Choose a version:
38% The original file has 114434 bytes (111.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 43842 bytes (42.8k, 38%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  13059 bytes (12.8k)
 local copy
cdnjs
  13029 bytes (12.7k)
 CDN
gzip -9
  13004 bytes (12.7k)
 local copy
kzip -s0 -rn -b2
  12597 bytes (12.3k)
 local copy
libdeflate -12
  12576 bytes (12.3k)
 local copy
7zip -mx=9 -tgzip
  12569 bytes (12.3k)
 local copy
zultra
  12568 bytes (12.3k)
 local copy
pigz -11 -n
  12556 bytes (12.3k)
 local copy
Zopfli
  12540 bytes (12.2k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.5.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.5.2 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 --i100000 --mb8 --mls4096 --bsr17 --lazy --ohh

(found September 20, 2022)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 17  --bsr17
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/4.5.2/socket.io.min.js --location | md5sum
6459b916c000f9f2a61952f876f3c960  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.2.min.zopfli.js.gz | md5sum
6459b916c000f9f2a61952f876f3c960  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.5.2/socket.io.min.js --location | sha1sum
d5b2f99722f6041db23c2f42371f589287973c64  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.2.min.zopfli.js.gz | sha1sum
d5b2f99722f6041db23c2f42371f589287973c64  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 13029 bytes 6459b916c000f9f2a61952f876f3c960 (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
12540 bytes -1 byte zopfli --i100000 --mls4096 --bsr17 --lazy --ohh September 20, 2022 @ 14:17
12541 bytes -1 byte zopfli --i10000 --mls8192 --bsr18 --lazy --ohh September 19, 2022 @ 16:24
12542 bytes -1 byte zopfli --i1000 --mls8192 --bsr18 --lazy --ohh September 19, 2022 @ 15:21
12543 bytes -4 bytes zopfli --i1000 --mls1024 --bsr7 --lazy --ohh September 19, 2022 @ 15:18
12547 bytes – zopfli --i100 --mls8192 --bsr18 --lazy --ohh September 19, 2022 @ 14:44

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

Most recent activity on September 20, 2022 @ 17:47.

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, 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 12555 12555 12555 12546 12544 12544 12544 12543 12544 12545 12548 12547 12545 12548 12546
4 12550 12544 12546 12551 12548 12551 12552 12550 12546 12546 12545 12543 12550 12545 12548
6 12548 12551 12548 12546 12546 12546 12547 12546 12547 12543 12547 12547 12549 12550 12545
7 12551 12546 12548 12550 12547 12546 12549 12543 12547 12543 12544 12543 12549 12543 12543
8 12545 12547 12546 12546 12545 12544 12544 12543 12544 12545 12541 12544 12543 12550 12546
9 12551 12550 12545 12547 12546 12545 12545 12548 12550 12548 12546 12541 12550 12548 12548
10 12556 12547 12547 12544 12547 12552 12543 12544 12546 12543 12543 12543 12545 12543 12542
11 12545 12547 12547 12550 12546 12546 12549 12549 12548 12547 12546 12541 12543 12548 12549
12 12549 12544 12546 12545 12546 12551 12547 12545 12550 12545 12544 12543 12549 12550 12549
13 12549 12543 12545 12550 12550 12546 12549 12549 12546 12543 12543 12543 12550 12550 12550
14 12545 12545 12548 12546 12546 12545 12549 12546 12547 12547 12544 12541 12547 12549 12550
15 12551 12545 12546 12550 12546 12547 12543 12543 12547 12543 12543 12543 12545 12543 12543
16 12546 12551 12549 12550 12543 12550 12543 12543 12548 12547 12543 12541 12548 12547 12550
17 12544 12549 12548 12550 12549 12547 12543 12543 12546 12545 12544 12540 12546 12546 12543
18 12551 12553 12546 12549 12549 12545 12548 12545 12548 12548 12543 12543 12541 12549 12550
19 12545 12548 12544 12550 12546 12548 12543 12543 12548 12547 12544 12543 12543 12550 12551
20 12549 12551 12545 12546 12546 12549 12552 12549 12548 12543 12543 12543 12543 12550 12552
21 12551 12544 12546 12550 12546 12549 12543 12543 12545 12547 12544 12549 12543 12549 12548
22 12547 12545 12548 12550 12546 12548 12550 12543 12547 12549 12543 12543 12548 12542 12548
23 12546 12545 12547 12547 12546 12547 12543 12549 12548 12548 12544 12541 12543 12549 12549
25 12547 12551 12548 12547 12546 12549 12549 12549 12549 12551 12544 12543 12547 12543 12550
30 12550 12543 12547 12550 12546 12549 12543 12543 12545 12548 12545 12543 12548 12543 12549
40 12551 12550 12549 12547 12546 12547 12550 12543 12547 12541 12543 12543 12547 12543 12543

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 12547 bytes – 100%
1,000 12542 bytes -5 bytes 100%
10,000 12541 bytes -1 byte 100%
100,000 12540 bytes -1 byte 3.19%
1,000,000 12540 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) 12600 bytes +60 bytes (+0.48%) +3 bytes
1 12600 bytes +60 bytes (+0.48%) +3 bytes
2 12597 bytes +57 bytes (+0.45%) –
3 12640 bytes +100 bytes (+0.80%) +43 bytes
4 12629 bytes +89 bytes (+0.71%) +32 bytes
5 12659 bytes +119 bytes (+0.95%) +62 bytes
6 12691 bytes +151 bytes (+1.20%) +94 bytes
7 12710 bytes +170 bytes (+1.36%) +113 bytes
8 12744 bytes +204 bytes (+1.63%) +147 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 10676 bytes -1864 bytes (-14.86%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 11076 bytes -1464 bytes (-11.67%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11601 bytes -939 bytes (-7.49%)
Brotli (Wikipedia) brotli brotli -q 11 11740 bytes -800 bytes (-6.38%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 12243 bytes -297 bytes (-2.37%)
LZMA2 (Wikipedia) xz xz -9 12464 bytes -76 bytes (-0.61%)
Zstandard (Wikipedia) zstd zstd -19 12497 bytes -43 bytes (-0.34%)

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.