Choose a version:
85% The original file has 62395 bytes (60.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 53019 bytes (51.8k, 85%).

After GZIP compression these minified files vary in size:
Boot
  16510 bytes (16.1k)
CDN
cdnjs
  15932 bytes (15.6k)
CDN
gzip -6 (default)
  15783 bytes (15.4k)
local copy
gzip -9
  15718 bytes (15.3k)
local copy
libdeflate -12
  15180 bytes (14.8k)
local copy
7zip -mx=9 -tgzip
  15153 bytes (14.8k)
local copy
kzip -s0 -rn -b4
  15145 bytes (14.8k)
local copy
pigz -11 -n
  15120 bytes (14.8k)
local copy
Zopfli
  15093 bytes (14.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.1.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 2.1.0 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 --mls32 --bsr23 --lazy --ohh

(found June 26, 2018)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 23  --bsr23
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/2.1.0/socket.io.min.js --location | md5sum
a6526d0b994f410826635a73dee567ea  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.1.0.min.zopfli.js.gz | md5sum
a6526d0b994f410826635a73dee567ea  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.1.0/socket.io.min.js --location | sha1sum
51208b9039c7244ee15b7c785095f6a283970679  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.1.0.min.zopfli.js.gz | sha1sum
51208b9039c7244ee15b7c785095f6a283970679  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 16510 bytes a6526d0b994f410826635a73dee567ea (invalid)
cdnjs 15932 bytes a6526d0b994f410826635a73dee567ea (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
15093 bytes -2 bytes zopfli --i1000000 --mls32 --bsr23 --lazy --ohh June 26, 2018 @ 19:30
15095 bytes -1 byte zopfli --i100000 --mls32 --bsr23 --lazy --ohh June 26, 2018 @ 16:23
15096 bytes -1 byte zopfli --i10000 --mls32 --bsr23 --lazy --ohh June 26, 2018 @ 11:59
15097 bytes -2 bytes zopfli --i10000 --mls4 --bsr4 --lazy --ohh June 26, 2018 @ 11:45
15099 bytes -1 byte zopfli --i1000 --mls4 --bsr14 --lazy --ohh June 26, 2018 @ 11:21
15100 bytes -4 bytes zopfli --i1000 --mls16 --bsr30 --lazy --ohh June 26, 2018 @ 11:20
15104 bytes zopfli --i100 --mls8 --bsr4 --lazy --ohh June 26, 2018 @ 11:13

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

Most recent activity on June 26, 2018 @ 21:14.

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
15109 15110 15108 15107 15108 15109 15111 15109 15114 15110 15108 15109 15112 15110 15113
15102 15094 15098 15101 15107 15109 15109 15099 15101 15104 15104 15121 15111 15121 15104
15110 15108 15108 15098 15096 15103 15098 15107 15109 15107 15118 15119 15106 15109 15107
15109 15110 15108 15109 15102 15109 15099 15105 15103 15105 15108 15109 15107 15109 15109
15110 15110 15107 15110 15110 15104 15108 15103 15113 15102 15108 15103 15110 15108 15103
15102 15096 15103 15108 15104 15106 15106 15103 15103 15107 15112 15109 15107 15109 15105
15106 15109 15111 15107 15102 15108 15109 15103 15102 15106 15108 15108 15110 15106 15106
15106 15107 15110 15104 15108 15105 15106 15109 15110 15107 15108 15120 15120 15107 15106
15107 15099 15106 15104 15104 15102 15106 15109 15102 15107 15101 15103 15111 15108 15102
15104 15105 15109 15106 15102 15104 15110 15100 15107 15104 15104 15103 15120 15108 15105
15106 15094 15108 15108 15093 15106 15109 15109 15103 15105 15104 15103 15107 15106 15104
15105 15102 15106 15107 15102 15106 15101 15102 15102 15108 15107 15109 15108 15112 15107
15106 15098 15109 15099 15104 15108 15109 15104 15104 15105 15109 15108 15111 15108 15106
15106 15098 15108 15110 15102 15108 15104 15107 15110 15105 15108 15103 15107 15108 15104
15107 15108 15108 15109 15099 15106 15110 15102 15102 15106 15120 15121 15106 15108 15109
15107 15096 15096 15107 15102 15108 15109 15102 15112 15104 15107 15109 15109 15108 15108
15101 15099 15108 15110 15096 15104 15100 15102 15102 15104 15111 15103 15110 15106 15104
15103 15094 15096 15106 15103 15108 15108 15103 15111 15105 15120 15105 15109 15109 15108
15100 15100 15108 15102 15103 15107 15109 15107 15109 15104 15119 15108 15107 15109 15105
15110 15110 15105 15107 15093 15107 15108 15103 15101 15104 15107 15108 15107 15108 15106
15102 15100 15098 15098 15103 15104 15107 15105 15103 15104 15118 15099 15109 15108 15105
15103 15109 15104 15098 15102 15104 15107 15103 15102 15104 15102 15102 15109 15109 15104
15103 15101 15107 15105 15104 15104 15103 15103 15103 15108 15108 15108 15110 15108 15102

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 15104 bytes 100%
1,000 15099 bytes -5 bytes 100%
10,000 15096 bytes -3 bytes 100%
100,000 15095 bytes -1 byte 3.19%
1,000,000 15093 bytes -2 bytes 1.45%
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
15161 bytes +68 bytes (+0.45%) +16 bytes
15161 bytes +68 bytes (+0.45%) +16 bytes
15158 bytes +65 bytes (+0.43%) +13 bytes
15186 bytes +93 bytes (+0.62%) +41 bytes
15145 bytes +52 bytes (+0.34%)
15162 bytes +69 bytes (+0.46%) +17 bytes
15197 bytes +104 bytes (+0.69%) +52 bytes
15232 bytes +139 bytes (+0.92%) +87 bytes
15260 bytes +167 bytes (+1.11%) +115 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
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12620 bytes -2473 bytes (-16.39%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12803 bytes -2290 bytes (-15.17%)
Brotli (Wikipedia) brotli brotli -q 11 13936 bytes -1157 bytes (-7.67%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13980 bytes -1113 bytes (-7.37%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14379 bytes -714 bytes (-4.73%)
LZMA2 (Wikipedia) xz xz -9 14660 bytes -433 bytes (-2.87%)
Zstandard (Wikipedia) zstd zstd -19 14926 bytes -167 bytes (-1.11%)

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