Choose a version:
39% The original file has 106043 bytes (103.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 40916 bytes (40.0k, 39%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  12435 bytes (12.1k)
 local copy
gzip -9
  12388 bytes (12.1k)
 local copy
7zip -mx=9 -tgzip
  11938 bytes (11.7k)
 local copy
libdeflate -12
  11938 bytes (11.7k)
 local copy
zultra
  11935 bytes (11.7k)
 local copy
kzip -s0 -rn -b2
  11929 bytes (11.6k)
 local copy
pigz -11 -n
  11915 bytes (11.6k)
 local copy
Zopfli
  11906 bytes (11.6k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.3.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.3.1 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 --i1000 --mb8 --mls32768 --bsr15 --lazy --ohh

(found October 22, 2021)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 32768  --mls32768
block splitting recursion 15  --bsr15
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/4.3.1/socket.io.min.js --location | md5sum
c200e2cd564c6aa68ae19ec51b019b88  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.3.1.min.zopfli.js.gz | md5sum
c200e2cd564c6aa68ae19ec51b019b88  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.3.1/socket.io.min.js --location | sha1sum
0aadbeab2010938300797afaed4b5ef26d644268  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.3.1.min.zopfli.js.gz | sha1sum
0aadbeab2010938300797afaed4b5ef26d644268  -

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
11906 bytes -1 byte zopfli --i1000 --mls32768 --bsr15 --lazy --ohh October 22, 2021 @ 15:49
11907 bytes -1 byte zopfli --i1000 --mls2 --bsr15 --lazy --ohh October 22, 2021 @ 15:46
11908 bytes -1 byte zopfli --i1000 --mls8 --bsr10 --lazy --ohh October 22, 2021 @ 15:46
11909 bytes -2 bytes zopfli --i1000 --mls4 --bsr11 --lazy --ohh October 22, 2021 @ 15:46
11911 bytes – zopfli --i100 --mls128 --bsr40 --lazy --ohh October 22, 2021 @ 15:42

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

Most recent activity on October 23, 2021 @ 03:34.

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 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 11918 11917 11916 11918 11918 11916 11917 11916 11915 11916 11915 11919 11917 11917 11921
4 11912 11917 11915 11920 11908 11909 11911 11908 11915 11909 11909 11915 11918 11918 11921
6 11908 11908 11908 11908 11907 11907 11917 11908 11908 11907 11914 11915 11916 11908 11908
7 11906 11906 11908 11907 11908 11920 11907 11910 11907 11907 11916 11916 11907 11908 11906
8 11907 11908 11909 11908 11908 11918 11917 11907 11907 11915 11917 11908 11916 11908 11908
9 11917 11908 11908 11907 11919 11918 11907 11907 11907 11907 11907 11916 11916 11907 11920
10 11907 11908 11908 11908 11908 11907 11908 11907 11907 11915 11915 11917 11908 11908 11908
11 11907 11907 11908 11908 11919 11918 11919 11919 11919 11914 11915 11915 11915 11909 11908
12 11907 11906 11917 11907 11919 11918 11907 11907 11915 11907 11914 11916 11908 11907 11909
13 11906 11908 11915 11921 11919 11918 11919 11916 11918 11915 11915 11916 11915 11908 11908
14 11906 11906 11917 11908 11915 11918 11909 11910 11907 11909 11908 11907 11907 11909 11908
15 11906 11914 11922 11920 11920 11919 11917 11915 11916 11915 11914 11915 11917 11909 11906
16 11907 11907 11908 11907 11918 11918 11907 11907 11907 11907 11913 11915 11916 11908 11908
17 11918 11916 11921 11919 11917 11917 11917 11914 11918 11915 11916 11916 11915 11908 11908
18 11920 11907 11908 11908 11919 11917 11908 11906 11914 11915 11914 11915 11916 11908 11908
19 11906 11908 11913 11918 11920 11919 11909 11909 11914 11907 11906 11915 11916 11909 11908
20 11917 11920 11915 11908 11918 11918 11918 11915 11918 11915 11915 11916 11916 11916 11920
21 11909 11908 11908 11907 11918 11918 11907 11907 11907 11908 11914 11915 11915 11909 11907
22 11914 11917 11920 11918 11919 11918 11918 11907 11919 11915 11914 11909 11916 11917 11916
23 11906 11907 11908 11907 11919 11918 11917 11907 11918 11915 11915 11915 11916 11909 11908
25 11907 11907 11921 11920 11919 11919 11917 11916 11907 11915 11915 11915 11916 11916 11908
30 11906 11906 11908 11907 11919 11916 11908 11916 11915 11915 11915 11918 11917 11917 11908
40 11906 11907 11914 11907 11916 11917 11908 11907 11919 11915 11916 11917 11916 11908 11908

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 11911 bytes – 100%
1,000 11906 bytes -5 bytes 100%
10,000 11906 bytes – 100%
100,000 11906 bytes – 4.64%
1,000,000 11906 bytes – 4.64%
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) 11979 bytes +73 bytes (+0.61%) +50 bytes
1 11978 bytes +72 bytes (+0.60%) +49 bytes
2 11929 bytes +23 bytes (+0.19%) –
3 11946 bytes +40 bytes (+0.34%) +17 bytes
4 11993 bytes +87 bytes (+0.73%) +64 bytes
5 12024 bytes +118 bytes (+0.99%) +95 bytes
6 12051 bytes +145 bytes (+1.22%) +122 bytes
7 12064 bytes +158 bytes (+1.33%) +135 bytes
8 12094 bytes +188 bytes (+1.58%) +165 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 10181 bytes -1725 bytes (-14.49%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 10606 bytes -1300 bytes (-10.92%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 10999 bytes -907 bytes (-7.62%)
Brotli (Wikipedia) brotli brotli -q 11 11168 bytes -738 bytes (-6.20%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 11617 bytes -289 bytes (-2.43%)
LZMA2 (Wikipedia) xz xz -9 11856 bytes -50 bytes (-0.42%)
Zstandard (Wikipedia) zstd zstd -19 11891 bytes -15 bytes (-0.13%)

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.