Choose a version:
39% The original file has 105970 bytes (103.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 40868 bytes (39.9k, 39%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  12417 bytes (12.1k)
 local copy
Boot
  12381 bytes (12.1k)
 CDN
cdnjs
  12381 bytes (12.1k)
 CDN
gzip -9
  12370 bytes (12.1k)
 local copy
7zip -mx=9 -tgzip
  11920 bytes (11.6k)
 local copy
libdeflate -12
  11920 bytes (11.6k)
 local copy
zultra
  11916 bytes (11.6k)
 local copy
kzip -s0 -rn -b2
  11912 bytes (11.6k)
 local copy
pigz -11 -n
  11900 bytes (11.6k)
 local copy
Zopfli
  11888 bytes (11.6k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.3.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.3.2 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 --mls256 --bsr16 --lazy --ohh

(found October 25, 2021)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 256  --mls256
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/4.3.2/socket.io.min.js --location | md5sum
5cbe62664e9305f8871de91a6d8d9a08  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.3.2.min.zopfli.js.gz | md5sum
5cbe62664e9305f8871de91a6d8d9a08  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.3.2/socket.io.min.js --location | sha1sum
2afe29828d4ecd270162fd7d636e5e7d3ae4d64e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.3.2.min.zopfli.js.gz | sha1sum
2afe29828d4ecd270162fd7d636e5e7d3ae4d64e  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 12381 bytes 5cbe62664e9305f8871de91a6d8d9a08 (invalid)
cdnjs 12381 bytes 5cbe62664e9305f8871de91a6d8d9a08 (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
11888 bytes -1 byte zopfli --i10000 --mls256 --bsr16 --lazy --ohh October 25, 2021 @ 12:46
11889 bytes -1 byte zopfli --i1000 --mls256 --bsr16 --lazy --ohh October 25, 2021 @ 12:41
11890 bytes -2 bytes zopfli --i1000 --mls2 --bsr2 --lazy --ohh October 25, 2021 @ 12:40
11892 bytes – zopfli --i100 --mls16 --bsr14 --lazy --ohh October 25, 2021 @ 12:09

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

Most recent activity on October 25, 2021 @ 18:09.

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 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 11889 11901 11897 11889 11898 11897 11899 11896 11896 11896 11896 11901 11903 11899 11905
4 11894 11897 11893 11893 11889 11900 11891 11890 11896 11890 11889 11897 11889 11890 11890
6 11890 11891 11890 11889 11891 11889 11889 11889 11890 11889 11892 11896 11889 11890 11890
7 11889 11889 11889 11889 11891 11898 11889 11889 11889 11890 11889 11897 11889 11890 11889
8 11889 11889 11889 11890 11890 11890 11889 11890 11889 11897 11894 11899 11896 11889 11890
9 11889 11889 11890 11892 11899 11899 11891 11890 11896 11889 11889 11897 11896 11890 11889
10 11888 11889 11889 11889 11896 11890 11900 11889 11889 11897 11893 11898 11889 11890 11890
11 11890 11890 11890 11890 11900 11898 11889 11889 11893 11896 11896 11893 11896 11890 11889
12 11889 11889 11889 11896 11889 11899 11899 11890 11893 11897 11895 11899 11889 11890 11890
13 11889 11899 11901 11900 11900 11899 11899 11899 11895 11895 11892 11897 11896 11890 11889
14 11889 11889 11889 11889 11897 11889 11890 11891 11889 11889 11889 11898 11889 11891 11890
15 11889 11897 11892 11899 11899 11899 11899 11899 11893 11897 11892 11896 11899 11891 11890
16 11889 11889 11890 11890 11889 11889 11889 11888 11889 11889 11891 11899 11889 11889 11890
17 11901 11896 11893 11899 11899 11900 11899 11898 11893 11896 11891 11892 11901 11890 11903
18 11896 11889 11890 11889 11900 11893 11900 11890 11899 11897 11897 11893 11900 11891 11890
19 11897 11900 11894 11899 11899 11900 11891 11891 11897 11889 11889 11896 11901 11890 11889
20 11896 11904 11893 11892 11890 11899 11899 11889 11893 11895 11895 11896 11901 11890 11900
21 11891 11889 11889 11890 11891 11898 11889 11890 11889 11889 11889 11891 11901 11890 11890
22 11897 11896 11894 11899 11899 11898 11898 11889 11893 11897 11895 11891 11901 11898 11890
23 11889 11890 11889 11889 11900 11898 11889 11889 11896 11897 11896 11896 11899 11890 11889
25 11889 11889 11889 11898 11900 11900 11899 11899 11893 11897 11896 11896 11899 11898 11889
30 11889 11889 11889 11889 11890 11889 11889 11899 11896 11895 11895 11896 11901 11889 11890
40 11889 11889 11896 11889 11897 11889 11889 11889 11893 11897 11891 11899 11899 11890 11890

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 11892 bytes – 100%
1,000 11889 bytes -3 bytes 100%
10,000 11888 bytes -1 byte 100%
100,000 11888 bytes – 0.58%
1,000,000 11888 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) 11961 bytes +73 bytes (+0.61%) +49 bytes
1 11964 bytes +76 bytes (+0.64%) +52 bytes
2 11912 bytes +24 bytes (+0.20%) –
3 11927 bytes +39 bytes (+0.33%) +15 bytes
4 11975 bytes +87 bytes (+0.73%) +63 bytes
5 12002 bytes +114 bytes (+0.96%) +90 bytes
6 12030 bytes +142 bytes (+1.19%) +118 bytes
7 12043 bytes +155 bytes (+1.30%) +131 bytes
8 12074 bytes +186 bytes (+1.56%) +162 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 10174 bytes -1714 bytes (-14.42%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 10597 bytes -1291 bytes (-10.86%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 10981 bytes -907 bytes (-7.63%)
Brotli (Wikipedia) brotli brotli -q 11 11163 bytes -725 bytes (-6.10%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 11628 bytes -260 bytes (-2.19%)
LZMA2 (Wikipedia) xz xz -9 11844 bytes -44 bytes (-0.37%)
Zstandard (Wikipedia) zstd zstd -19 11878 bytes -10 bytes (-0.08%)

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.