Choose a version:
34% The original file has 196420 bytes (191.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 67623 bytes (66.0k, 34%).

After GZIP compression these minified files vary in size:
Boot
  24959 bytes (24.4k)
CDN
cdnjs
  21323 bytes (20.8k)
CDN
gzip -6 (default)
  21155 bytes (20.7k)
local copy
cdnhttps
  21105 bytes (20.6k)
CDN
gzip -9
  21062 bytes (20.6k)
local copy
7zip -mx=9 -tgzip
  20353 bytes (19.9k)
local copy
libdeflate -12
  20325 bytes (19.8k)
local copy
kzip -s0 -rn -b2
  20269 bytes (19.8k)
local copy
pigz -11 -n
  20266 bytes (19.8k)
local copy
Zopfli
  20240 bytes (19.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.5.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.5.0 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 --mls512 --bsr7 --lazy --ohh

(found October 12, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 7  --bsr7
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/1.5.0/socket.io.min.js --location | md5sum
1c26483e45e4d6f35d836bb86751dddd  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.5.0.min.zopfli.js.gz | md5sum
1c26483e45e4d6f35d836bb86751dddd  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.5.0/socket.io.min.js --location | sha1sum
7697441ea0492fcf2b6729a2b970fac4de7e93cf  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.5.0.min.zopfli.js.gz | sha1sum
7697441ea0492fcf2b6729a2b970fac4de7e93cf  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 24959 bytes 1c26483e45e4d6f35d836bb86751dddd (invalid)
cdnjs 21323 bytes 1c26483e45e4d6f35d836bb86751dddd (invalid)
cdnhttps 21105 bytes 1c26483e45e4d6f35d836bb86751dddd November 22, 2016 @ 16:41

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
20240 bytes -1 byte zopfli --i1000000 --mls512 --bsr7 --lazy --ohh October 12, 2016 @ 17:16
20241 bytes -1 byte zopfli --i100000 --mls512 --bsr7 --lazy --ohh October 11, 2016 @ 06:42
20242 bytes -2 bytes zopfli --i10000 --mls512 --bsr7 --lazy --ohh October 11, 2016 @ 06:23
20244 bytes -1 byte zopfli --i1000 --mls512 --bsr7 --lazy --ohh October 11, 2016 @ 06:19
20245 bytes -3 bytes zopfli --i1000 --mls8192 --bsr8 --lazy --ohh October 11, 2016 @ 06:18
20248 bytes -1 byte zopfli --i1000 --mls2048 --bsr16 --lazy --ohh October 11, 2016 @ 06:18
20249 bytes zopfli --i100 --mls2048 --bsr16 --lazy --ohh October 11, 2016 @ 06:03

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

Most recent activity on January 25, 2017 @ 15:29.

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
20269 20270 20269 20270 20274 20277 20278 20273 20269 20273 20261 20277 20274 20277 20279
20249 20249 20250 20246 20253 20252 20252 20244 20244 20248 20251 20241 20260 20253 20245
20249 20251 20254 20255 20244 20249 20246 20252 20242 20247 20245 20242 20265 20246 20246
20259 20252 20253 20253 20255 20251 20252 20248 20240 20247 20244 20262 20260 20251 20252
20249 20251 20250 20246 20249 20251 20253 20247 20245 20249 20244 20244 20242 20252 20251
20249 20251 20249 20251 20258 20251 20255 20252 20241 20246 20244 20253 20260 20253 20260
20250 20252 20250 20254 20250 20252 20240 20241 20242 20247 20244 20260 20260 20246 20252
20250 20252 20248 20254 20249 20251 20252 20244 20246 20248 20252 20245 20261 20253 20254
20249 20252 20250 20255 20250 20251 20251 20250 20242 20246 20254 20253 20253 20248 20261
20250 20245 20254 20251 20248 20244 20252 20246 20242 20246 20244 20244 20261 20251 20251
20251 20251 20253 20247 20245 20251 20252 20245 20244 20248 20244 20241 20253 20246 20252
20249 20251 20253 20247 20249 20251 20252 20247 20244 20247 20245 20241 20247 20254 20255
20250 20251 20249 20246 20245 20250 20246 20246 20244 20246 20242 20244 20260 20246 20253
20249 20251 20244 20252 20249 20250 20250 20246 20244 20247 20253 20251 20260 20247 20248
20250 20252 20249 20251 20244 20253 20251 20246 20242 20246 20244 20260 20260 20247 20246
20250 20249 20248 20255 20253 20251 20251 20246 20249 20247 20244 20245 20240 20247 20250
20251 20251 20249 20250 20244 20249 20242 20251 20242 20246 20244 20250 20256 20247 20246
20250 20251 20250 20247 20260 20251 20246 20246 20248 20247 20247 20255 20247 20253 20253
20250 20253 20247 20253 20256 20252 20252 20250 20244 20247 20244 20244 20240 20246 20246
20250 20251 20248 20251 20249 20253 20246 20246 20248 20246 20253 20242 20260 20253 20251
20250 20251 20247 20251 20249 20251 20251 20246 20248 20246 20244 20260 20260 20250 20245
20249 20258 20247 20254 20250 20250 20252 20248 20244 20248 20244 20253 20257 20246 20246
20251 20252 20248 20252 20250 20253 20253 20253 20242 20248 20245 20245 20263 20248 20245

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 20249 bytes 100%
1,000 20244 bytes -5 bytes 100%
10,000 20242 bytes -2 bytes 100%
100,000 20241 bytes -1 byte 6.09%
1,000,000 20240 bytes -1 byte 2.61%
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
20343 bytes +103 bytes (+0.51%) +74 bytes
20343 bytes +103 bytes (+0.51%) +74 bytes
20269 bytes +29 bytes (+0.14%)
20274 bytes +34 bytes (+0.17%) +5 bytes
20296 bytes +56 bytes (+0.28%) +27 bytes
20292 bytes +52 bytes (+0.26%) +23 bytes
20313 bytes +73 bytes (+0.36%) +44 bytes
20357 bytes +117 bytes (+0.58%) +88 bytes
20378 bytes +138 bytes (+0.68%) +109 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
ZPAQ (Wikipedia) zpaq zpaq -method 69 16828 bytes -3412 bytes (-16.86%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 17173 bytes -3067 bytes (-15.15%)
Brotli (Wikipedia) brotli brotli -q 11 18544 bytes -1696 bytes (-8.38%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 18593 bytes -1647 bytes (-8.14%)
LZMA2 (Wikipedia) xz xz -9 19328 bytes -912 bytes (-4.51%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19418 bytes -822 bytes (-4.06%)
Zstandard (Wikipedia) zstd zstd -19 19746 bytes -494 bytes (-2.44%)

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