Choose a version:
85% The original file has 62421 bytes (61.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 53044 bytes (51.8k, 85%).

After GZIP compression these minified files vary in size:
Boot
  15931 bytes (15.6k)
CDN
cdnjs
  15931 bytes (15.6k)
CDN
gzip -6 (default)
  15776 bytes (15.4k)
local copy
gzip -9
  15707 bytes (15.3k)
local copy
libdeflate -12
  15170 bytes (14.8k)
local copy
7zip -mx=9 -tgzip
  15144 bytes (14.8k)
local copy
kzip -s0 -rn -b4
  15124 bytes (14.8k)
local copy
pigz -11 -n
  15114 bytes (14.8k)
local copy
Zopfli
  15086 bytes (14.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.2.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 2.2.0 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 --bsr6 --lazy --ohh

(found November 29, 2018)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 6  --bsr6
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/2.2.0/socket.io.min.js --location | md5sum
f0a7d5df9122b3d7ab4de80f36db8475  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.2.0.min.zopfli.js.gz | md5sum
f0a7d5df9122b3d7ab4de80f36db8475  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.2.0/socket.io.min.js --location | sha1sum
16b7a23f32c78a449964085ca6ca541989bc3154  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-2.2.0.min.zopfli.js.gz | sha1sum
16b7a23f32c78a449964085ca6ca541989bc3154  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15931 bytes f0a7d5df9122b3d7ab4de80f36db8475 (invalid)
cdnjs 15931 bytes f0a7d5df9122b3d7ab4de80f36db8475 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available Socket.IO versions at minime.stephan-brumme.com:

2.2.0, 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
15086 bytes -1 byte zopfli --i1000000 --mls32 --bsr6 --lazy --ohh November 29, 2018 @ 18:15
15087 bytes -1 byte zopfli --i100000 --mls32 --bsr6 --lazy --ohh November 29, 2018 @ 15:20
15088 bytes -4 bytes zopfli --i10000 --mls32 --bsr6 --lazy --ohh November 29, 2018 @ 14:49
15092 bytes -1 byte zopfli --i10000 --mls8 --bsr18 --lazy --ohh November 29, 2018 @ 14:46
15093 bytes -1 byte zopfli --i1000 --mls32 --bsr6 --lazy --ohh November 29, 2018 @ 14:33
15094 bytes -1 byte zopfli --i1000 --mls8 --bsr23 --lazy --ohh November 29, 2018 @ 14:33
15095 bytes zopfli --i100 --mls16 --bsr19 --lazy --ohh November 29, 2018 @ 14:31

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

Most recent activity on November 29, 2018 @ 18:15.

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
15096 15096 15098 15100 15100 15100 15100 15098 15099 15099 15108 15101 15104 15102 15102
15094 15101 15094 15096 15099 15096 15097 15095 15099 15098 15097 15096 15095 15106 15098
15096 15096 15095 15095 15086 15099 15094 15095 15096 15097 15094 15101 15107 15101 15108
15099 15101 15095 15097 15101 15097 15098 15094 15100 15101 15101 15100 15098 15100 15100
15097 15099 15097 15096 15095 15099 15095 15094 15099 15097 15101 15100 15096 15101 15095
15096 15098 15095 15095 15095 15095 15094 15096 15095 15103 15094 15097 15097 15100 15101
15097 15099 15095 15097 15095 15095 15094 15095 15093 15095 15102 15106 15102 15105 15103
15097 15099 15093 15094 15095 15095 15093 15094 15095 15095 15102 15098 15101 15100 15101
15095 15097 15094 15097 15097 15095 15097 15100 15094 15101 15095 15098 15103 15102 15102
15094 15094 15097 15093 15095 15095 15095 15094 15093 15094 15100 15100 15101 15100 15097
15096 15097 15094 15097 15095 15095 15096 15096 15094 15098 15102 15101 15105 15105 15093
15097 15098 15093 15094 15097 15094 15095 15094 15095 15101 15097 15100 15098 15099 15102
15096 15097 15094 15092 15094 15097 15094 15096 15093 15098 15094 15096 15101 15101 15102
15094 15098 15099 15093 15094 15094 15094 15096 15094 15095 15104 15096 15095 15099 15093
15095 15097 15092 15097 15095 15094 15096 15102 15093 15097 15098 15101 15097 15100 15101
15098 15096 15095 15092 15096 15094 15096 15094 15093 15094 15096 15101 15102 15101 15101
15097 15097 15097 15095 15096 15096 15096 15094 15094 15095 15102 15095 15105 15100 15101
15094 15095 15098 15098 15096 15101 15095 15094 15095 15094 15101 15101 15098 15105 15097
15096 15097 15097 15098 15095 15095 15089 15089 15094 15095 15094 15095 15101 15101 15101
15094 15096 15092 15096 15095 15095 15094 15098 15093 15095 15095 15098 15101 15107 15097
15095 15095 15102 15096 15095 15095 15095 15096 15096 15100 15100 15095 15101 15102 15101
15097 15098 15095 15096 15095 15095 15094 15096 15092 15097 15100 15099 15102 15102 15100
15098 15096 15098 15092 15094 15096 15094 15094 15096 15096 15095 15099 15101 15100 15101

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 15095 bytes 100%
1,000 15093 bytes -2 bytes 100%
10,000 15088 bytes -5 bytes 100%
100,000 15087 bytes -1 byte 0.87%
1,000,000 15086 bytes -1 byte 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
15147 bytes +61 bytes (+0.40%) +23 bytes
15148 bytes +62 bytes (+0.41%) +24 bytes
15146 bytes +60 bytes (+0.40%) +22 bytes
15164 bytes +78 bytes (+0.52%) +40 bytes
15124 bytes +38 bytes (+0.25%)
15140 bytes +54 bytes (+0.36%) +16 bytes
15172 bytes +86 bytes (+0.57%) +48 bytes
15201 bytes +115 bytes (+0.76%) +77 bytes
15226 bytes +140 bytes (+0.93%) +102 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 12605 bytes -2481 bytes (-16.45%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12792 bytes -2294 bytes (-15.21%)
Brotli (Wikipedia) brotli brotli -q 11 13928 bytes -1158 bytes (-7.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13974 bytes -1112 bytes (-7.37%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14379 bytes -707 bytes (-4.69%)
LZMA2 (Wikipedia) xz xz -9 14648 bytes -438 bytes (-2.90%)
Zstandard (Wikipedia) zstd zstd -19 14851 bytes -235 bytes (-1.56%)

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