Choose a version:
35% The original file has 185790 bytes (181.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 64504 bytes (63.0k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15311 bytes (15.0k)
 local copy
gzip -9
  15236 bytes (14.9k)
 local copy
Boot
  15219 bytes (14.9k)
 CDN
cdnjs
  15219 bytes (14.9k)
 CDN
7zip -mx=9 -tgzip
  14793 bytes (14.4k)
 local copy
libdeflate -12
  14772 bytes (14.4k)
 local copy
zultra
  14708 bytes (14.4k)
 local copy
kzip -s0 -rn -b2
  14699 bytes (14.4k)
 local copy
pigz -11 -n
  14696 bytes (14.4k)
 local copy
Zopfli
  14685 bytes (14.3k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.1.3.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.1.3 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 --mls16384 --bsr14 --lazy --ohh

(found July 13, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 16384  --mls16384
block splitting recursion 14  --bsr14
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/4.1.3/socket.io.min.js --location | md5sum
0d3d1c891c2b7a65c68738a66334c9ef  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.1.3.min.zopfli.js.gz | md5sum
0d3d1c891c2b7a65c68738a66334c9ef  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.1.3/socket.io.min.js --location | sha1sum
41ab03b10b8b9f01444e73bebc29923a420b4940  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.1.3.min.zopfli.js.gz | sha1sum
41ab03b10b8b9f01444e73bebc29923a420b4940  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15219 bytes 0d3d1c891c2b7a65c68738a66334c9ef (invalid)
cdnjs 15219 bytes 0d3d1c891c2b7a65c68738a66334c9ef (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
14685 bytes -2 bytes zopfli --i100000 --mls16384 --bsr14 --lazy --ohh July 13, 2021 @ 14:39
14687 bytes -1 byte zopfli --i10000 --mls16384 --bsr14 --lazy --ohh July 13, 2021 @ 13:15
14688 bytes -1 byte zopfli --i1000 --mls2 --bsr13 --lazy --ohh July 13, 2021 @ 12:52
14689 bytes – zopfli --i100 --mls16384 --bsr14 --lazy --ohh July 13, 2021 @ 12:50

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

Most recent activity on July 19, 2021 @ 14:09.

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, 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
2 14689 14689 14689 14690 14690 14688 14689 14686 14686 14690 14690 14691 14689 14686 14690
4 14688 14688 14688 14686 14688 14686 14686 14689 14691 14688 14689 14689 14689 14688 14705
6 14688 14688 14688 14686 14689 14689 14690 14686 14686 14686 14689 14688 14689 14709 14708
7 14688 14688 14689 14686 14689 14689 14686 14686 14686 14688 14689 14689 14689 14705 14705
8 14688 14688 14688 14686 14689 14689 14686 14691 14686 14690 14688 14689 14689 14706 14690
9 14688 14688 14686 14686 14686 14688 14690 14686 14690 14688 14690 14689 14706 14705 14686
10 14688 14688 14688 14686 14688 14689 14686 14686 14687 14686 14688 14688 14699 14707 14686
11 14689 14689 14688 14688 14686 14689 14687 14688 14686 14688 14689 14688 14689 14685 14705
12 14688 14688 14686 14686 14688 14689 14686 14686 14686 14686 14689 14686 14688 14689 14689
13 14688 14688 14688 14686 14686 14689 14687 14686 14686 14688 14686 14691 14688 14689 14689
14 14686 14686 14686 14688 14686 14686 14688 14686 14687 14688 14689 14689 14706 14685 14687
15 14688 14688 14689 14686 14689 14689 14689 14686 14689 14688 14686 14689 14689 14707 14707
16 14688 14688 14687 14686 14688 14686 14687 14686 14686 14688 14689 14690 14688 14705 14687
17 14688 14688 14688 14686 14688 14688 14686 14686 14686 14686 14686 14689 14689 14705 14705
18 14688 14688 14686 14688 14688 14688 14687 14687 14687 14688 14686 14688 14689 14705 14705
19 14688 14688 14687 14687 14688 14688 14687 14686 14687 14686 14686 14689 14689 14705 14686
20 14688 14688 14689 14689 14688 14689 14686 14688 14686 14686 14688 14688 14689 14689 14706
21 14689 14689 14688 14686 14688 14688 14687 14686 14686 14689 14686 14689 14688 14705 14705
22 14688 14688 14687 14689 14689 14688 14687 14686 14686 14688 14689 14688 14689 14690 14689
23 14689 14689 14687 14687 14688 14688 14686 14686 14686 14689 14689 14688 14689 14689 14707
25 14688 14688 14688 14687 14686 14686 14686 14686 14686 14686 14686 14689 14688 14707 14707
30 14688 14688 14686 14686 14686 14686 14686 14686 14686 14688 14686 14689 14706 14686 14689
40 14688 14688 14688 14687 14688 14689 14689 14686 14688 14688 14689 14690 14706 14705 14706

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 14689 bytes – 100%
1,000 14688 bytes -1 byte 100%
10,000 14687 bytes -1 byte 100%
100,000 14685 bytes -2 bytes 34.49%
1,000,000 14685 bytes – 0.58%
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) 14733 bytes +48 bytes (+0.33%) +34 bytes
1 14733 bytes +48 bytes (+0.33%) +34 bytes
2 14699 bytes +14 bytes (+0.10%) –
3 14716 bytes +31 bytes (+0.21%) +17 bytes
4 14753 bytes +68 bytes (+0.46%) +54 bytes
5 14763 bytes +78 bytes (+0.53%) +64 bytes
6 14783 bytes +98 bytes (+0.67%) +84 bytes
7 14834 bytes +149 bytes (+1.01%) +135 bytes
8 14860 bytes +175 bytes (+1.19%) +161 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
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12333 bytes -2352 bytes (-16.02%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12590 bytes -2095 bytes (-14.27%)
Brotli (Wikipedia) brotli brotli -q 11 13294 bytes -1391 bytes (-9.47%)
LZMA2 (Wikipedia) xz xz -9 14100 bytes -585 bytes (-3.98%)
Zstandard (Wikipedia) zstd zstd -19 14202 bytes -483 bytes (-3.29%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14337 bytes -348 bytes (-2.37%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15818 bytes +1133 bytes (+7.72%)

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.