Choose a version:
33% The original file has 184112 bytes (179.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 61626 bytes (60.2k, 33%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  14916 bytes (14.6k)
 local copy
gzip -9
  14836 bytes (14.5k)
 local copy
Boot
  14831 bytes (14.5k)
 CDN
cdnjs
  14831 bytes (14.5k)
 CDN
7zip -mx=9 -tgzip
  14385 bytes (14.0k)
 local copy
libdeflate -12
  14382 bytes (14.0k)
 local copy
zultra
  14336 bytes (14.0k)
 local copy
kzip -s0 -rn -b2
  14318 bytes (14.0k)
 local copy
pigz -11 -n
  14304 bytes (14.0k)
 local copy
Zopfli
  14295 bytes (14.0k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.0.3.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.0.3 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 --i1000000 --mb8 --mls8192 --bsr25 --lazy --ohh

(found November 23, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 25  --bsr25
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/3.0.3/socket.io.min.js --location | md5sum
39e4a873315ff750781728f802a1b4c5  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.3.min.zopfli.js.gz | md5sum
39e4a873315ff750781728f802a1b4c5  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.0.3/socket.io.min.js --location | sha1sum
358a40995b92a72736df7db0c0b5ff6c276365f1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.3.min.zopfli.js.gz | sha1sum
358a40995b92a72736df7db0c0b5ff6c276365f1  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14831 bytes 39e4a873315ff750781728f802a1b4c5 November 25, 2020 @ 20:53
cdnjs 14831 bytes 39e4a873315ff750781728f802a1b4c5 November 25, 2020 @ 20:53

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
14295 bytes -1 byte zopfli --i1000000 --mls8192 --bsr25 --lazy --ohh November 23, 2020 @ 15:36
14296 bytes -1 byte zopfli --i100000 --mls8192 --bsr25 --lazy --ohh November 20, 2020 @ 11:39
14297 bytes -1 byte zopfli --i1000 --mls256 --bsr10 --lazy --ohh November 19, 2020 @ 14:03
14298 bytes – zopfli --i100 --mls256 --bsr15 --lazy --ohh November 19, 2020 @ 13:40

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

Most recent activity on January 28, 2021 @ 13:35.

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 14301 14300 14301 14300 14299 14298 14299 14300 14300 14298 14300 14303 14298 14301 14300
4 14300 14300 14299 14299 14299 14299 14296 14296 14300 14296 14298 14299 14302 14298 14303
6 14299 14299 14299 14299 14299 14299 14299 14296 14299 14296 14296 14297 14296 14296 14296
7 14300 14300 14299 14299 14299 14299 14301 14296 14296 14296 14300 14296 14296 14297 14302
8 14301 14301 14299 14299 14300 14298 14298 14300 14300 14297 14300 14298 14296 14300 14299
9 14298 14300 14299 14299 14299 14299 14301 14296 14296 14296 14296 14299 14302 14311 14298
10 14299 14299 14299 14299 14296 14299 14296 14296 14296 14296 14300 14298 14303 14297 14299
11 14303 14303 14299 14299 14299 14299 14301 14296 14300 14298 14300 14298 14298 14302 14299
12 14299 14300 14299 14299 14299 14299 14300 14296 14296 14296 14296 14298 14303 14296 14304
13 14300 14300 14299 14299 14299 14299 14299 14296 14296 14296 14296 14296 14302 14296 14302
14 14300 14300 14301 14300 14299 14298 14296 14296 14296 14296 14300 14296 14297 14296 14299
15 14298 14300 14299 14299 14299 14299 14301 14296 14296 14296 14300 14296 14296 14296 14302
16 14300 14300 14299 14299 14299 14299 14299 14296 14296 14296 14300 14298 14302 14296 14299
17 14299 14299 14299 14299 14300 14299 14298 14299 14296 14296 14301 14298 14296 14296 14301
18 14300 14300 14299 14299 14299 14299 14298 14296 14296 14296 14300 14296 14302 14296 14302
19 14299 14299 14299 14299 14299 14299 14300 14296 14296 14296 14300 14298 14303 14296 14298
20 14300 14300 14299 14299 14299 14299 14299 14296 14299 14296 14296 14298 14298 14296 14299
21 14300 14296 14299 14298 14296 14299 14298 14296 14299 14296 14296 14298 14296 14297 14300
22 14298 14298 14299 14299 14296 14299 14299 14296 14300 14296 14296 14298 14296 14296 14299
23 14299 14299 14299 14299 14299 14299 14296 14296 14296 14298 14296 14296 14296 14296 14298
25 14300 14300 14299 14296 14299 14299 14299 14296 14299 14296 14296 14296 14295 14296 14299
30 14300 14300 14299 14299 14299 14299 14296 14296 14296 14296 14296 14296 14296 14296 14299
40 14300 14300 14300 14299 14299 14298 14301 14296 14296 14298 14298 14297 14296 14298 14299

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 14298 bytes – 100%
1,000 14297 bytes -1 byte 100%
10,000 14297 bytes – 100%
100,000 14296 bytes -1 byte 32.75%
1,000,000 14295 bytes -1 byte 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) 14341 bytes +46 bytes (+0.32%) +23 bytes
1 14344 bytes +49 bytes (+0.34%) +26 bytes
2 14318 bytes +23 bytes (+0.16%) –
3 14345 bytes +50 bytes (+0.35%) +27 bytes
4 14359 bytes +64 bytes (+0.45%) +41 bytes
5 14382 bytes +87 bytes (+0.61%) +64 bytes
6 14412 bytes +117 bytes (+0.82%) +94 bytes
7 14429 bytes +134 bytes (+0.94%) +111 bytes
8 14459 bytes +164 bytes (+1.15%) +141 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 12011 bytes -2284 bytes (-15.98%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12330 bytes -1965 bytes (-13.75%)
Brotli (Wikipedia) brotli brotli -q 11 12966 bytes -1329 bytes (-9.30%)
LZMA2 (Wikipedia) xz xz -9 13760 bytes -535 bytes (-3.74%)
Zstandard (Wikipedia) zstd zstd -19 13866 bytes -429 bytes (-3.00%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13928 bytes -367 bytes (-2.57%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15026 bytes +731 bytes (+5.11%)

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.