Choose a version:
34% The original file has 184986 bytes (180.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 62076 bytes (60.6k, 34%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  15058 bytes (14.7k)
 local copy
gzip -9
  14973 bytes (14.6k)
 local copy
Boot
  14963 bytes (14.6k)
 CDN
cdnjs
  14963 bytes (14.6k)
 CDN
7zip -mx=9 -tgzip
  14525 bytes (14.2k)
 local copy
libdeflate -12
  14516 bytes (14.2k)
 local copy
zultra
  14471 bytes (14.1k)
 local copy
kzip -s0 -rn -b2
  14450 bytes (14.1k)
 local copy
pigz -11 -n
  14440 bytes (14.1k)
 local copy
Zopfli
  14428 bytes (14.1k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.0.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.0.1 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 --i1000000 --mb8 --mls256 --bsr6 --lazy --ohh

(found November 10, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 6  --bsr6
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/3.0.1/socket.io.min.js --location | md5sum
40ab217559e57f953aa43823f5db4634  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.1.min.zopfli.js.gz | md5sum
40ab217559e57f953aa43823f5db4634  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.0.1/socket.io.min.js --location | sha1sum
0b65935ef35641d488d3e68eac4e9d8af22fb641  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.1.min.zopfli.js.gz | sha1sum
0b65935ef35641d488d3e68eac4e9d8af22fb641  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14963 bytes 40ab217559e57f953aa43823f5db4634 (invalid)
cdnjs 14963 bytes 40ab217559e57f953aa43823f5db4634 (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
14428 bytes -1 byte zopfli --i1000000 --mls256 --bsr6 --lazy --ohh November 10, 2020 @ 16:12
14429 bytes -1 byte zopfli --i10000 --mls256 --bsr6 --lazy --ohh November 9, 2020 @ 18:36
14430 bytes -1 byte zopfli --i1000 --mls256 --bsr6 --lazy --ohh November 9, 2020 @ 18:23
14431 bytes -2 bytes zopfli --i1000 --mls256 --bsr2 --lazy --ohh November 9, 2020 @ 18:23
14433 bytes – zopfli --i100 --mls256 --bsr4 --lazy --ohh November 9, 2020 @ 18:13

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 @ 13:34.

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 14433 14433 14432 14434 14434 14434 14432 14430 14431 14435 14436 14436 14436 14430 14433
4 14434 14434 14433 14432 14431 14432 14430 14430 14434 14433 14434 14433 14434 14436 14438
6 14441 14433 14441 14441 14440 14432 14431 14428 14434 14430 14434 14433 14436 14430 14430
7 14433 14433 14433 14432 14430 14432 14431 14429 14433 14430 14431 14433 14435 14430 14430
8 14433 14433 14432 14434 14434 14433 14432 14430 14430 14433 14429 14434 14437 14430 14433
9 14433 14433 14433 14433 14432 14432 14430 14429 14433 14430 14429 14433 14436 14445 14433
10 14433 14433 14433 14433 14431 14432 14431 14432 14432 14430 14431 14433 14437 14437 14431
11 14433 14433 14434 14430 14431 14432 14432 14431 14432 14430 14429 14432 14438 14431 14430
12 14433 14433 14432 14431 14431 14432 14431 14431 14433 14430 14433 14432 14436 14430 14430
13 14433 14433 14433 14430 14431 14432 14431 14431 14432 14434 14429 14433 14438 14437 14430
14 14433 14433 14434 14430 14431 14432 14430 14432 14433 14430 14430 14433 14437 14431 14430
15 14433 14433 14433 14432 14430 14432 14431 14429 14432 14433 14429 14433 14436 14430 14430
16 14433 14433 14434 14430 14431 14432 14430 14430 14432 14433 14429 14432 14437 14438 14431
17 14433 14433 14433 14432 14430 14433 14432 14431 14434 14431 14431 14433 14436 14430 14433
18 14434 14431 14433 14430 14430 14432 14431 14432 14433 14433 14430 14433 14431 14430 14430
19 14434 14434 14433 14431 14431 14432 14430 14432 14433 14430 14431 14432 14436 14430 14430
20 14432 14431 14433 14432 14433 14432 14431 14430 14432 14430 14429 14432 14431 14430 14433
21 14433 14433 14433 14433 14431 14432 14430 14429 14433 14430 14429 14433 14432 14430 14430
22 14433 14433 14433 14440 14431 14432 14431 14429 14434 14430 14429 14432 14432 14430 14430
23 14433 14431 14434 14432 14432 14432 14430 14431 14433 14432 14433 14433 14436 14431 14430
25 14433 14433 14432 14430 14431 14432 14430 14431 14433 14430 14429 14432 14432 14430 14430
30 14433 14433 14433 14430 14430 14432 14430 14429 14432 14430 14429 14433 14435 14430 14430
40 14432 14432 14433 14430 14430 14433 14431 14432 14434 14430 14429 14433 14432 14430 14430

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 14433 bytes – 100%
1,000 14430 bytes -3 bytes 100%
10,000 14429 bytes -1 byte 100%
100,000 14429 bytes – 5.51%
1,000,000 14428 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) 14473 bytes +45 bytes (+0.31%) +23 bytes
1 14473 bytes +45 bytes (+0.31%) +23 bytes
2 14450 bytes +22 bytes (+0.15%) –
3 14480 bytes +52 bytes (+0.36%) +30 bytes
4 14489 bytes +61 bytes (+0.42%) +39 bytes
5 14512 bytes +84 bytes (+0.58%) +62 bytes
6 14539 bytes +111 bytes (+0.77%) +89 bytes
7 14561 bytes +133 bytes (+0.92%) +111 bytes
8 14596 bytes +168 bytes (+1.16%) +146 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 12108 bytes -2320 bytes (-16.08%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12432 bytes -1996 bytes (-13.83%)
Brotli (Wikipedia) brotli brotli -q 11 13092 bytes -1336 bytes (-9.26%)
LZMA2 (Wikipedia) xz xz -9 13880 bytes -548 bytes (-3.80%)
Zstandard (Wikipedia) zstd zstd -19 13998 bytes -430 bytes (-2.98%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14060 bytes -368 bytes (-2.55%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15180 bytes +752 bytes (+5.21%)

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.