Choose a version:
35% The original file has 177958 bytes (173.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 61701 bytes (60.3k, 35%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  14925 bytes (14.6k)
 local copy
Boot
  14840 bytes (14.5k)
 CDN
cdnjs
  14840 bytes (14.5k)
 CDN
gzip -9
  14839 bytes (14.5k)
 local copy
libdeflate -12
  14400 bytes (14.1k)
 local copy
7zip -mx=9 -tgzip
  14399 bytes (14.1k)
 local copy
zultra
  14356 bytes (14.0k)
 local copy
kzip -s0 -rn -b2
  14328 bytes (14.0k)
 local copy
pigz -11 -n
  14321 bytes (14.0k)
 local copy
Zopfli
  14306 bytes (14.0k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.1.2.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.1.2 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 --mls8 --bsr6 --lazy --ohh

(found February 27, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 8  --mls8
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/3.1.2/socket.io.min.js --location | md5sum
b78ca320934c2ea600f868fde602f965  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.1.2.min.zopfli.js.gz | md5sum
b78ca320934c2ea600f868fde602f965  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.1.2/socket.io.min.js --location | sha1sum
1a0f9dadbce0ebd398fd91db872a2fd78d8b461c  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.1.2.min.zopfli.js.gz | sha1sum
1a0f9dadbce0ebd398fd91db872a2fd78d8b461c  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14840 bytes b78ca320934c2ea600f868fde602f965 (invalid)
cdnjs 14840 bytes b78ca320934c2ea600f868fde602f965 (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
14306 bytes -1 byte zopfli --i100000 --mls8 --bsr6 --lazy --ohh February 27, 2021 @ 04:41
14307 bytes -1 byte zopfli --i10000 --mls2 --bsr6 --lazy --ohh February 26, 2021 @ 17:02
14308 bytes -1 byte zopfli --i1000 --mls512 --bsr4 --lazy --ohh February 26, 2021 @ 15:32
14309 bytes -1 byte zopfli --i1000 --mls64 --bsr23 --lazy --ohh February 26, 2021 @ 14:46
14310 bytes -3 bytes zopfli --i1000 --mls8 --bsr25 --lazy --ohh February 26, 2021 @ 14:45
14313 bytes – zopfli --i100 --mls64 --bsr10 --lazy --ohh February 26, 2021 @ 13:47

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

Most recent activity on March 1, 2021 @ 15:28.

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 14312 14313 14314 14314 14314 14316 14316 14316 14316 14318 14317 14316 14317 14317 14312
4 14312 14312 14309 14309 14310 14311 14309 14308 14308 14308 14311 14313 14310 14312 14308
6 14307 14307 14306 14308 14311 14312 14307 14308 14308 14308 14308 14308 14310 14308 14308
7 14309 14309 14310 14308 14307 14310 14310 14308 14308 14308 14308 14308 14310 14307 14308
8 14307 14307 14312 14311 14307 14310 14307 14307 14308 14310 14308 14308 14311 14307 14310
9 14308 14308 14309 14308 14307 14311 14309 14308 14308 14307 14308 14308 14313 14307 14308
10 14309 14309 14311 14307 14307 14308 14307 14307 14309 14309 14308 14308 14311 14308 14308
11 14309 14309 14309 14308 14310 14309 14307 14307 14308 14308 14308 14308 14310 14308 14308
12 14309 14309 14306 14309 14307 14310 14309 14308 14308 14308 14308 14308 14310 14308 14308
13 14307 14307 14309 14309 14309 14309 14307 14307 14308 14308 14308 14308 14310 14307 14308
14 14309 14309 14309 14308 14309 14310 14308 14308 14308 14308 14308 14308 14310 14307 14308
15 14309 14309 14309 14308 14307 14310 14309 14308 14308 14308 14308 14308 14315 14307 14308
16 14309 14309 14309 14309 14309 14308 14309 14308 14308 14308 14308 14308 14313 14308 14308
17 14309 14309 14309 14309 14307 14310 14309 14308 14308 14309 14308 14308 14311 14307 14308
18 14309 14309 14310 14308 14307 14310 14309 14307 14308 14308 14308 14308 14314 14307 14308
19 14309 14309 14311 14310 14309 14312 14309 14307 14308 14308 14308 14310 14311 14308 14308
20 14309 14309 14310 14307 14309 14308 14307 14308 14308 14309 14308 14308 14310 14308 14308
21 14309 14309 14310 14308 14309 14308 14309 14308 14308 14308 14308 14308 14310 14308 14308
22 14309 14309 14309 14309 14310 14310 14307 14308 14309 14308 14308 14308 14310 14307 14308
23 14307 14307 14310 14308 14309 14308 14312 14308 14308 14308 14308 14308 14310 14307 14308
25 14309 14309 14306 14308 14309 14308 14309 14308 14308 14308 14308 14308 14310 14308 14308
30 14309 14309 14309 14308 14310 14310 14309 14308 14308 14308 14308 14308 14310 14307 14308
40 14310 14310 14306 14308 14309 14308 14308 14308 14308 14309 14307 14309 14311 14307 14308

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 14313 bytes – 100%
1,000 14308 bytes -5 bytes 100%
10,000 14307 bytes -1 byte 100%
100,000 14306 bytes -1 byte 14.20%
1,000,000 14306 bytes – 1.16%
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) 14360 bytes +54 bytes (+0.38%) +32 bytes
1 14359 bytes +53 bytes (+0.37%) +31 bytes
2 14328 bytes +22 bytes (+0.15%) –
3 14359 bytes +53 bytes (+0.37%) +31 bytes
4 14390 bytes +84 bytes (+0.59%) +62 bytes
5 14411 bytes +105 bytes (+0.73%) +83 bytes
6 14433 bytes +127 bytes (+0.89%) +105 bytes
7 14460 bytes +154 bytes (+1.08%) +132 bytes
8 14490 bytes +184 bytes (+1.29%) +162 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 12023 bytes -2283 bytes (-15.96%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12368 bytes -1938 bytes (-13.55%)
Brotli (Wikipedia) brotli brotli -q 11 12971 bytes -1335 bytes (-9.33%)
LZMA2 (Wikipedia) xz xz -9 13780 bytes -526 bytes (-3.68%)
Zstandard (Wikipedia) zstd zstd -19 13880 bytes -426 bytes (-2.98%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13980 bytes -326 bytes (-2.28%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15065 bytes +759 bytes (+5.31%)

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.