Choose a version:
38% The original file has 114397 bytes (111.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 43623 bytes (42.6k, 38%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  13055 bytes (12.7k)
local copy
cdnjs
  13024 bytes (12.7k)
CDN
gzip -9
  13002 bytes (12.7k)
local copy
7zip -mx=9 -tgzip
  12572 bytes (12.3k)
local copy
libdeflate -12
  12568 bytes (12.3k)
local copy
zultra
  12558 bytes (12.3k)
local copy
pigz -11 -n
  12552 bytes (12.3k)
local copy
Zopfli
  12532 bytes (12.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.5.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.5.0 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 --mls16 --bsr30 --lazy --ohh

(found May 4, 2022)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 16  --mls16
block splitting recursion 30  --bsr30
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/4.5.0/socket.io.min.js --location | md5sum
2dd2f05aaab352fc37751c4f46f66b44  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.0.min.zopfli.js.gz | md5sum
2dd2f05aaab352fc37751c4f46f66b44  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.5.0/socket.io.min.js --location | sha1sum
7f8f08299f1ff467b7ad75477f6b9324abdb311e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.0.min.zopfli.js.gz | sha1sum
7f8f08299f1ff467b7ad75477f6b9324abdb311e  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 13024 bytes 2dd2f05aaab352fc37751c4f46f66b44 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available Socket.IO versions at minime.stephan-brumme.com:

4.5.1, 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
12532 bytes -1 byte zopfli --i1000000 --mls16 --bsr30 --lazy --ohh May 4, 2022 @ 15:08
12533 bytes -1 byte zopfli --i100000 --mls16 --bsr30 --lazy --ohh May 4, 2022 @ 12:47
12534 bytes -1 byte zopfli --i10000 --mls16384 --bsr15 --lazy --ohh May 4, 2022 @ 05:21
12535 bytes -1 byte zopfli --i1000 --mls16 --bsr30 --lazy --ohh May 2, 2022 @ 17:53
12536 bytes -1 byte zopfli --i1000 --mls16384 --bsr8 --lazy --ohh May 2, 2022 @ 17:14
12537 bytes -1 byte zopfli --i1000 --mls2048 --bsr10 --lazy --ohh May 2, 2022 @ 16:32
12538 bytes -1 byte zopfli --i1000 --mls32768 --bsr19 --lazy --ohh May 2, 2022 @ 16:24
12539 bytes zopfli --i100 --mls1024 --bsr4 --lazy --ohh May 2, 2022 @ 16:03

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

Most recent activity on May 4, 2022 @ 15:08.

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
12542 12543 12536 12536 12536 12535 12535 12536 12536 12537 12535 12535 12536 12536 12536
12540 12538 12537 12540 12539 12540 12540 12539 12540 12537 12536 12537 12538 12540 12538
12538 12538 12535 12532 12540 12542 12540 12541 12539 12536 12535 12537 12543 12541 12535
12541 12540 12536 12540 12539 12539 12539 12542 12535 12534 12538 12535 12542 12535 12546
12540 12538 12540 12536 12536 12536 12535 12536 12535 12535 12535 12535 12535 12535 12536
12539 12541 12541 12538 12538 12539 12539 12540 12541 12536 12538 12546 12539 12541 12542
12537 12538 12541 12535 12542 12535 12535 12536 12535 12535 12535 12536 12538 12547 12535
12544 12535 12536 12536 12535 12539 12541 12541 12539 12537 12538 12535 12543 12547 12538
12541 12540 12543 12537 12538 12535 12535 12536 12541 12539 12538 12535 12544 12546 12535
12539 12538 12536 12535 12538 12540 12541 12541 12540 12542 12535 12535 12536 12536 12543
12538 12539 12542 12539 12538 12540 12535 12536 12539 12537 12536 12535 12543 12536 12550
12541 12542 12543 12539 12543 12541 12541 12540 12535 12536 12536 12534 12542 12534 12547
12543 12536 12542 12535 12536 12535 12541 12536 12541 12536 12538 12538 12545 12537 12547
12542 12541 12540 12537 12540 12540 12536 12537 12535 12535 12537 12536 12546 12546 12535
12540 12537 12542 12544 12540 12541 12537 12540 12541 12540 12535 12535 12541 12547 12536
12543 12537 12536 12535 12539 12535 12535 12540 12539 12535 12538 12535 12539 12535 12536
12535 12538 12543 12539 12540 12540 12539 12541 12541 12536 12535 12536 12538 12543 12539
12537 12537 12542 12535 12535 12535 12541 12535 12539 12535 12537 12536 12544 12546 12535
12539 12538 12542 12541 12539 12540 12535 12540 12541 12536 12539 12537 12544 12546 12535
12539 12540 12536 12539 12540 12540 12539 12536 12535 12535 12538 12537 12543 12535 12535
12539 12538 12540 12537 12540 12539 12535 12542 12541 12540 12537 12535 12543 12535 12536
12536 12538 12540 12532 12538 12535 12541 12541 12535 12536 12537 12535 12544 12535 12535
12540 12540 12541 12540 12539 12539 12539 12544 12535 12537 12536 12535 12543 12535 12535

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 12539 bytes 100%
1,000 12535 bytes -4 bytes 100%
10,000 12534 bytes -1 byte 100%
100,000 12533 bytes -1 byte 1.45%
1,000,000 12532 bytes -1 byte 0.58%
10,000,000

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 10658 bytes -1874 bytes (-14.95%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 11060 bytes -1472 bytes (-11.75%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11549 bytes -983 bytes (-7.84%)
Brotli (Wikipedia) brotli brotli -q 11 11747 bytes -785 bytes (-6.26%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 12196 bytes -336 bytes (-2.68%)
LZMA2 (Wikipedia) xz xz -9 12444 bytes -88 bytes (-0.70%)
Zstandard (Wikipedia) zstd zstd -19 12492 bytes -40 bytes (-0.32%)

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):


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 2022.
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.