Choose a version:
53% The original file has 172531 bytes (168.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 90645 bytes (88.5k, 53%).

After GZIP compression these minified files vary in size:
Boot
  23650 bytes (23.1k)
CDN
gzip -6 (default)
  23606 bytes (23.1k)
local copy
gzip -9
  23508 bytes (23.0k)
local copy
libdeflate -12
  22670 bytes (22.1k)
local copy
7zip -mx=9 -tgzip
  22643 bytes (22.1k)
local copy
pigz -11 -n
  22603 bytes (22.1k)
local copy
kzip -s0 -rn -b2
  22600 bytes (22.1k)
local copy
Zopfli
  22584 bytes (22.1k)
local copy
cdnjs
  20261 bytes (19.8k)
CDN
cdnhttps
  20086 bytes (19.6k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.3.2.min.js

You will automatically get the smallest Socket.IO 1.3.2 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 --mls2048 --bsr11 --lazy --ohh

(found June 30, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 11  --bsr11
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/1.3.2/socket.io.min.js --location | md5sum
3e618f924c63851dce1372ddefc2c5ec  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.2.min.zopfli.js.gz | md5sum
3e618f924c63851dce1372ddefc2c5ec  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.3.2/socket.io.min.js --location | sha1sum
c894b0c8854b493d1b041ce22365ebcde837815f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.2.min.zopfli.js.gz | sha1sum
c894b0c8854b493d1b041ce22365ebcde837815f  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 23650 bytes 45dbaf4089495584b97f030ea958da2f < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< byteString+=encodeCodePoint(codePoint)}return byteString}f [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> });return n.call(t,e,o)};u.parse=function(t,e){var n,r;ret [...]
(invalid)
cdnjs 20261 bytes 45dbaf4089495584b97f030ea958da2f < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< byteString+=encodeCodePoint(codePoint)}return byteString}f [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> });return n.call(t,e,o)};u.parse=function(t,e){var n,r;ret [...]
(invalid)
cdnhttps 20086 bytes 45dbaf4089495584b97f030ea958da2f < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< byteString+=encodeCodePoint(codePoint)}return byteString}f [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> });return n.call(t,e,o)};u.parse=function(t,e){var n,r;ret [...]
(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
22584 bytes -1 byte zopfli --i100000 --mls2048 --bsr11 --lazy --ohh June 30, 2016 @ 09:44
22585 bytes -1 byte zopfli --i10000 --mls2048 --bsr11 --lazy --ohh June 30, 2016 @ 08:03
22586 bytes -3 bytes zopfli --i1000 --mls2048 --bsr11 --lazy --ohh June 29, 2016 @ 19:46
22589 bytes -1 byte zopfli --i10000 --mls512 --bsr18 --lazy --ohh June 29, 2016 @ 17:05
22590 bytes -1 byte zopfli --i1000 --mls1024 --bsr15 --lazy --ohh June 29, 2016 @ 15:25
22591 bytes zopfli --i100 --mls1024 --bsr15 --lazy --ohh June 29, 2016 @ 12:29

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

Most recent activity on July 2, 2016 @ 23:31.

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
22593 22593 22594 22601 22603 22603 22592 22592 22591 22592 22594 22603 22606 22607 22605
22604 22618 22617 22620 22621 22599 22600 22600 22590 22594 22591 22607 22604 22605 22605
22591 22590 22590 22591 22592 22592 22591 22591 22591 22590 22589 22605 22606 22604 22606
22593 22597 22590 22594 22590 22592 22592 22592 22589 22589 22594 22604 22606 22605 22603
22591 22590 22589 22589 22590 22590 22588 22591 22590 22590 22594 22604 22606 22605 22605
22591 22591 22598 22598 22597 22599 22601 22591 22590 22589 22590 22603 22605 22605 22605
22593 22597 22599 22589 22590 22595 22589 22597 22591 22590 22594 22604 22605 22605 22606
22592 22587 22590 22592 22589 22596 22592 22597 22589 22593 22584 22604 22603 22605 22606
22594 22599 22599 22602 22600 22596 22599 22590 22590 22590 22589 22604 22602 22605 22604
22589 22590 22587 22591 22589 22591 22590 22591 22590 22590 22589 22603 22606 22605 22609
22590 22590 22589 22591 22593 22591 22589 22589 22590 22590 22590 22602 22603 22610 22609
22595 22588 22590 22590 22591 22592 22592 22593 22589 22588 22591 22604 22605 22605 22604
22588 22587 22599 22599 22591 22597 22596 22596 22591 22589 22591 22605 22603 22603 22605
22590 22590 22588 22589 22590 22590 22589 22590 22590 22590 22591 22604 22605 22605 22609
22590 22589 22590 22589 22590 22590 22589 22589 22589 22589 22590 22605 22604 22605 22604
22590 22590 22588 22588 22591 22590 22589 22591 22590 22590 22591 22603 22605 22607 22605
22589 22598 22598 22599 22602 22598 22597 22598 22590 22588 22590 22605 22605 22607 22604
22590 22590 22599 22600 22601 22598 22589 22590 22590 22588 22590 22604 22607 22607 22605
22589 22588 22588 22589 22589 22591 22590 22589 22590 22590 22590 22604 22603 22607 22607
22591 22590 22590 22589 22591 22590 22589 22589 22590 22589 22590 22604 22606 22605 22605
22590 22600 22599 22597 22600 22597 22599 22591 22591 22589 22590 22602 22607 22605 22606
22593 22594 22600 22598 22598 22597 22588 22589 22591 22589 22591 22604 22605 22605 22606
22589 22599 22598 22590 22588 22590 22590 22591 22590 22591 22591 22602 22606 22604 22603

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 22591 bytes 100%
1,000 22586 bytes -5 bytes 100%
10,000 22585 bytes -1 byte 100%
100,000 22584 bytes -1 byte 0.29%
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
22608 bytes +24 bytes (+0.11%) +8 bytes
22623 bytes +39 bytes (+0.17%) +23 bytes
22600 bytes +16 bytes (+0.07%)
22654 bytes +70 bytes (+0.31%) +54 bytes
22685 bytes +101 bytes (+0.45%) +85 bytes
22716 bytes +132 bytes (+0.58%) +116 bytes
22748 bytes +164 bytes (+0.73%) +148 bytes
22773 bytes +189 bytes (+0.84%) +173 bytes
22781 bytes +197 bytes (+0.87%) +181 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 18266 bytes -4318 bytes (-19.12%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 19273 bytes -3311 bytes (-14.66%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 20142 bytes -2442 bytes (-10.81%)
Brotli (Wikipedia) brotli brotli -q 11 21334 bytes -1250 bytes (-5.53%)
LZMA2 (Wikipedia) xz xz -9 22056 bytes -528 bytes (-2.34%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 22247 bytes -337 bytes (-1.49%)
ZSTD (Wikipedia) zstd zstd -19 22385 bytes -199 bytes (-0.88%)

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