Choose a version:
39% The original file has 105718 bytes (103.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 40817 bytes (39.9k, 39%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  12384 bytes (12.1k)
 local copy
Boot
  12349 bytes (12.1k)
 CDN
cdnjs
  12349 bytes (12.1k)
 CDN
gzip -9
  12336 bytes (12.0k)
 local copy
libdeflate -12
  11888 bytes (11.6k)
 local copy
7zip -mx=9 -tgzip
  11883 bytes (11.6k)
 local copy
zultra
  11880 bytes (11.6k)
 local copy
kzip -s0 -rn -b2
  11876 bytes (11.6k)
 local copy
pigz -11 -n
  11872 bytes (11.6k)
 local copy
Zopfli
  11855 bytes (11.6k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.3.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.3.0 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 --i10000 --mb8 --mls8192 --bsr16 --lazy --ohh

(found October 21, 2021)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
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/4.3.0/socket.io.min.js --location | md5sum
5f75a5b9bebd4b3d7cbca8f1bfbf4171  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.3.0.min.zopfli.js.gz | md5sum
5f75a5b9bebd4b3d7cbca8f1bfbf4171  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.3.0/socket.io.min.js --location | sha1sum
77f4cd806a6734b56b8e447d7e295cd228704aa8  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.3.0.min.zopfli.js.gz | sha1sum
77f4cd806a6734b56b8e447d7e295cd228704aa8  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 12349 bytes 5f75a5b9bebd4b3d7cbca8f1bfbf4171 (invalid)
cdnjs 12349 bytes 5f75a5b9bebd4b3d7cbca8f1bfbf4171 (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
11855 bytes -1 byte zopfli --i10000 --mls8192 --bsr16 --lazy --ohh October 21, 2021 @ 13:09
11856 bytes -1 byte zopfli --i1000 --mls8192 --bsr16 --lazy --ohh October 21, 2021 @ 13:08
11857 bytes -1 byte zopfli --i1000 --mls512 --bsr8 --lazy --ohh October 21, 2021 @ 13:07
11858 bytes -1 byte zopfli --i1000 --mls16 --bsr6 --lazy --ohh October 21, 2021 @ 13:07
11859 bytes – zopfli --i100 --mls512 --bsr8 --lazy --ohh October 21, 2021 @ 13:04

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

Most recent activity on October 22, 2021 @ 17:27.

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 11856 11864 11855 11855 11868 11865 11866 11865 11865 11865 11864 11868 11871 11867 11869
4 11856 11865 11864 11869 11858 11868 11867 11859 11861 11864 11857 11864 11858 11857 11859
6 11858 11856 11857 11857 11858 11859 11857 11857 11861 11857 11863 11860 11856 11865 11857
7 11856 11856 11856 11859 11867 11867 11857 11857 11856 11857 11856 11865 11855 11855 11857
8 11856 11857 11857 11867 11857 11867 11857 11857 11857 11857 11865 11857 11856 11864 11858
9 11856 11856 11856 11870 11867 11867 11864 11857 11861 11864 11857 11865 11867 11857 11858
10 11856 11856 11857 11858 11857 11857 11870 11865 11860 11864 11864 11865 11867 11864 11857
11 11857 11857 11857 11866 11867 11867 11858 11868 11861 11864 11860 11863 11866 11857 11857
12 11856 11856 11857 11867 11864 11868 11865 11865 11862 11864 11864 11860 11857 11857 11858
13 11855 11860 11859 11869 11866 11867 11866 11866 11861 11864 11864 11865 11867 11857 11855
14 11855 11856 11868 11858 11857 11857 11858 11857 11861 11857 11857 11866 11856 11857 11857
15 11856 11860 11864 11867 11865 11867 11865 11866 11861 11864 11864 11865 11865 11857 11859
16 11857 11857 11857 11859 11857 11857 11858 11857 11864 11857 11863 11865 11855 11857 11858
17 11867 11863 11868 11867 11867 11867 11865 11865 11862 11864 11865 11865 11866 11857 11871
18 11870 11856 11857 11858 11858 11857 11867 11865 11862 11864 11863 11865 11866 11857 11857
19 11869 11865 11865 11867 11866 11867 11866 11857 11862 11864 11857 11865 11867 11857 11858
20 11869 11856 11865 11859 11857 11867 11865 11868 11866 11863 11863 11866 11864 11857 11871
21 11857 11856 11856 11859 11858 11867 11857 11856 11862 11863 11857 11865 11855 11857 11858
22 11864 11860 11864 11866 11857 11869 11865 11866 11864 11864 11863 11866 11868 11865 11858
23 11856 11856 11857 11867 11867 11868 11858 11866 11864 11863 11863 11865 11867 11855 11855
25 11857 11857 11857 11868 11867 11867 11866 11865 11862 11864 11863 11865 11867 11865 11858
30 11856 11856 11857 11858 11868 11857 11857 11865 11861 11864 11864 11865 11866 11857 11858
40 11857 11857 11857 11864 11864 11857 11857 11857 11861 11865 11865 11865 11867 11857 11857

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 11859 bytes – 100%
1,000 11856 bytes -3 bytes 100%
10,000 11855 bytes -1 byte 100%
100,000 11855 bytes – 11.88%
1,000,000 11855 bytes – 3.19%
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) 11927 bytes +72 bytes (+0.61%) +51 bytes
1 11927 bytes +72 bytes (+0.61%) +51 bytes
2 11876 bytes +21 bytes (+0.18%) –
3 11892 bytes +37 bytes (+0.31%) +16 bytes
4 11941 bytes +86 bytes (+0.73%) +65 bytes
5 11970 bytes +115 bytes (+0.97%) +94 bytes
6 11999 bytes +144 bytes (+1.21%) +123 bytes
7 12011 bytes +156 bytes (+1.32%) +135 bytes
8 12040 bytes +185 bytes (+1.56%) +164 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 10143 bytes -1712 bytes (-14.44%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 10565 bytes -1290 bytes (-10.88%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 10951 bytes -904 bytes (-7.63%)
Brotli (Wikipedia) brotli brotli -q 11 11131 bytes -724 bytes (-6.11%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 11607 bytes -248 bytes (-2.09%)
LZMA2 (Wikipedia) xz xz -9 11812 bytes -43 bytes (-0.36%)
Zstandard (Wikipedia) zstd zstd -19 11848 bytes -7 bytes (-0.06%)

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.