Choose a version:
52% The original file has 151573 bytes (148.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 78682 bytes (76.8k, 52%).

After GZIP compression these minified files vary in size:
Boot
  24785 bytes (24.2k)
 CDN
cdnjs
  20725 bytes (20.2k)
 CDN
gzip -6 (default)
  20560 bytes (20.1k)
 local copy
gzip -9
  20484 bytes (20.0k)
 local copy
libdeflate -12
  19801 bytes (19.3k)
 local copy
7zip -mx=9 -tgzip
  19759 bytes (19.3k)
 local copy
zultra
  19739 bytes (19.3k)
 local copy
pigz -11 -n
  19729 bytes (19.3k)
 local copy
kzip -s0 -rn -b2
  19715 bytes (19.3k)
 local copy
Zopfli
  19695 bytes (19.2k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.0.4.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.0.4 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 --mls64 --bsr7 --lazy --ohh

(found June 29, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 64  --mls64
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/1.0.4/socket.io.min.js --location | md5sum
3885f948b7c10f0c35d719f9e92c7ef6  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.4.min.zopfli.js.gz | md5sum
3885f948b7c10f0c35d719f9e92c7ef6  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.0.4/socket.io.min.js --location | sha1sum
cfde03455917c7f995cc2ec280e8bb6361eceb9b  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.4.min.zopfli.js.gz | sha1sum
cfde03455917c7f995cc2ec280e8bb6361eceb9b  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 24785 bytes 3885f948b7c10f0c35d719f9e92c7ef6 (invalid)
cdnjs 20725 bytes 3885f948b7c10f0c35d719f9e92c7ef6 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available Socket.IO versions at minime.stephan-brumme.com:

4.5.4, 4.5.3, 4.5.2, 4.5.1, 4.5.0, 4.4.1, 4.4.0, 4.3.2, 4.3.1, 4.3.0, 4.2.0, 4.1.3, 4.1.2, 4.1.1, 4.1.0, 4.0.2, 4.0.1, 4.0.0,
3.1.2, 3.1.1, 3.1.0, 3.0.5, 3.0.4, 3.0.3, 3.0.2, 3.0.1, 3.0.0,
2.4.0, 2.3.1, 2.3.0, 2.2.0, 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
19695 bytes -2 bytes zopfli --i100000 --mls64 --bsr7 --lazy --ohh June 29, 2016 @ 16:44
19697 bytes -2 bytes zopfli --i10000 --mls64 --bsr7 --lazy --ohh June 29, 2016 @ 15:39
19699 bytes -3 bytes zopfli --i1000 --mls64 --bsr7 --lazy --ohh June 29, 2016 @ 15:23
19702 bytes – zopfli --i100 --mls64 --bsr7 --lazy --ohh June 29, 2016 @ 13:34

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

Most recent activity on July 20, 2020 @ 12:49.

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
2 19708 19707 19707 19708 19706 19707 19719 19706 19718 19718 19722 19719 19718 19712 19711
4 19705 19705 19707 19706 19707 19709 19722 19708 19721 19723 19721 19720 19719 19711 19721
6 19709 19709 19712 19713 19709 19709 19722 19709 19711 19719 19726 19711 19711 19715 19719
7 19709 19711 19709 19710 19709 19695 19720 19709 19711 19711 19722 19711 19712 19714 19715
8 19709 19704 19706 19706 19708 19706 19720 19709 19720 19720 19721 19719 19718 19711 19711
9 19709 19709 19716 19709 19710 19716 19718 19720 19718 19719 19721 19712 19711 19711 19711
10 19708 19709 19709 19706 19710 19713 19719 19711 19712 19713 19714 19712 19702 19711 19711
11 19710 19704 19709 19706 19709 19708 19721 19707 19720 19720 19721 19722 19711 19711 19711
12 19709 19708 19708 19707 19705 19712 19720 19709 19718 19722 19721 19722 19711 19718 19727
13 19709 19707 19706 19706 19708 19708 19720 19720 19711 19719 19721 19711 19718 19711 19714
14 19709 19708 19708 19709 19708 19707 19719 19706 19711 19725 19721 19711 19711 19711 19714
15 19709 19710 19710 19710 19710 19699 19719 19708 19718 19719 19722 19711 19711 19711 19714
16 19709 19707 19704 19707 19709 19708 19720 19710 19718 19719 19721 19722 19711 19711 19711
17 19709 19709 19707 19708 19708 19708 19720 19707 19720 19719 19721 19714 19719 19711 19711
18 19709 19707 19714 19709 19710 19712 19719 19719 19719 19718 19722 19719 19711 19720 19711
19 19707 19708 19710 19705 19708 19709 19719 19710 19710 19719 19721 19711 19711 19714 19712
20 19708 19709 19709 19706 19710 19708 19721 19709 19704 19720 19721 19720 19711 19711 19711
21 19709 19705 19708 19707 19711 19709 19721 19707 19705 19722 19722 19711 19720 19711 19715
22 19708 19708 19710 19707 19708 19709 19721 19709 19705 19719 19721 19714 19711 19714 19711
23 19709 19708 19710 19705 19710 19699 19719 19707 19710 19719 19721 19711 19711 19711 19711
25 19708 19706 19709 19705 19708 19708 19720 19710 19705 19719 19721 19721 19719 19714 19711
30 19709 19709 19708 19706 19709 19708 19721 19710 19720 19719 19722 19722 19711 19711 19711
40 19709 19705 19707 19708 19709 19708 19721 19709 19717 19719 19721 19721 19711 19711 19711

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 19702 bytes – 100%
1,000 19699 bytes -3 bytes 100%
10,000 19697 bytes -2 bytes 100%
100,000 19695 bytes -2 bytes 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
(any) 19720 bytes +25 bytes (+0.13%) +5 bytes
1 19736 bytes +41 bytes (+0.21%) +21 bytes
2 19715 bytes +20 bytes (+0.10%) –
3 19759 bytes +64 bytes (+0.32%) +44 bytes
4 19788 bytes +93 bytes (+0.47%) +73 bytes
5 19823 bytes +128 bytes (+0.65%) +108 bytes
6 19858 bytes +163 bytes (+0.83%) +143 bytes
7 19856 bytes +161 bytes (+0.82%) +141 bytes
8 19896 bytes +201 bytes (+1.02%) +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.
However, support for Brotli is constantly growing - for example, your browser actually supports it !
Algorithm Program Parameters Size Compared To Best Zopfli
ZPAQ (Wikipedia) zpaq zpaq -method 69 16108 bytes -3587 bytes (-18.21%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 16832 bytes -2863 bytes (-14.54%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 17563 bytes -2132 bytes (-10.83%)
Brotli (Wikipedia) brotli brotli -q 11 18646 bytes -1049 bytes (-5.33%)
LZMA2 (Wikipedia) xz xz -9 19312 bytes -383 bytes (-1.94%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19366 bytes -329 bytes (-1.67%)
Zstandard (Wikipedia) zstd zstd -19 19590 bytes -105 bytes (-0.53%)

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):
Load and Parse Data


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 2026.
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

All trademarks are property of their respective owners. You know, the boring legal stuff.