea8c79b4e83be9d3bc7d3a09d0  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 18751 bytes b662b0a146686b726716c16f62eb5efe (invalid)
cdnjs 15900 bytes b662b0a146686b726716c16f62eb5efe (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
15059 bytes -2 bytes zopfli --i100000 --mls16384 --bsr16 --lazy --ohh September 7, 2017 @ 08:21
15061 bytes -3 bytes zopfli --i10000 --mls16384 --bsr16 --lazy --ohh September 6, 2017 @ 17:45
15064 bytes -2 bytes zopfli --i1000 --mls16384 --bsr16 --lazy --ohh September 6, 2017 @ 12:47
15066 bytes -2 bytes zopfli --i1000 --mls4096 --bsr2 --lazy --ohh September 6, 2017 @ 12:43
15068 bytes zopfli --i100 --mls4096 --bsr2 --lazy --ohh September 6, 2017 @ 11:23

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 @ 13:56.

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
15073 15073 15074 15075 15076 15076 15074 15075 15078 15078 15094 15064 15089 15081 15083
15077 15078 15070 15073 15073 15071 15074 15072 15076 15072 15089 15073 15088 15077 15075
15065 15064 15065 15070 15065 15065 15065 15064 15064 15067 15071 15065 15077 15072 15073
15069 15069 15069 15068 15065 15068 15066 15069 15069 15090 15088 15064 15078 15072 15071
15070 15069 15075 15068 15070 15065 15066 15069 15069 15067 15064 15065 15069 15069 15072
15069 15069 15074 15071 15074 15065 15068 15073 15068 15066 15065 15064 15090 15071 15075
15070 15070 15070 15066 15065 15065 15061 15069 15071 15065 15066 15069 15068 15073 15073
15070 15071 15070 15071 15068 15068 15074 15070 15076 15069 15069 15072 15081 15067 15074
15073 15075 15069 15064 15072 15068 15066 15068 15076 15070 15065 15071 15069 15071 15076
15070 15069 15068 15063 15068 15066 15071 15070 15068 15068 15065 15064 15063 15077 15072
15069 15069 15063 15074 15064 15064 15067 15070 15069 15069 15065 15064 15069 15071 15080
15069 15069 15068 15066 15065 15065 15064 15069 15070 15068 15068 15067 15068 15072 15070
15069 15070 15069 15063 15072 15066 15065 15067 15069 15068 15064 15067 15078 15059 15075
15063 15070 15076 15065 15069 15069 15070 15073 15069 15068 15064 15063 15068 15067 15065
15069 15069 15075 15061 15064 15069 15067 15074 15069 15068 15069 15068 15069 15071 15080
15063 15063 15063 15064 15067 15065 15064 15065 15068 15067 15072 15069 15068 15067 15077
15065 15071 15065 15065 15064 15065 15066 15068 15072 15068 15063 15069 15074 15071 15073
15070 15071 15068 15066 15068 15068 15069 15068 15069 15071 15071 15072 15069 15072 15072
15069 15069 15070 15068 15069 15068 15071 15064 15069 15068 15065 15066 15078 15067 15075
15069 15069 15068 15064 15064 15063 15068 15069 15069 15068 15069 15070 15077 15071 15077
15064 15069 15068 15063 15069 15063 15066 15063 15068 15068 15065 15069 15069 15072 15071
15070 15069 15069 15069 15072 15065 15071 15068 15069 15068 15069 15070 15070 15067 15071
15069 15068 15068 15069 15064 15066 15074 15068 15068 15068 15065 15068 15069 15070 15071

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 15068 bytes 100%
1,000 15064 bytes -4 bytes 100%
10,000 15061 bytes -3 bytes 100%
100,000 15059 bytes -2 bytes 1.16%
1,000,000 15059 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
15123 bytes +64 bytes (+0.42%) +9 bytes
15127 bytes +68 bytes (+0.45%) +13 bytes
15133 bytes +74 bytes (+0.49%) +19 bytes
15150 bytes +91 bytes (+0.60%) +36 bytes
15114 bytes +55 bytes (+0.37%)
15133 bytes +74 bytes (+0.49%) +19 bytes
15163 bytes +104 bytes (+0.69%) +49 bytes
15197 bytes +138 bytes (+0.92%) +83 bytes
15230 bytes +171 bytes (+1.14%) +116 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
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12630 bytes -2429 bytes (-16.13%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12807 bytes -2252 bytes (-14.95%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13894 bytes -1165 bytes (-7.74%)
Brotli (Wikipedia) brotli brotli -q 11 13975 bytes -1084 bytes (-7.20%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14413 bytes -646 bytes (-4.29%)
LZMA2 (Wikipedia) xz xz -9 14692 bytes -367 bytes (-2.44%)
ZSTD (Wikipedia) zstd zstd -19 14918 bytes -141 bytes (-0.94%)

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