Choose a version:
53% The original file has 173978 bytes (169.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 91544 bytes (89.4k, 53%).

After GZIP compression these minified files vary in size:
Boot
  23748 bytes (23.2k)
CDN
gzip -6 (default)
  23690 bytes (23.1k)
local copy
gzip -9
  23585 bytes (23.0k)
local copy
libdeflate -12
  22776 bytes (22.2k)
local copy
7zip -mx=9 -tgzip
  22710 bytes (22.2k)
local copy
kzip -s0 -rn -b0
  22670 bytes (22.1k)
local copy
pigz -11 -n
  22670 bytes (22.1k)
local copy
Zopfli
  22653 bytes (22.1k)
local copy
cdnjs
  20325 bytes (19.8k)
CDN
cdnhttps
  20148 bytes (19.7k)
CDN

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.3.4.min.js

You will automatically get the smallest Socket.IO 1.3.4 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 --mls512 --bsr16 --lazy --ohh

(found June 30, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 512  --mls512
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/1.3.4/socket.io.min.js --location | md5sum
aac1813ca2d75f46874b401ea60e7ff0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.4.min.zopfli.js.gz | md5sum
aac1813ca2d75f46874b401ea60e7ff0  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.3.4/socket.io.min.js --location | sha1sum
e01d1f0baa8c58d38724d7b32beefaeeb15007c7  -
curl --silent --compressed http://minime.stephan-brumme.com/files/socket.io/socket.io-1.3.4.min.zopfli.js.gz | sha1sum
e01d1f0baa8c58d38724d7b32beefaeeb15007c7  -

CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Boot 23748 bytes 5e4f2832183a08a8d609d4fe401a0c4d < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< output+=stringFromCharCode(value>>>10&1023|55296);value=56 [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> if("null"==i.slice(P,P+4))return P+=4,null;q()}return"$"}, [...]
(invalid)
cdnjs 20325 bytes 5e4f2832183a08a8d609d4fe401a0c4d < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< output+=stringFromCharCode(value>>>10&1023|55296);value=56 [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> if("null"==i.slice(P,P+4))return P+=4,null;q()}return"$"}, [...]
(invalid)
cdnhttps 20148 bytes 5e4f2832183a08a8d609d4fe401a0c4d < !function(e){if("object"==typeof exports&&"undefined"!=typ [...]
< this.query=this.query||{};if(!callbacks){if(!global.___eio [...]
< output+=stringFromCharCode(value>>>10&1023|55296);value=56 [...]
> !function(t){if("object"==typeof exports&&"undefined"!=typ [...]
> (!c||e)&&(this.supportsBinary=!1),r.call(this,t)}var r=t(" [...]
> if("null"==i.slice(P,P+4))return P+=4,null;q()}return"$"}, [...]
(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
22653 bytes -1 byte zopfli --i100000 --mls512 --bsr16 --lazy --ohh June 30, 2016 @ 16:46
22654 bytes -1 byte zopfli --i1000 --mls256 --bsr6 --lazy --ohh June 29, 2016 @ 16:46
22655 bytes -1 byte zopfli --i1000 --mls1024 --bsr17 --lazy --ohh June 29, 2016 @ 16:08
22656 bytes -1 byte zopfli --i1000 --mls128 --bsr14 --lazy --ohh June 29, 2016 @ 16:07
22657 bytes -3 bytes zopfli --i1000 --mls256 --bsr17 --lazy --ohh June 29, 2016 @ 15:24
22660 bytes zopfli --i100 --mls128 --bsr23 --lazy --ohh June 29, 2016 @ 13:57

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 @ 07:46.

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
22660 22660 22660 22660 22660 22667 22659 22659 22662 22663 22671 22672 22672 22671 22669
22684 22682 22682 22684 22686 22688 22665 22664 22657 22657 22675 22670 22670 22671 22671
22656 22656 22657 22658 22656 22657 22656 22654 22655 22656 22656 22670 22671 22671 22670
22659 22656 22656 22657 22657 22656 22655 22656 22653 22655 22660 22669 22669 22672 22670
22656 22656 22656 22657 22656 22657 22656 22655 22658 22655 22672 22672 22670 22672 22671
22661 22661 22656 22656 22663 22664 22663 22655 22656 22657 22655 22669 22670 22671 22670
22655 22656 22663 22664 22662 22664 22662 22662 22656 22660 22660 22673 22671 22670 22671
22660 22660 22656 22664 22655 22664 22664 22662 22656 22655 22658 22672 22671 22671 22671
22655 22666 22664 22663 22656 22665 22656 22656 22656 22655 22658 22672 22669 22671 22670
22656 22656 22656 22656 22656 22657 22656 22655 22657 22660 22661 22670 22671 22671 22669
22656 22655 22656 22656 22665 22655 22655 22655 22656 22659 22656 22669 22670 22670 22674
22656 22657 22659 22657 22657 22657 22660 22657 22659 22655 22662 22670 22670 22672 22670
22661 22660 22662 22657 22662 22664 22662 22662 22653 22655 22658 22670 22670 22670 22671
22661 22656 22656 22656 22658 22657 22655 22656 22655 22654 22662 22670 22670 22671 22669
22661 22655 22656 22656 22657 22658 22657 22657 22656 22659 22656 22670 22670 22671 22670
22660 22656 22664 22656 22659 22657 22656 22656 22657 22655 22657 22671 22670 22672 22670
22667 22663 22663 22664 22662 22663 22663 22656 22656 22655 22656 22672 22670 22672 22671
22660 22655 22663 22663 22657 22657 22656 22656 22656 22655 22663 22671 22671 22671 22674
22661 22655 22656 22655 22660 22660 22655 22656 22656 22655 22671 22669 22670 22672 22670
22659 22656 22656 22657 22656 22656 22656 22656 22655 22655 22656 22669 22670 22671 22670
22660 22656 22665 22664 22658 22664 22656 22656 22654 22655 22656 22669 22670 22671 22670
22661 22661 22664 22663 22656 22658 22655 22656 22659 22656 22662 22672 22671 22671 22670
22662 22657 22657 22657 22657 22656 22656 22656 22658 22657 22656 22671 22671 22671 22671

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 22660 bytes 100%
1,000 22654 bytes -6 bytes 100%
10,000 22654 bytes 100%
100,000 22653 bytes -1 byte 1.45%
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
22670 bytes +17 bytes (+0.08%)
22693 bytes +40 bytes (+0.18%) +23 bytes
22671 bytes +18 bytes (+0.08%) +1 byte
22723 bytes +70 bytes (+0.31%) +53 bytes
22749 bytes +96 bytes (+0.42%) +79 bytes
22777 bytes +124 bytes (+0.55%) +107 bytes
22813 bytes +160 bytes (+0.71%) +143 bytes
22843 bytes +190 bytes (+0.84%) +173 bytes
22843 bytes +190 bytes (+0.84%) +173 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 18318 bytes -4335 bytes (-19.14%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 19318 bytes -3335 bytes (-14.72%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 20279 bytes -2374 bytes (-10.48%)
Brotli (Wikipedia) brotli brotli -q 11 21409 bytes -1244 bytes (-5.49%)
LZMA2 (Wikipedia) xz xz -9 22116 bytes -537 bytes (-2.37%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 22320 bytes -333 bytes (-1.47%)
ZSTD (Wikipedia) zstd zstd -19 22446 bytes -207 bytes (-0.91%)

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