Choose a version:
86% The original file has 61211 bytes (59.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 52900 bytes (51.7k, 86%).

After GZIP compression these minified files vary in size:
Boot
  18767 bytes (18.3k)
CDN
cdnjs
  15915 bytes (15.5k)
CDN
gzip -6 (default)
  15755 bytes (15.4k)
local copy
gzip -9
  15687 bytes (15.3k)
local copy
libdeflate -12
  15160 bytes (14.8k)
local copy
7zip -mx=9 -tgzip
  15143 bytes (14.8k)
local copy
kzip -s0 -rn -b4
  15124 bytes (14.8k)
local copy
pigz -11 -n
  15095 bytes (14.7k)
local copy
Zopfli
  15074 bytes (14.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-2.0.3.min.js

You will automatically get the smallest Socket.IO 2.0.3 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 --mls8192 --bsr16 --lazy --ohh

(found September 7, 2017)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
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/2.0.3/socket.io.min.js --location | md5sum
272b6bb1aee2b475fd08dbe3c5017dae  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.0.3.min.zopfli.js.gz | md5sum
272b6bb1aee2b475fd08dbe3c5017dae  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/2.0.3/socket.io.min.js --location | sha1sum
3fd8983b6ff74f0801650cdd498b4ea41966add5  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-2.0.3.min.zopfli.js.gz | sha1sum
3fd8983b6ff74f0801650cdd498b4ea41966add5  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 18767 bytes 272b6bb1aee2b475fd08dbe3c5017dae (invalid)
cdnjs 15915 bytes 272b6bb1aee2b475fd08dbe3c5017dae (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
15074 bytes -1 byte zopfli --i100000 --mls8192 --bsr16 --lazy --ohh September 7, 2017 @ 07:46
15075 bytes -1 byte zopfli --i10000 --mls8192 --bsr16 --lazy --ohh September 6, 2017 @ 19:47
15076 bytes -1 byte zopfli --i10000 --mls16 --bsr17 --lazy --ohh September 6, 2017 @ 19:35
15077 bytes -1 byte zopfli --i10000 --mls2 --bsr10 --lazy --ohh September 6, 2017 @ 18:22
15078 bytes -1 byte zopfli --i1000 --mls32768 --bsr14 --lazy --ohh September 6, 2017 @ 13:24
15079 bytes -1 byte zopfli --i1000 --mls128 --bsr4 --lazy --ohh September 6, 2017 @ 12:45
15080 bytes zopfli --i100 --mls128 --bsr40 --lazy --ohh September 6, 2017 @ 11:56

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

Most recent activity on October 23, 2017 @ 14:28.

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
15087 15087 15088 15088 15088 15087 15088 15087 15087 15088 15103 15102 15103 15091 15094
15080 15085 15083 15085 15085 15080 15077 15084 15087 15085 15098 15105 15088 15086 15095
15078 15083 15083 15080 15077 15080 15079 15084 15079 15082 15097 15101 15087 15084 15082
15080 15081 15082 15078 15078 15079 15078 15087 15083 15085 15081 15079 15085 15085 15082
15080 15080 15082 15082 15079 15078 15081 15086 15080 15083 15097 15098 15092 15085 15090
15080 15081 15087 15085 15079 15086 15078 15083 15083 15088 15097 15081 15087 15084 15081
15077 15080 15084 15080 15078 15079 15078 15080 15084 15086 15086 15097 15089 15087 15082
15078 15080 15085 15085 15078 15077 15077 15085 15083 15087 15085 15097 15087 15086 15080
15080 15082 15081 15086 15079 15077 15078 15080 15080 15086 15097 15098 15083 15083 15086
15080 15089 15079 15079 15079 15079 15078 15083 15085 15086 15097 15082 15091 15088 15087
15077 15080 15086 15077 15079 15079 15078 15078 15083 15079 15083 15098 15086 15080 15075
15077 15080 15082 15077 15077 15081 15078 15084 15083 15081 15078 15080 15091 15082 15082
15077 15081 15084 15078 15080 15078 15079 15078 15083 15085 15083 15084 15074 15084 15091
15078 15081 15084 15075 15079 15080 15078 15086 15081 15087 15097 15100 15089 15083 15090
15077 15081 15085 15078 15078 15078 15078 15083 15081 15086 15098 15097 15086 15081 15085
15083 15085 15082 15079 15077 15079 15085 15085 15081 15084 15099 15100 15086 15091 15091
15078 15082 15077 15085 15078 15077 15081 15084 15083 15087 15083 15098 15086 15083 15085
15077 15082 15081 15078 15077 15078 15078 15080 15080 15080 15087 15086 15081 15085 15084
15081 15081 15082 15081 15079 15076 15078 15080 15083 15081 15097 15097 15091 15085 15090
15077 15080 15084 15077 15077 15078 15078 15085 15081 15084 15083 15100 15087 15080 15081
15077 15080 15079 15079 15079 15078 15078 15078 15082 15086 15097 15097 15091 15084 15090
15080 15080 15086 15084 15082 15081 15078 15082 15086 15086 15097 15098 15084 15094 15080
15087 15081 15084 15087 15082 15080 15079 15085 15082 15085 15084 15100 15089 15082 15091

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 15080 bytes 100%
1,000 15078 bytes -2 bytes 100%
10,000 15075 bytes -3 bytes 100%
100,000 15074 bytes -1 byte 1.16%
1,000,000 15074 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
15138 bytes +64 bytes (+0.42%) +14 bytes
15135 bytes +61 bytes (+0.40%) +11 bytes
15142 bytes +68 bytes (+0.45%) +18 bytes
15161 bytes +87 bytes (+0.58%) +37 bytes
15124 bytes +50 bytes (+0.33%)
15139 bytes +65 bytes (+0.43%) +15 bytes
15174 bytes +100 bytes (+0.66%) +50 bytes
15213 bytes +139 bytes (+0.92%) +89 bytes
15241 bytes +167 bytes (+1.11%) +117 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 12619 bytes -2455 bytes (-16.29%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12794 bytes -2280 bytes (-15.13%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13913 bytes -1161 bytes (-7.70%)
Brotli (Wikipedia) brotli brotli -q 11 13936 bytes -1138 bytes (-7.55%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14398 bytes -676 bytes (-4.48%)
LZMA2 (Wikipedia) xz xz -9 14668 bytes -406 bytes (-2.69%)
ZSTD (Wikipedia) zstd zstd -19 14912 bytes -162 bytes (-1.07%)

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 2017.
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