Choose a version:
35% The original file has 184751 bytes (180.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 64333 bytes (62.8k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15272 bytes (14.9k)
 local copy
gzip -9
  15189 bytes (14.8k)
 local copy
Boot
  15181 bytes (14.8k)
 CDN
cdnjs
  15181 bytes (14.8k)
 CDN
7zip -mx=9 -tgzip
  14736 bytes (14.4k)
 local copy
libdeflate -12
  14724 bytes (14.4k)
 local copy
zultra
  14680 bytes (14.3k)
 local copy
pigz -11 -n
  14653 bytes (14.3k)
 local copy
kzip -s0 -rn -b2
  14652 bytes (14.3k)
 local copy
Zopfli
  14640 bytes (14.3k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.0.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.0.0 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 --i1000000 --mb8 --mls2048 --bsr25 --lazy --ohh

(found March 16, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 25  --bsr25
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/4.0.0/socket.io.min.js --location | md5sum
d5593b94cdc1b8d03f7e079046b3c6ff  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.0.0.min.zopfli.js.gz | md5sum
d5593b94cdc1b8d03f7e079046b3c6ff  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.0.0/socket.io.min.js --location | sha1sum
5655d395bb4c2231f795bf9ec6e81e54f7301c3d  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.0.0.min.zopfli.js.gz | sha1sum
5655d395bb4c2231f795bf9ec6e81e54f7301c3d  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15181 bytes d5593b94cdc1b8d03f7e079046b3c6ff (invalid)
cdnjs 15181 bytes d5593b94cdc1b8d03f7e079046b3c6ff (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
14640 bytes -1 byte zopfli --i1000000 --mls2048 --bsr25 --lazy --ohh March 16, 2021 @ 22:27
14641 bytes -1 byte zopfli --i100000 --mls256 --bsr14 --lazy --ohh March 15, 2021 @ 14:37
14642 bytes -1 byte zopfli --i1000 --mls256 --bsr40 --lazy --ohh March 12, 2021 @ 14:10
14643 bytes -3 bytes zopfli --i1000 --mls64 --bsr4 --lazy --ohh March 12, 2021 @ 14:07
14646 bytes – zopfli --i100 --mls2048 --bsr13 --lazy --ohh March 12, 2021 @ 13:09

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

Most recent activity on March 17, 2021 @ 17:46.

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 14647 14647 14646 14646 14646 14646 14645 14645 14644 14646 14647 14647 14647 14645 14665
4 14644 14643 14644 14644 14643 14642 14644 14644 14645 14642 14642 14642 14644 14645 14661
6 14643 14642 14644 14643 14643 14642 14643 14641 14642 14643 14642 14645 14659 14644 14644
7 14644 14643 14644 14642 14642 14642 14644 14640 14642 14642 14640 14645 14662 14644 14660
8 14644 14642 14642 14642 14642 14643 14643 14641 14642 14648 14642 14642 14646 14645 14644
9 14644 14642 14644 14642 14644 14642 14644 14640 14642 14642 14645 14644 14644 14644 14661
10 14644 14644 14644 14642 14644 14642 14644 14641 14642 14643 14642 14642 14660 14660 14644
11 14644 14643 14644 14642 14643 14643 14644 14642 14643 14642 14642 14642 14644 14644 14661
12 14643 14643 14644 14642 14644 14643 14646 14640 14642 14642 14644 14644 14659 14664 14644
13 14644 14643 14644 14643 14642 14642 14643 14642 14643 14642 14642 14642 14644 14644 14660
14 14644 14644 14644 14642 14642 14642 14642 14641 14643 14642 14640 14642 14659 14662 14660
15 14644 14644 14644 14642 14644 14642 14644 14640 14645 14643 14640 14642 14658 14644 14660
16 14644 14642 14644 14643 14642 14642 14642 14641 14644 14642 14642 14642 14644 14660 14661
17 14644 14644 14643 14642 14642 14642 14642 14640 14643 14642 14642 14642 14644 14662 14661
18 14643 14644 14643 14643 14643 14643 14642 14642 14641 14642 14642 14642 14644 14644 14644
19 14644 14643 14644 14644 14643 14643 14644 14642 14642 14642 14642 14642 14644 14644 14661
20 14644 14642 14644 14643 14642 14642 14643 14642 14642 14642 14642 14642 14644 14644 14644
21 14644 14644 14643 14642 14643 14643 14642 14640 14643 14642 14642 14642 14644 14644 14644
22 14644 14642 14643 14643 14644 14643 14644 14642 14642 14642 14644 14642 14644 14644 14661
23 14644 14644 14644 14643 14643 14642 14644 14641 14643 14642 14642 14644 14644 14662 14644
25 14644 14642 14644 14643 14642 14642 14644 14640 14642 14642 14640 14642 14644 14661 14644
30 14643 14642 14644 14642 14644 14642 14644 14643 14642 14642 14640 14644 14644 14663 14660
40 14644 14643 14643 14643 14642 14643 14644 14642 14642 14642 14642 14642 14645 14644 14660

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 14646 bytes – 100%
1,000 14642 bytes -4 bytes 100%
10,000 14642 bytes – 100%
100,000 14641 bytes -1 byte 38.84%
1,000,000 14640 bytes -1 byte 5.51%
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) 14686 bytes +46 bytes (+0.31%) +34 bytes
1 14686 bytes +46 bytes (+0.31%) +34 bytes
2 14652 bytes +12 bytes (+0.08%) –
3 14673 bytes +33 bytes (+0.23%) +21 bytes
4 14708 bytes +68 bytes (+0.46%) +56 bytes
5 14721 bytes +81 bytes (+0.55%) +69 bytes
6 14740 bytes +100 bytes (+0.68%) +88 bytes
7 14786 bytes +146 bytes (+1.00%) +134 bytes
8 14815 bytes +175 bytes (+1.20%) +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 12272 bytes -2368 bytes (-16.17%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12540 bytes -2100 bytes (-14.34%)
Brotli (Wikipedia) brotli brotli -q 11 13237 bytes -1403 bytes (-9.58%)
LZMA2 (Wikipedia) xz xz -9 14028 bytes -612 bytes (-4.18%)
Zstandard (Wikipedia) zstd zstd -19 14156 bytes -484 bytes (-3.31%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14248 bytes -392 bytes (-2.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15734 bytes +1094 bytes (+7.47%)

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.