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
kzip -s0 -rn -b3
  12577 bytes (12.3k)
 local copy
7zip -mx=9 -tgzip
  12572 bytes (12.3k)
 local copy
libdeflate -12
  12572 bytes (12.3k)
 local copy
zultra
  12558 bytes (12.3k)
 local copy
pigz -11 -n
  12545 bytes (12.3k)
 local copy
Zopfli
  12533 bytes (12.2k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.5.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.5.1 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 --i100000 --mb8 --mls512 --bsr40 --lazy --ohh

(found May 23, 2022)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 40  --bsr40
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/4.5.1/socket.io.min.js --location | md5sum
4df396b4096952347cb72e07967520e1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.1.min.zopfli.js.gz | md5sum
4df396b4096952347cb72e07967520e1  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.5.1/socket.io.min.js --location | sha1sum
091a63ca2707250264cd7e2ef74e76381cea9328  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.1.min.zopfli.js.gz | sha1sum
091a63ca2707250264cd7e2ef74e76381cea9328  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 13024 bytes 4df396b4096952347cb72e07967520e1 (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
12533 bytes -1 byte zopfli --i100000 --mls512 --bsr40 --lazy --ohh May 23, 2022 @ 08:49
12534 bytes -1 byte zopfli --i10000 --mls512 --bsr17 --lazy --ohh May 23, 2022 @ 08:07
12535 bytes -1 byte zopfli --i10000 --mls8 --bsr6 --lazy --ohh May 23, 2022 @ 08:06
12536 bytes -1 byte zopfli --i1000 --mls8 --bsr6 --lazy --ohh May 23, 2022 @ 08:04
12537 bytes -2 bytes zopfli --i1000 --mls64 --bsr16 --lazy --ohh May 23, 2022 @ 08:04
12539 bytes – zopfli --i100 --mls1024 --bsr20 --lazy --ohh May 23, 2022 @ 07:53

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

Most recent activity on May 23, 2022 @ 13:33.

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 12542 12542 12536 12536 12536 12535 12535 12536 12536 12537 12535 12535 12537 12536 12536
4 12540 12537 12537 12539 12539 12541 12540 12538 12539 12537 12537 12537 12538 12539 12537
6 12538 12538 12535 12535 12540 12542 12540 12541 12539 12536 12535 12537 12543 12540 12535
7 12541 12540 12536 12540 12538 12540 12539 12542 12535 12534 12537 12535 12542 12535 12546
8 12540 12538 12540 12536 12536 12535 12535 12536 12535 12536 12535 12535 12535 12535 12536
9 12539 12541 12541 12540 12538 12542 12539 12537 12537 12536 12537 12546 12539 12538 12537
10 12537 12538 12541 12535 12542 12535 12535 12536 12535 12533 12535 12536 12537 12535 12535
11 12544 12535 12536 12536 12535 12540 12541 12538 12538 12537 12538 12536 12543 12547 12538
12 12541 12539 12542 12537 12538 12535 12535 12536 12540 12540 12538 12535 12544 12547 12535
13 12539 12537 12536 12535 12538 12540 12540 12540 12541 12541 12535 12536 12536 12535 12544
14 12538 12539 12543 12538 12538 12540 12535 12536 12540 12537 12536 12535 12543 12535 12550
15 12541 12542 12543 12538 12543 12538 12541 12540 12535 12536 12536 12534 12543 12535 12535
16 12543 12536 12542 12535 12536 12535 12541 12536 12541 12536 12538 12538 12545 12537 12547
17 12542 12541 12540 12537 12539 12540 12536 12537 12534 12535 12537 12536 12546 12546 12535
18 12540 12537 12542 12544 12540 12539 12538 12539 12540 12534 12535 12536 12537 12547 12536
19 12543 12537 12536 12535 12539 12535 12535 12539 12538 12535 12538 12536 12538 12535 12536
20 12535 12538 12543 12538 12540 12540 12539 12541 12542 12536 12535 12536 12544 12542 12539
21 12537 12537 12542 12535 12535 12535 12540 12535 12539 12535 12537 12536 12544 12546 12546
22 12538 12538 12543 12541 12539 12540 12535 12539 12541 12535 12539 12537 12544 12547 12535
23 12539 12540 12536 12538 12539 12540 12539 12536 12535 12535 12537 12537 12543 12546 12535
25 12539 12537 12539 12537 12539 12540 12535 12542 12541 12540 12540 12535 12543 12535 12536
30 12536 12538 12540 12535 12538 12535 12541 12539 12535 12535 12537 12535 12544 12535 12535
40 12540 12540 12541 12539 12539 12541 12538 12544 12533 12536 12536 12535 12543 12535 12536

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 12536 bytes -3 bytes 100%
10,000 12534 bytes -2 bytes 100%
100,000 12533 bytes -1 byte 1.74%
1,000,000 12533 bytes – 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) 12593 bytes +60 bytes (+0.48%) +16 bytes
1 12593 bytes +60 bytes (+0.48%) +16 bytes
2 12588 bytes +55 bytes (+0.44%) +11 bytes
3 12577 bytes +44 bytes (+0.35%) –
4 12620 bytes +87 bytes (+0.69%) +43 bytes
5 12647 bytes +114 bytes (+0.91%) +70 bytes
6 12681 bytes +148 bytes (+1.18%) +104 bytes
7 12699 bytes +166 bytes (+1.32%) +122 bytes
8 12737 bytes +204 bytes (+1.63%) +160 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 10656 bytes -1877 bytes (-14.98%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 11060 bytes -1473 bytes (-11.75%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11549 bytes -984 bytes (-7.85%)
Brotli (Wikipedia) brotli brotli -q 11 11748 bytes -785 bytes (-6.26%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 12195 bytes -338 bytes (-2.70%)
LZMA2 (Wikipedia) xz xz -9 12436 bytes -97 bytes (-0.77%)
Zstandard (Wikipedia) zstd zstd -19 12492 bytes -41 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 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.