Choose a version:
35% The original file has 185291 bytes (180.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 64273 bytes (62.8k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15236 bytes (14.9k)
 local copy
gzip -9
  15162 bytes (14.8k)
 local copy
Boot
  15154 bytes (14.8k)
 CDN
cdnjs
  15154 bytes (14.8k)
 CDN
7zip -mx=9 -tgzip
  14712 bytes (14.4k)
 local copy
libdeflate -12
  14704 bytes (14.4k)
 local copy
zultra
  14676 bytes (14.3k)
 local copy
pigz -11 -n
  14627 bytes (14.3k)
 local copy
kzip -s0 -rn -b2
  14625 bytes (14.3k)
 local copy
Zopfli
  14616 bytes (14.3k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.1.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.1.2 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 --i1000000 --mb8 --mls2048 --bsr16 --lazy --ohh

(found May 22, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
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/4.1.2/socket.io.min.js --location | md5sum
ab93646dca3693bc7271d9d41bbe31ed  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.1.2.min.zopfli.js.gz | md5sum
ab93646dca3693bc7271d9d41bbe31ed  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.1.2/socket.io.min.js --location | sha1sum
29c7b38769106020e58d2c61e2fff35ee49ef483  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.1.2.min.zopfli.js.gz | sha1sum
29c7b38769106020e58d2c61e2fff35ee49ef483  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 15154 bytes ab93646dca3693bc7271d9d41bbe31ed June 8, 2021 @ 05:33
cdnjs 15154 bytes ab93646dca3693bc7271d9d41bbe31ed June 8, 2021 @ 05:33

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
14616 bytes -2 bytes zopfli --i1000000 --mls2048 --bsr16 --lazy --ohh May 22, 2021 @ 04:33
14618 bytes -2 bytes zopfli --i10000 --mls2048 --bsr13 --lazy --ohh May 21, 2021 @ 15:56
14620 bytes -1 byte zopfli --i1000 --mls2048 --bsr13 --lazy --ohh May 21, 2021 @ 15:51
14621 bytes -1 byte zopfli --i1000 --mls128 --bsr20 --lazy --ohh May 21, 2021 @ 15:47
14622 bytes – zopfli --i100 --mls16 --bsr7 --lazy --ohh May 21, 2021 @ 15:23

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

Most recent activity on July 13, 2021 @ 12:47.

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 14623 14623 14623 14622 14622 14622 14623 14622 14622 14623 14623 14624 14623 14622 14641
4 14619 14619 14620 14620 14619 14621 14620 14619 14621 14619 14616 14619 14620 14640 14638
6 14622 14622 14618 14617 14621 14620 14620 14619 14619 14621 14619 14620 14639 14637 14623
7 14620 14620 14619 14619 14620 14620 14620 14619 14619 14619 14619 14619 14636 14635 14623
8 14620 14620 14620 14617 14621 14620 14620 14619 14619 14617 14619 14619 14623 14622 14640
9 14620 14620 14621 14620 14619 14619 14620 14619 14619 14619 14620 14619 14641 14624 14619
10 14620 14620 14620 14619 14622 14620 14620 14619 14619 14620 14619 14619 14641 14638 14620
11 14620 14620 14619 14619 14621 14620 14620 14618 14619 14617 14619 14619 14638 14622 14640
12 14620 14620 14618 14619 14619 14620 14620 14619 14620 14619 14621 14620 14640 14635 14622
13 14620 14620 14618 14617 14617 14620 14620 14619 14620 14617 14616 14619 14638 14639 14639
14 14620 14620 14620 14617 14620 14620 14620 14619 14619 14619 14619 14619 14636 14639 14639
15 14620 14620 14619 14619 14619 14620 14620 14619 14619 14619 14619 14619 14636 14635 14640
16 14620 14620 14620 14619 14620 14620 14619 14618 14619 14621 14616 14621 14621 14639 14624
17 14620 14620 14620 14619 14621 14620 14620 14618 14619 14620 14620 14621 14639 14635 14620
18 14620 14620 14620 14620 14621 14619 14620 14620 14620 14619 14619 14616 14621 14641 14621
19 14620 14620 14619 14620 14621 14620 14620 14619 14619 14620 14620 14619 14640 14641 14639
20 14620 14620 14620 14619 14619 14620 14619 14618 14617 14617 14619 14619 14622 14639 14633
21 14621 14621 14619 14620 14620 14620 14620 14619 14619 14620 14619 14619 14636 14635 14621
22 14620 14620 14620 14617 14621 14620 14620 14618 14620 14619 14619 14619 14639 14639 14639
23 14620 14620 14620 14617 14621 14620 14620 14619 14619 14617 14619 14619 14622 14640 14640
25 14620 14620 14618 14619 14621 14620 14620 14619 14620 14617 14619 14619 14636 14640 14640
30 14621 14621 14618 14619 14621 14620 14620 14619 14619 14619 14619 14619 14636 14623 14623
40 14620 14620 14621 14617 14620 14620 14620 14619 14621 14619 14619 14619 14637 14640 14640

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 14622 bytes – 100%
1,000 14620 bytes -2 bytes 100%
10,000 14618 bytes -2 bytes 100%
100,000 14618 bytes – 8.41%
1,000,000 14616 bytes -2 bytes 8.41%
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) 14663 bytes +47 bytes (+0.32%) +38 bytes
1 14663 bytes +47 bytes (+0.32%) +38 bytes
2 14625 bytes +9 bytes (+0.06%) –
3 14648 bytes +32 bytes (+0.22%) +23 bytes
4 14683 bytes +67 bytes (+0.46%) +58 bytes
5 14693 bytes +77 bytes (+0.53%) +68 bytes
6 14712 bytes +96 bytes (+0.66%) +87 bytes
7 14759 bytes +143 bytes (+0.98%) +134 bytes
8 14787 bytes +171 bytes (+1.17%) +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 12256 bytes -2360 bytes (-16.15%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12535 bytes -2081 bytes (-14.24%)
Brotli (Wikipedia) brotli brotli -q 11 13236 bytes -1380 bytes (-9.44%)
LZMA2 (Wikipedia) xz xz -9 14020 bytes -596 bytes (-4.08%)
Zstandard (Wikipedia) zstd zstd -19 14135 bytes -481 bytes (-3.29%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14273 bytes -343 bytes (-2.35%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15729 bytes +1113 bytes (+7.61%)

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.