Choose a version:
34% The original file has 183196 bytes (178.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 61563 bytes (60.1k, 34%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  14888 bytes (14.5k)
 local copy
Boot
  14802 bytes (14.5k)
 CDN
cdnjs
  14802 bytes (14.5k)
 CDN
gzip -9
  14799 bytes (14.5k)
 local copy
7zip -mx=9 -tgzip
  14373 bytes (14.0k)
 local copy
libdeflate -12
  14361 bytes (14.0k)
 local copy
zultra
  14322 bytes (14.0k)
 local copy
pigz -11 -n
  14293 bytes (14.0k)
 local copy
Zopfli
  14277 bytes (13.9k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.1.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.1.0 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 --i1000000 --mb8 --mls512 --bsr14 --lazy --ohh

(found January 21, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 14  --bsr14
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/3.1.0/socket.io.min.js --location | md5sum
24c5b6ac56d8d9cc8c194801b852a674  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.1.0.min.zopfli.js.gz | md5sum
24c5b6ac56d8d9cc8c194801b852a674  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.1.0/socket.io.min.js --location | sha1sum
d940ce59fccb16662bd5114a30ac23d169fbd642  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.1.0.min.zopfli.js.gz | sha1sum
d940ce59fccb16662bd5114a30ac23d169fbd642  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14802 bytes 24c5b6ac56d8d9cc8c194801b852a674 (invalid)
cdnjs 14802 bytes 24c5b6ac56d8d9cc8c194801b852a674 (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
14277 bytes -1 byte zopfli --i1000000 --mls512 --bsr14 --lazy --ohh January 21, 2021 @ 17:08
14278 bytes -1 byte zopfli --i100000 --mls16384 --bsr7 --lazy --ohh January 16, 2021 @ 12:19
14279 bytes -1 byte zopfli --i10000 --mls16384 --bsr7 --lazy --ohh January 15, 2021 @ 16:15
14280 bytes -4 bytes zopfli --i1000 --mls256 --bsr8 --lazy --ohh January 15, 2021 @ 12:51
14284 bytes – zopfli --i100 --mls2048 --bsr9 --lazy --ohh January 15, 2021 @ 11:00

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

Most recent activity on January 22, 2021 @ 06:55.

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 14285 14285 14285 14285 14285 14285 14287 14289 14288 14290 14289 14289 14290 14288 14285
4 14283 14283 14280 14283 14279 14282 14281 14281 14282 14279 14279 14282 14279 14282 14284
6 14279 14279 14279 14280 14282 14282 14279 14282 14279 14279 14279 14279 14279 14280 14279
7 14279 14278 14279 14280 14279 14282 14279 14283 14283 14279 14279 14278 14279 14278 14279
8 14280 14280 14280 14282 14279 14282 14279 14279 14279 14279 14279 14282 14280 14279 14284
9 14280 14280 14282 14279 14279 14281 14279 14279 14283 14279 14279 14282 14279 14279 14279
10 14279 14279 14280 14279 14282 14282 14280 14279 14279 14279 14279 14282 14279 14299 14279
11 14278 14279 14279 14280 14282 14282 14279 14281 14279 14278 14279 14279 14279 14279 14279
12 14279 14279 14279 14280 14279 14282 14279 14283 14282 14279 14279 14283 14279 14279 14279
13 14280 14280 14280 14279 14279 14282 14279 14283 14279 14279 14279 14279 14279 14280 14279
14 14280 14280 14279 14279 14279 14284 14279 14283 14277 14279 14279 14280 14279 14279 14279
15 14279 14279 14279 14280 14281 14282 14279 14281 14279 14279 14279 14279 14279 14279 14279
16 14281 14281 14281 14282 14279 14282 14279 14279 14282 14278 14279 14279 14278 14279 14279
17 14279 14279 14282 14279 14281 14282 14280 14281 14282 14279 14279 14282 14279 14279 14279
18 14280 14280 14279 14280 14279 14282 14279 14281 14279 14279 14279 14282 14279 14279 14279
19 14279 14279 14281 14281 14280 14285 14279 14280 14283 14279 14279 14282 14279 14279 14279
20 14281 14280 14281 14279 14282 14282 14280 14279 14282 14279 14279 14279 14279 14279 14279
21 14279 14279 14283 14280 14282 14282 14280 14281 14282 14279 14280 14279 14281 14279 14279
22 14282 14282 14279 14279 14280 14282 14280 14279 14283 14279 14279 14282 14279 14279 14279
23 14280 14280 14282 14279 14282 14282 14280 14281 14282 14280 14279 14279 14279 14279 14279
25 14278 14279 14280 14278 14282 14282 14279 14281 14282 14279 14279 14279 14279 14279 14279
30 14279 14279 14279 14279 14279 14282 14279 14279 14282 14279 14279 14279 14279 14279 14279
40 14278 14278 14281 14279 14280 14282 14280 14279 14280 14279 14280 14279 14279 14279 14279

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 14284 bytes – 100%
1,000 14280 bytes -4 bytes 100%
10,000 14278 bytes -2 bytes 100%
100,000 14278 bytes – 4.06%
1,000,000 14277 bytes -1 byte 3.48%
10,000,000 – – –

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 -2266 bytes (-15.87%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12356 bytes -1921 bytes (-13.46%)
Brotli (Wikipedia) brotli brotli -q 11 12940 bytes -1337 bytes (-9.36%)
LZMA2 (Wikipedia) xz xz -9 13756 bytes -521 bytes (-3.65%)
Zstandard (Wikipedia) zstd zstd -19 13853 bytes -424 bytes (-2.97%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13951 bytes -326 bytes (-2.28%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15036 bytes +759 bytes (+5.32%)

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.