Choose a version:
34% The original file has 185191 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 62172 bytes (60.7k, 34%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15011 bytes (14.7k)
 local copy
gzip -9
  14918 bytes (14.6k)
 local copy
Boot
  14913 bytes (14.6k)
 CDN
cdnjs
  14913 bytes (14.6k)
 CDN
7zip -mx=9 -tgzip
  14477 bytes (14.1k)
 local copy
libdeflate -12
  14472 bytes (14.1k)
 local copy
zultra
  14421 bytes (14.1k)
 local copy
kzip -s0 -rn -b2
  14402 bytes (14.1k)
 local copy
pigz -11 -n
  14400 bytes (14.1k)
 local copy
Zopfli
  14380 bytes (14.0k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.0.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.0.0 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 --i100000 --mb8 --mls256 --bsr40 --lazy --ohh

(found November 10, 2020)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 256  --mls256
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/3.0.0/socket.io.min.js --location | md5sum
508a736853f79cae3e5c99042616b6f5  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.0.min.zopfli.js.gz | md5sum
508a736853f79cae3e5c99042616b6f5  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.0.0/socket.io.min.js --location | sha1sum
fad291683141930418a52106fe116f71716105b0  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.0.min.zopfli.js.gz | sha1sum
fad291683141930418a52106fe116f71716105b0  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14913 bytes 508a736853f79cae3e5c99042616b6f5 (invalid)
cdnjs 14913 bytes 508a736853f79cae3e5c99042616b6f5 (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
14380 bytes -1 byte zopfli --i100000 --mls256 --bsr40 --lazy --ohh November 10, 2020 @ 13:13
14381 bytes -1 byte zopfli --i1000 --mls256 --bsr11 --lazy --ohh November 9, 2020 @ 18:23
14382 bytes -4 bytes zopfli --i1000 --mls16 --bsr4 --lazy --ohh November 9, 2020 @ 18:22
14386 bytes – zopfli --i100 --mls16384 --bsr8 --lazy --ohh November 9, 2020 @ 18:12

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

Most recent activity on November 10, 2020 @ 17:44.

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 14385 14385 14384 14385 14385 14384 14385 14381 14384 14387 14387 14388 14387 14382 14385
4 14384 14384 14382 14382 14385 14383 14384 14390 14381 14382 14381 14384 14384 14383 14385
6 14381 14381 14382 14382 14384 14381 14381 14390 14381 14382 14381 14384 14384 14397 14381
7 14381 14381 14384 14382 14384 14381 14381 14389 14381 14391 14382 14385 14384 14384 14381
8 14384 14384 14384 14384 14385 14384 14385 14381 14384 14391 14384 14384 14384 14382 14384
9 14381 14381 14384 14382 14385 14389 14381 14389 14381 14391 14382 14384 14384 14398 14399
10 14382 14383 14384 14382 14385 14389 14381 14390 14389 14390 14381 14384 14384 14384 14383
11 14381 14381 14384 14382 14385 14385 14383 14381 14382 14390 14381 14384 14384 14384 14381
12 14381 14381 14384 14382 14382 14382 14381 14389 14381 14389 14382 14385 14384 14396 14398
13 14382 14382 14384 14382 14384 14382 14381 14389 14381 14389 14382 14385 14384 14384 14381
14 14381 14381 14384 14382 14384 14382 14385 14390 14383 14382 14382 14385 14384 14384 14381
15 14381 14381 14384 14382 14384 14381 14381 14389 14381 14389 14382 14385 14384 14384 14381
16 14382 14382 14384 14382 14385 14381 14382 14389 14382 14389 14382 14384 14384 14397 14381
17 14382 14382 14385 14385 14385 14385 14383 14381 14382 14391 14381 14384 14384 14384 14384
18 14382 14382 14384 14382 14384 14382 14382 14390 14381 14382 14382 14385 14384 14384 14381
19 14384 14382 14384 14382 14385 14382 14382 14389 14381 14382 14381 14384 14385 14396 14381
20 14384 14381 14384 14382 14385 14382 14382 14381 14381 14382 14381 14384 14384 14384 14381
21 14381 14381 14382 14382 14384 14382 14381 14389 14381 14382 14382 14384 14384 14384 14381
22 14384 14381 14384 14382 14382 14382 14381 14389 14381 14382 14381 14384 14384 14398 14381
23 14381 14381 14385 14382 14384 14382 14381 14389 14382 14391 14382 14384 14381 14382 14381
25 14381 14381 14384 14382 14382 14382 14382 14381 14381 14382 14381 14385 14384 14397 14381
30 14382 14381 14384 14382 14384 14381 14381 14389 14381 14382 14382 14385 14384 14385 14381
40 14382 14381 14384 14382 14385 14383 14382 14380 14382 14382 14382 14385 14384 14384 14382

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 14386 bytes – 100%
1,000 14381 bytes -5 bytes 100%
10,000 14381 bytes – 100%
100,000 14380 bytes -1 byte 24.93%
1,000,000 14380 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) 14420 bytes +40 bytes (+0.28%) +18 bytes
1 14420 bytes +40 bytes (+0.28%) +18 bytes
2 14402 bytes +22 bytes (+0.15%) –
3 14432 bytes +52 bytes (+0.36%) +30 bytes
4 14460 bytes +80 bytes (+0.56%) +58 bytes
5 14481 bytes +101 bytes (+0.70%) +79 bytes
6 14502 bytes +122 bytes (+0.85%) +100 bytes
7 14545 bytes +165 bytes (+1.15%) +143 bytes
8 14567 bytes +187 bytes (+1.30%) +165 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 12128 bytes -2252 bytes (-15.66%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12442 bytes -1938 bytes (-13.48%)
Brotli (Wikipedia) brotli brotli -q 11 13113 bytes -1267 bytes (-8.81%)
LZMA2 (Wikipedia) xz xz -9 13884 bytes -496 bytes (-3.45%)
Zstandard (Wikipedia) zstd zstd -19 14011 bytes -369 bytes (-2.57%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14075 bytes -305 bytes (-2.12%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15471 bytes +1091 bytes (+7.59%)

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.