Choose a version:
38% The original file has 107516 bytes (105.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 41370 bytes (40.4k, 38%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  12576 bytes (12.3k)
 local copy
Boot
  12544 bytes (12.3k)
 CDN
cdnjs
  12544 bytes (12.3k)
 CDN
gzip -9
  12532 bytes (12.2k)
 local copy
7zip -mx=9 -tgzip
  12072 bytes (11.8k)
 local copy
libdeflate -12
  12071 bytes (11.8k)
 local copy
zultra
  12066 bytes (11.8k)
 local copy
kzip -s0 -rn -b2
  12062 bytes (11.8k)
 local copy
pigz -11 -n
  12052 bytes (11.8k)
 local copy
Zopfli
  12040 bytes (11.8k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.4.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.4.1 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 --i1000000 --mb8 --mls16384 --bsr16 --lazy --ohh

(found January 18, 2022)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 16384  --mls16384
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/4.4.1/socket.io.min.js --location | md5sum
a51b4692d112bfa2f54ba01e7d42d0df  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.4.1.min.zopfli.js.gz | md5sum
a51b4692d112bfa2f54ba01e7d42d0df  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.4.1/socket.io.min.js --location | sha1sum
cd0a16d338ae78a347112816a2cac2d0e177bdcc  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.4.1.min.zopfli.js.gz | sha1sum
cd0a16d338ae78a347112816a2cac2d0e177bdcc  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 12544 bytes a51b4692d112bfa2f54ba01e7d42d0df (invalid)
cdnjs 12544 bytes a51b4692d112bfa2f54ba01e7d42d0df (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
12040 bytes -1 byte zopfli --i1000000 --mls16384 --bsr16 --lazy --ohh January 18, 2022 @ 15:41
12041 bytes -1 byte zopfli --i10000 --mls16384 --bsr16 --lazy --ohh January 17, 2022 @ 13:14
12042 bytes -1 byte zopfli --i10000 --mls8 --bsr14 --lazy --ohh January 17, 2022 @ 13:08
12043 bytes -1 byte zopfli --i1000 --mls16384 --bsr19 --lazy --ohh January 17, 2022 @ 12:18
12044 bytes -1 byte zopfli --i1000 --mls256 --bsr14 --lazy --ohh January 17, 2022 @ 12:17
12045 bytes -1 byte zopfli --i1000 --mls16384 --bsr9 --lazy --ohh January 17, 2022 @ 12:17
12046 bytes – zopfli --i100 --mls32768 --bsr40 --lazy --ohh January 17, 2022 @ 11:47

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

Most recent activity on January 18, 2022 @ 15:41.

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 12049 12050 12053 12049 12055 12048 12049 12049 12050 12049 12050 12053 12051 12055 12055
4 12045 12044 12044 12043 12042 12054 12042 12043 12042 12042 12044 12042 12044 12044 12057
6 12043 12051 12050 12052 12050 12050 12054 12042 12048 12050 12047 12050 12056 12043 12043
7 12042 12043 12042 12043 12052 12052 12052 12042 12052 12050 12044 12049 12056 12043 12043
8 12043 12042 12042 12043 12043 12052 12052 12042 12051 12049 12049 12043 12052 12050 12058
9 12051 12054 12043 12052 12055 12043 12043 12043 12044 12042 12044 12049 12042 12042 12052
10 12052 12052 12043 12042 12043 12051 12043 12042 12047 12048 12050 12043 12043 12043 12042
11 12053 12052 12042 12052 12043 12043 12052 12042 12050 12049 12048 12049 12053 12042 12043
12 12043 12052 12051 12052 12051 12051 12052 12051 12051 12043 12044 12042 12044 12043 12043
13 12052 12052 12051 12052 12053 12051 12050 12051 12047 12050 12048 12049 12056 12042 12042
14 12054 12043 12042 12042 12052 12052 12042 12042 12044 12042 12044 12043 12054 12057 12057
15 12052 12052 12051 12052 12054 12051 12052 12051 12050 12050 12048 12048 12054 12043 12044
16 12042 12041 12043 12043 12053 12053 12043 12042 12043 12043 12050 12049 12054 12040 12043
17 12052 12054 12050 12052 12053 12051 12053 12051 12048 12049 12049 12043 12055 12049 12056
18 12054 12042 12043 12042 12053 12051 12051 12042 12047 12050 12048 12049 12050 12052 12042
19 12051 12051 12043 12053 12042 12054 12042 12044 12044 12043 12048 12049 12056 12042 12042
20 12052 12052 12051 12051 12042 12051 12052 12050 12047 12049 12049 12043 12056 12049 12051
21 12042 12052 12043 12052 12042 12051 12052 12043 12043 12043 12049 12048 12048 12042 12043
22 12052 12054 12051 12052 12051 12051 12051 12051 12047 12049 12043 12043 12044 12043 12051
23 12054 12048 12043 12051 12043 12043 12052 12044 12047 12049 12048 12048 12048 12049 12042
25 12054 12047 12042 12051 12052 12051 12051 12042 12047 12050 12049 12043 12055 12049 12049
30 12052 12052 12047 12043 12043 12051 12052 12051 12047 12050 12049 12043 12056 12052 12043
40 12042 12043 12042 12042 12053 12050 12052 12049 12047 12049 12049 12043 12043 12055 12043

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 12046 bytes – 100%
1,000 12043 bytes -3 bytes 100%
10,000 12041 bytes -2 bytes 100%
100,000 12041 bytes – 0.58%
1,000,000 12040 bytes -1 byte 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) 12114 bytes +74 bytes (+0.61%) +52 bytes
1 12114 bytes +74 bytes (+0.61%) +52 bytes
2 12062 bytes +22 bytes (+0.18%) –
3 12079 bytes +39 bytes (+0.32%) +17 bytes
4 12126 bytes +86 bytes (+0.71%) +64 bytes
5 12156 bytes +116 bytes (+0.96%) +94 bytes
6 12182 bytes +142 bytes (+1.18%) +120 bytes
7 12196 bytes +156 bytes (+1.30%) +134 bytes
8 12225 bytes +185 bytes (+1.54%) +163 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 10284 bytes -1756 bytes (-14.58%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 10705 bytes -1335 bytes (-11.09%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11104 bytes -936 bytes (-7.77%)
Brotli (Wikipedia) brotli brotli -q 11 11292 bytes -748 bytes (-6.21%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 11765 bytes -275 bytes (-2.28%)
LZMA2 (Wikipedia) xz xz -9 11980 bytes -60 bytes (-0.50%)
Zstandard (Wikipedia) zstd zstd -19 12024 bytes -16 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.