Choose a version:
52% The original file has 151328 bytes (147.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 78563 bytes (76.7k, 52%).

After GZIP compression these minified files vary in size:
Boot
  24742 bytes (24.2k)
CDN
cdnjs
  20693 bytes (20.2k)
CDN
gzip -6 (default)
  20530 bytes (20.0k)
local copy
cdnhttps
  20507 bytes (20.0k)
CDN
gzip -9
  20454 bytes (20.0k)
local copy
libdeflate -12
  19774 bytes (19.3k)
local copy
7zip -mx=9 -tgzip
  19728 bytes (19.3k)
local copy
pigz -11 -n
  19698 bytes (19.2k)
local copy
kzip -s0 -rn -b2
  19687 bytes (19.2k)
local copy
Zopfli
  19665 bytes (19.2k)
local copy
Zopfli (defluff)
  19664 bytes (19.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.0.3.min.js

You will automatically get the smallest Socket.IO 1.0.3 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 --mls64 --bsr15 --lazy --ohh

(found June 29, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 15  --bsr15
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/1.0.3/socket.io.min.js --location | md5sum
df1a565da9518817bce65b5846aebd12  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.3.min.zopfli.js.gz | md5sum
df1a565da9518817bce65b5846aebd12  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.0.3/socket.io.min.js --location | sha1sum
926b6c73dd6c8ea67d724f5bcd19bb48d732cc06  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.3.min.zopfli.js.gz | sha1sum
926b6c73dd6c8ea67d724f5bcd19bb48d732cc06  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 24742 bytes df1a565da9518817bce65b5846aebd12 (invalid)
cdnjs 20693 bytes df1a565da9518817bce65b5846aebd12 (invalid)
cdnhttps 20507 bytes df1a565da9518817bce65b5846aebd12 (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
19665 bytes -2 bytes zopfli --i100000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 16:05
19667 bytes -3 bytes zopfli --i10000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 15:33
19670 bytes -2 bytes zopfli --i1000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 15:24
19672 bytes zopfli --i100 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 11:11

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

Most recent activity on July 4, 2016 @ 11:09.

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 or 100,000.
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
19678 19677 19678 19678 19677 19678 19689 19678 19689 19690 19692 19689 19678 19682 19668
19676 19676 19675 19677 19677 19678 19692 19677 19692 19690 19690 19697 19682 19693 19681
19679 19680 19683 19682 19676 19676 19693 19678 19681 19690 19691 19681 19682 19682 19683
19679 19682 19678 19684 19679 19680 19692 19679 19688 19682 19692 19685 19683 19683 19685
19678 19676 19676 19676 19676 19676 19692 19679 19691 19689 19692 19689 19682 19682 19689
19679 19680 19682 19680 19680 19690 19689 19690 19689 19690 19691 19687 19682 19682 19669
19679 19680 19680 19678 19680 19666 19692 19689 19683 19682 19683 19682 19673 19682 19669
19679 19677 19675 19676 19677 19675 19692 19676 19691 19692 19692 19682 19682 19681 19682
19679 19676 19679 19676 19677 19676 19692 19692 19692 19692 19691 19692 19682 19688 19678
19679 19677 19676 19676 19678 19678 19692 19679 19691 19689 19693 19682 19689 19682 19685
19679 19678 19676 19678 19675 19675 19689 19679 19682 19690 19693 19682 19682 19682 19667
19678 19681 19678 19677 19680 19665 19689 19683 19688 19690 19681 19682 19682 19685 19682
19679 19674 19677 19678 19677 19677 19692 19676 19692 19689 19692 19692 19682 19682 19668
19680 19677 19678 19677 19677 19676 19692 19679 19693 19690 19692 19685 19681 19685 19676
19681 19679 19679 19678 19680 19689 19689 19690 19689 19690 19691 19682 19682 19690 19672
19679 19676 19679 19675 19678 19678 19690 19680 19682 19690 19691 19682 19682 19685 19669
19679 19678 19679 19678 19682 19676 19691 19677 19690 19690 19691 19692 19682 19685 19669
19678 19675 19678 19675 19677 19676 19692 19677 19690 19689 19691 19682 19682 19681 19669
19677 19676 19676 19676 19677 19676 19691 19693 19692 19689 19691 19682 19682 19685 19670
19679 19679 19678 19676 19681 19681 19689 19678 19692 19689 19691 19682 19682 19682 19669
19679 19675 19678 19675 19676 19677 19692 19678 19691 19689 19691 19692 19681 19682 19669
19678 19678 19676 19676 19675 19675 19691 19679 19690 19689 19691 19682 19682 19682 19667
19682 19677 19678 19675 19677 19676 19691 19694 19690 19691 19693 19689 19682 19682 19668

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 19672 bytes 100%
1,000 19667 bytes -5 bytes 100%
10,000 19667 bytes 100%
100,000 19665 bytes -2 bytes 1.16%
1,000,000
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
19690 bytes +25 bytes (+0.13%) +3 bytes
19710 bytes +45 bytes (+0.23%) +23 bytes
19687 bytes +22 bytes (+0.11%)
19728 bytes +63 bytes (+0.32%) +41 bytes
19758 bytes +93 bytes (+0.47%) +71 bytes
19790 bytes +125 bytes (+0.64%) +103 bytes
19823 bytes +158 bytes (+0.80%) +136 bytes
19827 bytes +162 bytes (+0.82%) +140 bytes
19861 bytes +196 bytes (+1.00%) +174 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
ZPAQ (Wikipedia) zpaq zpaq -method 69 16092 bytes -3573 bytes (-18.17%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 16818 bytes -2847 bytes (-14.48%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 17539 bytes -2126 bytes (-10.81%)
Brotli (Wikipedia) brotli brotli -q 11 18589 bytes -1076 bytes (-5.47%)
LZMA2 (Wikipedia) xz xz -9 19280 bytes -385 bytes (-1.96%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19341 bytes -324 bytes (-1.65%)
ZSTD (Wikipedia) zstd zstd -19 19558 bytes -107 bytes (-0.54%)

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