Choose a version:
37% The original file has 118682 bytes (115.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 44004 bytes (43.0k, 37%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  13092 bytes (12.8k)
 local copy
cdnjs
  13060 bytes (12.8k)
 CDN
gzip -9
  13035 bytes (12.7k)
 local copy
kzip -s0 -rn -b2
  12622 bytes (12.3k)
 local copy
libdeflate -12
  12599 bytes (12.3k)
 local copy
7zip -mx=9 -tgzip
  12596 bytes (12.3k)
 local copy
zultra
  12584 bytes (12.3k)
 local copy
pigz -11 -n
  12580 bytes (12.3k)
 local copy
Zopfli
  12561 bytes (12.3k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.5.3.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.5.3 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 --mls8192 --bsr7 --lazy --ohh

(found January 5, 2023)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 7  --bsr7
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/4.5.3/socket.io.min.js --location | md5sum
a54f5222914cb0b3138dd3daf13cb4d6  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.3.min.zopfli.js.gz | md5sum
a54f5222914cb0b3138dd3daf13cb4d6  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.5.3/socket.io.min.js --location | sha1sum
ba59fb413d25ae1b27813e5c0cd0b337e8d3ff09  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.3.min.zopfli.js.gz | sha1sum
ba59fb413d25ae1b27813e5c0cd0b337e8d3ff09  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 13060 bytes a54f5222914cb0b3138dd3daf13cb4d6 (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
12561 bytes -1 byte zopfli --i100000 --mls8192 --bsr7 --lazy --ohh January 5, 2023 @ 14:54
12562 bytes -1 byte zopfli --i100000 --mls8192 --bsr10 --lazy --ohh January 4, 2023 @ 11:10
12563 bytes -1 byte zopfli --i1000 --mls8192 --bsr10 --lazy --ohh January 4, 2023 @ 03:49
12564 bytes -3 bytes zopfli --i1000 --mls8192 --bsr30 --lazy --ohh January 3, 2023 @ 18:42
12567 bytes -2 bytes zopfli --i1000 --mls16384 --bsr15 --lazy --ohh January 3, 2023 @ 18:19
12569 bytes – zopfli --i100 --mls4096 --bsr4 --lazy --ohh January 3, 2023 @ 16:37

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

Most recent activity on January 5, 2023 @ 21:25.

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 12569 12570 12569 12568 12567 12568 12567 12566 12565 12569 12570 12567 12565 12570 12567
4 12571 12572 12567 12572 12570 12571 12571 12572 12572 12568 12566 12562 12570 12567 12574
6 12567 12570 12569 12568 12569 12569 12569 12570 12569 12572 12570 12568 12564 12570 12569
7 12569 12572 12570 12567 12570 12567 12568 12576 12570 12567 12566 12565 12561 12565 12565
8 12569 12568 12569 12568 12567 12568 12567 12566 12565 12569 12566 12562 12564 12575 12567
9 12567 12570 12570 12567 12570 12568 12567 12569 12569 12570 12566 12563 12570 12570 12569
10 12570 12569 12565 12571 12568 12566 12565 12565 12569 12566 12566 12565 12562 12566 12565
11 12572 12569 12566 12566 12569 12569 12567 12569 12565 12568 12566 12562 12562 12572 12569
12 12570 12570 12569 12568 12569 12573 12572 12568 12566 12567 12566 12565 12573 12564 12572
13 12571 12574 12568 12571 12569 12567 12574 12571 12569 12566 12567 12565 12573 12572 12573
14 12572 12567 12570 12568 12568 12568 12574 12569 12568 12571 12566 12564 12571 12570 12572
15 12570 12573 12571 12570 12570 12569 12571 12575 12569 12567 12566 12565 12562 12565 12565
16 12569 12568 12566 12567 12571 12567 12571 12565 12568 12567 12566 12562 12569 12573 12573
17 12567 12570 12565 12570 12569 12569 12565 12565 12569 12567 12566 12564 12570 12569 12565
18 12570 12571 12566 12570 12569 12566 12567 12568 12568 12566 12566 12565 12570 12570 12570
19 12571 12567 12565 12566 12566 12570 12565 12569 12568 12569 12566 12565 12562 12571 12574
20 12568 12567 12570 12569 12568 12567 12566 12568 12570 12570 12567 12565 12562 12571 12570
21 12569 12569 12565 12565 12569 12569 12565 12569 12566 12567 12566 12570 12562 12569 12572
22 12570 12570 12570 12567 12570 12566 12568 12565 12570 12566 12566 12565 12569 12564 12569
23 12572 12570 12565 12570 12568 12572 12571 12568 12565 12568 12566 12564 12570 12573 12573
25 12570 12570 12569 12570 12567 12570 12571 12568 12570 12571 12566 12565 12564 12565 12573
30 12568 12569 12565 12568 12569 12573 12565 12568 12570 12567 12566 12565 12564 12570 12573
40 12569 12567 12572 12573 12569 12574 12567 12569 12569 12567 12566 12565 12562 12565 12565

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 12569 bytes – 100%
1,000 12563 bytes -6 bytes 100%
10,000 12563 bytes – 100%
100,000 12561 bytes -2 bytes 3.77%
1,000,000 12561 bytes – 0.29%
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) 12626 bytes +65 bytes (+0.52%) +4 bytes
1 12623 bytes +62 bytes (+0.49%) +1 byte
2 12622 bytes +61 bytes (+0.49%) –
3 12663 bytes +102 bytes (+0.81%) +41 bytes
4 12652 bytes +91 bytes (+0.72%) +30 bytes
5 12680 bytes +119 bytes (+0.95%) +58 bytes
6 12712 bytes +151 bytes (+1.20%) +90 bytes
7 12731 bytes +170 bytes (+1.35%) +109 bytes
8 12765 bytes +204 bytes (+1.62%) +143 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 - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 10695 bytes -1866 bytes (-14.86%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 11092 bytes -1469 bytes (-11.69%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11637 bytes -924 bytes (-7.36%)
Brotli (Wikipedia) brotli brotli -q 11 11754 bytes -807 bytes (-6.42%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 12273 bytes -288 bytes (-2.29%)
LZMA2 (Wikipedia) xz xz -9 12484 bytes -77 bytes (-0.61%)
Zstandard (Wikipedia) zstd zstd -19 12519 bytes -42 bytes (-0.33%)

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 2024.
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.