Choose a version:
34% The original file has 184250 bytes (179.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 61801 bytes (60.4k, 34%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  14999 bytes (14.6k)
 local copy
Boot
  14911 bytes (14.6k)
 CDN
cdnjs
  14911 bytes (14.6k)
 CDN
gzip -9
  14908 bytes (14.6k)
 local copy
7zip -mx=9 -tgzip
  14475 bytes (14.1k)
 local copy
libdeflate -12
  14458 bytes (14.1k)
 local copy
zultra
  14428 bytes (14.1k)
 local copy
kzip -s0 -rn -b2
  14404 bytes (14.1k)
 local copy
pigz -11 -n
  14394 bytes (14.1k)
 local copy
Zopfli
  14380 bytes (14.0k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-3.0.4.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 3.0.4 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 --mls1024 --bsr6 --lazy --ohh

(found January 25, 2021)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
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/3.0.4/socket.io.min.js --location | md5sum
76df66c7ea771bc7a8957cc029959caf  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.4.min.zopfli.js.gz | md5sum
76df66c7ea771bc7a8957cc029959caf  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/3.0.4/socket.io.min.js --location | sha1sum
101c8d57228ca18292c7cbd1f414dd4448618411  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-3.0.4.min.zopfli.js.gz | sha1sum
101c8d57228ca18292c7cbd1f414dd4448618411  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 14911 bytes 76df66c7ea771bc7a8957cc029959caf (invalid)
cdnjs 14911 bytes 76df66c7ea771bc7a8957cc029959caf (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
14380 bytes -1 byte zopfli --i1000000 --mls1024 --bsr6 --lazy --ohh January 25, 2021 @ 13:08
14381 bytes -1 byte zopfli --i10000 --mls1024 --bsr6 --lazy --ohh January 15, 2021 @ 22:43
14382 bytes -1 byte zopfli --i1000 --mls256 --bsr14 --lazy --ohh January 15, 2021 @ 13:00
14383 bytes -3 bytes zopfli --i1000 --mls8192 --bsr18 --lazy --ohh January 15, 2021 @ 12:50
14386 bytes – zopfli --i100 --mls32768 --bsr20 --lazy --ohh January 15, 2021 @ 10:59

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 @ 13:36.

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 14388 14388 14388 14390 14389 14390 14391 14392 14392 14392 14392 14390 14392 14392 14388
4 14382 14384 14382 14385 14384 14382 14382 14382 14382 14383 14387 14385 14382 14386 14385
6 14383 14383 14383 14382 14385 14387 14382 14382 14381 14380 14384 14384 14383 14382 14382
7 14383 14383 14383 14382 14387 14382 14382 14382 14383 14382 14385 14385 14383 14383 14382
8 14383 14383 14385 14382 14385 14386 14382 14383 14382 14382 14387 14382 14382 14384 14385
9 14383 14383 14383 14382 14385 14383 14382 14382 14385 14382 14387 14382 14383 14383 14382
10 14383 14383 14383 14382 14385 14386 14384 14382 14381 14381 14385 14382 14382 14384 14382
11 14383 14383 14383 14383 14387 14382 14382 14382 14381 14381 14382 14390 14382 14383 14382
12 14383 14383 14383 14384 14385 14382 14382 14382 14381 14382 14386 14387 14382 14383 14385
13 14383 14383 14382 14383 14385 14383 14382 14382 14381 14381 14385 14385 14383 14385 14382
14 14383 14383 14382 14382 14387 14382 14382 14382 14381 14382 14382 14382 14383 14385 14382
15 14383 14383 14383 14382 14385 14382 14382 14382 14381 14382 14382 14385 14383 14383 14385
16 14384 14384 14383 14382 14386 14383 14384 14382 14383 14381 14382 14385 14382 14382 14382
17 14383 14383 14383 14384 14385 14386 14382 14382 14382 14382 14385 14385 14382 14385 14385
18 14383 14384 14382 14383 14385 14382 14382 14382 14381 14382 14385 14382 14382 14385 14382
19 14383 14383 14383 14382 14385 14385 14382 14382 14381 14382 14382 14385 14382 14383 14385
20 14383 14383 14382 14383 14385 14382 14382 14382 14382 14382 14382 14384 14382 14384 14382
21 14383 14383 14383 14382 14385 14382 14382 14382 14381 14381 14382 14384 14383 14383 14385
22 14383 14383 14382 14383 14385 14383 14382 14382 14381 14382 14385 14382 14385 14382 14382
23 14384 14383 14384 14382 14387 14383 14382 14382 14381 14382 14385 14382 14383 14383 14385
25 14383 14383 14382 14382 14386 14383 14382 14382 14381 14382 14386 14382 14383 14384 14385
30 14383 14383 14383 14385 14387 14382 14382 14382 14381 14382 14382 14382 14383 14382 14382
40 14383 14383 14383 14382 14386 14382 14382 14382 14382 14382 14382 14382 14382 14385 14382

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 14386 bytes – 100%
1,000 14382 bytes -4 bytes 100%
10,000 14381 bytes -1 byte 100%
100,000 14381 bytes – 5.80%
1,000,000 14380 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) 14429 bytes +49 bytes (+0.34%) +25 bytes
1 14432 bytes +52 bytes (+0.36%) +28 bytes
2 14404 bytes +24 bytes (+0.17%) –
3 14435 bytes +55 bytes (+0.38%) +31 bytes
4 14448 bytes +68 bytes (+0.47%) +44 bytes
5 14474 bytes +94 bytes (+0.65%) +70 bytes
6 14497 bytes +117 bytes (+0.81%) +93 bytes
7 14527 bytes +147 bytes (+1.02%) +123 bytes
8 14555 bytes +175 bytes (+1.22%) +151 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 12066 bytes -2314 bytes (-16.09%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12420 bytes -1960 bytes (-13.63%)
Brotli (Wikipedia) brotli brotli -q 11 13054 bytes -1326 bytes (-9.22%)
LZMA2 (Wikipedia) xz xz -9 13856 bytes -524 bytes (-3.64%)
Zstandard (Wikipedia) zstd zstd -19 13957 bytes -423 bytes (-2.94%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14043 bytes -337 bytes (-2.34%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15111 bytes +731 bytes (+5.08%)

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.