Choose a version:
35% The original file has 177807 bytes (173.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 61636 bytes (60.2k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  14906 bytes (14.6k)
 local copy
Boot
  14821 bytes (14.5k)
 CDN
cdnjs
  14821 bytes (14.5k)
 CDN
gzip -9
  14818 bytes (14.5k)
 local copy
7zip -mx=9 -tgzip
  14383 bytes (14.0k)
 local copy
libdeflate -12
  14375 bytes (14.0k)
 local copy
zultra
  14342 bytes (14.0k)
 local copy
kzip -s0 -rn -b2
  14318 bytes (14.0k)
 local copy
pigz -11 -n
  14308 bytes (14.0k)
 local copy
Zopfli
  14294 bytes (14.0k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.1.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.1.1 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 --mls16 --bsr19 --lazy --ohh

(found February 8, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 16  --mls16
block splitting recursion 19  --bsr19
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/3.1.1/socket.io.min.js --location | md5sum
908692161d508b2c5348147cc1fe98e3  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.1.1.min.zopfli.js.gz | md5sum
908692161d508b2c5348147cc1fe98e3  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.1.1/socket.io.min.js --location | sha1sum
35003fa2cc8ff3c7787e4b1496700e8738a20b25  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.1.1.min.zopfli.js.gz | sha1sum
35003fa2cc8ff3c7787e4b1496700e8738a20b25  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14821 bytes 908692161d508b2c5348147cc1fe98e3 (invalid)
cdnjs 14821 bytes 908692161d508b2c5348147cc1fe98e3 (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
14294 bytes -1 byte zopfli --i1000000 --mls16 --bsr19 --lazy --ohh February 8, 2021 @ 19:02
14295 bytes -1 byte zopfli --i10000 --mls16 --bsr19 --lazy --ohh February 8, 2021 @ 14:27
14296 bytes -1 byte zopfli --i10000 --mls256 --bsr30 --lazy --ohh February 8, 2021 @ 13:31
14297 bytes -1 byte zopfli --i1000 --mls256 --bsr10 --lazy --ohh February 8, 2021 @ 13:14
14298 bytes -2 bytes zopfli --i1000 --mls2 --bsr19 --lazy --ohh February 8, 2021 @ 13:10
14300 bytes – zopfli --i100 --mls4 --bsr14 --lazy --ohh February 8, 2021 @ 13:01

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

Most recent activity on February 8, 2021 @ 21:54.

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 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 14302 14302 14301 14303 14302 14303 14304 14306 14305 14305 14305 14304 14304 14306 14302
4 14300 14300 14299 14299 14299 14299 14296 14299 14296 14296 14299 14299 14296 14299 14301
6 14299 14299 14299 14297 14299 14297 14297 14298 14296 14296 14298 14300 14299 14296 14296
7 14299 14296 14296 14296 14299 14299 14296 14299 14296 14296 14296 14298 14299 14296 14296
8 14300 14300 14299 14299 14299 14299 14297 14296 14296 14296 14296 14296 14300 14296 14296
9 14297 14296 14296 14296 14299 14300 14296 14296 14296 14296 14296 14296 14302 14296 14296
10 14300 14300 14299 14296 14299 14299 14296 14296 14296 14296 14298 14297 14302 14296 14296
11 14299 14297 14296 14296 14299 14299 14296 14296 14296 14296 14297 14296 14300 14296 14302
12 14299 14299 14297 14296 14299 14299 14296 14296 14296 14296 14296 14300 14299 14296 14296
13 14297 14297 14298 14296 14299 14299 14296 14298 14296 14296 14296 14296 14299 14296 14296
14 14297 14296 14299 14297 14299 14299 14296 14298 14296 14296 14296 14296 14302 14296 14296
15 14297 14296 14296 14296 14299 14299 14297 14299 14296 14296 14296 14299 14302 14296 14296
16 14299 14298 14299 14296 14299 14299 14297 14296 14296 14296 14296 14296 14302 14296 14296
17 14297 14297 14299 14298 14299 14299 14298 14298 14296 14296 14296 14296 14299 14296 14296
18 14299 14297 14299 14300 14297 14299 14296 14296 14296 14297 14296 14296 14302 14296 14296
19 14296 14296 14297 14294 14299 14299 14296 14296 14296 14296 14296 14296 14298 14296 14296
20 14299 14300 14298 14297 14299 14299 14296 14298 14296 14296 14296 14299 14299 14299 14296
21 14299 14298 14299 14299 14299 14299 14297 14298 14296 14296 14296 14299 14299 14299 14296
22 14297 14297 14296 14296 14299 14299 14297 14296 14296 14296 14296 14296 14299 14296 14296
23 14299 14299 14299 14297 14299 14300 14296 14298 14296 14296 14296 14296 14299 14299 14296
25 14299 14298 14297 14297 14299 14299 14296 14296 14296 14296 14296 14296 14299 14296 14296
30 14299 14296 14296 14296 14299 14299 14296 14294 14297 14296 14296 14296 14302 14296 14296
40 14296 14296 14297 14296 14299 14300 14297 14296 14296 14296 14297 14296 14300 14296 14296

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 14300 bytes – 100%
1,000 14297 bytes -3 bytes 100%
10,000 14295 bytes -2 bytes 100%
100,000 14295 bytes – 0.58%
1,000,000 14294 bytes -1 byte 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) 14346 bytes +52 bytes (+0.36%) +28 bytes
1 14347 bytes +53 bytes (+0.37%) +29 bytes
2 14318 bytes +24 bytes (+0.17%) –
3 14348 bytes +54 bytes (+0.38%) +30 bytes
4 14376 bytes +82 bytes (+0.57%) +58 bytes
5 14398 bytes +104 bytes (+0.73%) +80 bytes
6 14422 bytes +128 bytes (+0.90%) +104 bytes
7 14450 bytes +156 bytes (+1.09%) +132 bytes
8 14478 bytes +184 bytes (+1.29%) +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 12021 bytes -2273 bytes (-15.90%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12363 bytes -1931 bytes (-13.51%)
Brotli (Wikipedia) brotli brotli -q 11 12949 bytes -1345 bytes (-9.41%)
LZMA2 (Wikipedia) xz xz -9 13772 bytes -522 bytes (-3.65%)
Zstandard (Wikipedia) zstd zstd -19 13869 bytes -425 bytes (-2.97%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13990 bytes -304 bytes (-2.13%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15046 bytes +752 bytes (+5.26%)

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.