Choose a version:
35% The original file has 188407 bytes (184.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 66064 bytes (64.5k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15552 bytes (15.2k)
 local copy
gzip -9
  15470 bytes (15.1k)
 local copy
Boot
  15465 bytes (15.1k)
 CDN
cdnjs
  15465 bytes (15.1k)
 CDN
7zip -mx=9 -tgzip
  15025 bytes (14.7k)
 local copy
zultra
  14972 bytes (14.6k)
 local copy
libdeflate -12
  14964 bytes (14.6k)
 local copy
kzip -s0 -rn -b2
  14934 bytes (14.6k)
 local copy
pigz -11 -n
  14925 bytes (14.6k)
 local copy
Zopfli
  14916 bytes (14.6k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.2.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.2.0 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 --i100000 --mb8 --mls2048 --bsr19 --lazy --ohh

(found August 30, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 19  --bsr19
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/4.2.0/socket.io.min.js --location | md5sum
7cd95f2446c1bf91b7182bb7b0ec8712  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.2.0.min.zopfli.js.gz | md5sum
7cd95f2446c1bf91b7182bb7b0ec8712  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.2.0/socket.io.min.js --location | sha1sum
af6fa41ee6217eb3d5bb3aef7b7e9c32546b6b68  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.2.0.min.zopfli.js.gz | sha1sum
af6fa41ee6217eb3d5bb3aef7b7e9c32546b6b68  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15465 bytes 7cd95f2446c1bf91b7182bb7b0ec8712 (invalid)
cdnjs 15465 bytes 7cd95f2446c1bf91b7182bb7b0ec8712 (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
14916 bytes -1 byte zopfli --i100000 --mls2048 --bsr19 --lazy --ohh August 30, 2021 @ 16:30
14917 bytes -1 byte zopfli --i1000 --mls16384 --bsr12 --lazy --ohh August 30, 2021 @ 14:27
14918 bytes -1 byte zopfli --i1000 --mls32768 --bsr19 --lazy --ohh August 30, 2021 @ 14:26
14919 bytes – zopfli --i100 --mls32768 --bsr40 --lazy --ohh August 30, 2021 @ 12:48

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

Most recent activity on September 3, 2021 @ 01:38.

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 14922 14922 14922 14923 14923 14923 14923 14923 14923 14919 14919 14920 14919 14923 14922
4 14920 14920 14920 14919 14918 14918 14918 14918 14918 14916 14922 14917 14919 14919 14918
6 14934 14934 14918 14919 14918 14918 14921 14919 14916 14916 14918 14919 14920 14919 14918
7 14918 14918 14916 14919 14918 14918 14916 14918 14917 14916 14917 14918 14920 14919 14916
8 14918 14918 14916 14918 14918 14919 14920 14916 14919 14919 14916 14920 14919 14916 14918
9 14918 14918 14916 14919 14918 14918 14921 14918 14919 14919 14916 14919 14919 14932 14933
10 14919 14919 14919 14919 14919 14918 14921 14919 14918 14919 14916 14919 14919 14918 14919
11 14918 14918 14919 14919 14917 14920 14921 14918 14918 14918 14917 14919 14918 14916 14916
12 14918 14918 14923 14919 14918 14918 14919 14919 14917 14919 14916 14918 14919 14916 14919
13 14918 14918 14916 14919 14919 14918 14920 14916 14916 14916 14916 14919 14920 14916 14918
14 14918 14918 14916 14919 14919 14918 14916 14918 14919 14919 14916 14916 14916 14932 14918
15 14918 14918 14916 14919 14919 14918 14916 14918 14917 14919 14918 14919 14919 14919 14918
16 14918 14918 14919 14919 14918 14918 14919 14918 14918 14919 14918 14919 14919 14933 14919
17 14918 14918 14919 14919 14918 14918 14921 14921 14918 14918 14916 14919 14920 14918 14919
18 14918 14918 14916 14919 14919 14918 14920 14919 14916 14919 14916 14919 14916 14916 14916
19 14919 14919 14916 14919 14918 14918 14921 14918 14918 14916 14916 14919 14919 14932 14916
20 14918 14918 14918 14919 14918 14918 14916 14918 14916 14919 14918 14919 14919 14932 14918
21 14918 14918 14916 14919 14918 14918 14918 14918 14917 14919 14916 14919 14916 14918 14916
22 14931 14931 14918 14919 14918 14918 14921 14918 14917 14919 14916 14919 14919 14916 14918
23 14918 14918 14918 14919 14919 14918 14918 14918 14916 14919 14918 14919 14919 14942 14918
25 14918 14918 14916 14919 14918 14918 14921 14918 14917 14916 14918 14919 14919 14933 14919
30 14918 14918 14916 14919 14918 14918 14921 14918 14917 14919 14916 14918 14919 14916 14918
40 14919 14919 14916 14918 14918 14918 14920 14918 14918 14916 14918 14916 14918 14916 14919

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 14919 bytes – 100%
1,000 14917 bytes -2 bytes 100%
10,000 14917 bytes – 100%
100,000 14916 bytes -1 byte 20.58%
1,000,000 14916 bytes – 17.39%
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) 14972 bytes +56 bytes (+0.38%) +38 bytes
1 14970 bytes +54 bytes (+0.36%) +36 bytes
2 14934 bytes +18 bytes (+0.12%) –
3 14952 bytes +36 bytes (+0.24%) +18 bytes
4 14989 bytes +73 bytes (+0.49%) +55 bytes
5 14997 bytes +81 bytes (+0.54%) +63 bytes
6 15016 bytes +100 bytes (+0.67%) +82 bytes
7 15063 bytes +147 bytes (+0.99%) +129 bytes
8 15090 bytes +174 bytes (+1.17%) +156 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 12600 bytes -2316 bytes (-15.53%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12794 bytes -2122 bytes (-14.23%)
Brotli (Wikipedia) brotli brotli -q 11 13518 bytes -1398 bytes (-9.37%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14001 bytes -915 bytes (-6.13%)
LZMA2 (Wikipedia) xz xz -9 14324 bytes -592 bytes (-3.97%)
Zstandard (Wikipedia) zstd zstd -19 14447 bytes -469 bytes (-3.14%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14565 bytes -351 bytes (-2.35%)

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.