Choose a version:
38% The original file has 107516 bytes (105.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 41370 bytes (40.4k, 38%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  12576 bytes (12.3k)
 local copy
Boot
  12544 bytes (12.3k)
 CDN
cdnjs
  12544 bytes (12.3k)
 CDN
gzip -9
  12532 bytes (12.2k)
 local copy
7zip -mx=9 -tgzip
  12071 bytes (11.8k)
 local copy
libdeflate -12
  12071 bytes (11.8k)
 local copy
zultra
  12065 bytes (11.8k)
 local copy
kzip -s0 -rn -b2
  12059 bytes (11.8k)
 local copy
pigz -11 -n
  12052 bytes (11.8k)
 local copy
Zopfli
  12040 bytes (11.8k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.4.0.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.4.0 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 --i100000 --mb8 --mls32768 --bsr13 --lazy --ohh

(found November 25, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 32768  --mls32768
block splitting recursion 13  --bsr13
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/4.4.0/socket.io.min.js --location | md5sum
48ccda3fd4a88b7158f107b91b8a1071  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.4.0.min.zopfli.js.gz | md5sum
48ccda3fd4a88b7158f107b91b8a1071  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.4.0/socket.io.min.js --location | sha1sum
fd57b30d85c2acf9c36baa3e6cc9818cb2bea8ab  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.4.0.min.zopfli.js.gz | sha1sum
fd57b30d85c2acf9c36baa3e6cc9818cb2bea8ab  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 12544 bytes 48ccda3fd4a88b7158f107b91b8a1071 (invalid)
cdnjs 12544 bytes 48ccda3fd4a88b7158f107b91b8a1071 (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
12040 bytes -1 byte zopfli --i100000 --mls32768 --bsr13 --lazy --ohh November 25, 2021 @ 15:49
12041 bytes -2 bytes zopfli --i10000 --mls2 --bsr16 --lazy --ohh November 25, 2021 @ 14:57
12043 bytes -1 byte zopfli --i1000 --mls16384 --bsr19 --lazy --ohh November 25, 2021 @ 14:46
12044 bytes – zopfli --i100 --mls2048 --bsr7 --lazy --ohh November 25, 2021 @ 14:38

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

Most recent activity on November 26, 2021 @ 11:27.

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 12049 12050 12056 12049 12055 12049 12048 12049 12050 12049 12050 12053 12051 12055 12055
4 12045 12045 12044 12043 12042 12053 12041 12043 12042 12041 12043 12042 12057 12044 12056
6 12044 12051 12054 12051 12051 12050 12054 12042 12048 12049 12046 12049 12054 12043 12043
7 12042 12044 12041 12041 12052 12052 12051 12042 12052 12049 12042 12049 12051 12042 12043
8 12043 12041 12042 12043 12043 12052 12052 12042 12050 12049 12049 12043 12051 12050 12057
9 12042 12054 12043 12051 12055 12043 12043 12043 12043 12042 12044 12049 12054 12041 12056
10 12054 12044 12043 12042 12043 12051 12043 12042 12047 12048 12049 12043 12043 12042 12042
11 12053 12047 12043 12051 12042 12043 12052 12042 12050 12049 12048 12049 12053 12042 12043
12 12043 12051 12051 12051 12051 12053 12052 12051 12050 12049 12044 12042 12043 12042 12042
13 12043 12053 12053 12052 12051 12054 12050 12051 12046 12049 12048 12049 12053 12041 12040
14 12053 12043 12042 12042 12052 12052 12042 12042 12043 12042 12044 12042 12053 12057 12056
15 12054 12051 12053 12052 12054 12052 12051 12051 12050 12049 12048 12048 12051 12042 12044
16 12041 12042 12043 12043 12053 12053 12043 12042 12042 12043 12050 12050 12053 12042 12043
17 12053 12054 12053 12052 12053 12051 12053 12051 12048 12049 12049 12042 12051 12049 12055
18 12054 12041 12043 12043 12042 12051 12051 12042 12046 12049 12048 12049 12049 12055 12042
19 12042 12051 12043 12052 12042 12054 12042 12043 12043 12043 12048 12049 12055 12041 12042
20 12054 12052 12051 12051 12042 12053 12052 12049 12046 12049 12049 12042 12051 12049 12050
21 12042 12051 12043 12051 12042 12052 12052 12043 12043 12042 12049 12047 12048 12042 12043
22 12053 12054 12053 12051 12050 12053 12051 12051 12047 12049 12043 12042 12043 12042 12051
23 12043 12053 12043 12051 12042 12042 12052 12044 12046 12049 12048 12047 12053 12049 12042
25 12053 12046 12042 12051 12050 12053 12051 12042 12047 12049 12049 12042 12054 12049 12049
30 12054 12052 12054 12043 12042 12054 12052 12051 12046 12049 12049 12042 12047 12052 12042
40 12042 12043 12043 12042 12052 12050 12052 12049 12047 12049 12049 12043 12043 12055 12043

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 12044 bytes – 100%
1,000 12043 bytes -1 byte 100%
10,000 12041 bytes -2 bytes 100%
100,000 12040 bytes -1 byte 5.22%
1,000,000 12040 bytes – 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) 12112 bytes +72 bytes (+0.60%) +53 bytes
1 12112 bytes +72 bytes (+0.60%) +53 bytes
2 12059 bytes +19 bytes (+0.16%) –
3 12078 bytes +38 bytes (+0.32%) +19 bytes
4 12127 bytes +87 bytes (+0.72%) +68 bytes
5 12152 bytes +112 bytes (+0.93%) +93 bytes
6 12178 bytes +138 bytes (+1.15%) +119 bytes
7 12192 bytes +152 bytes (+1.26%) +133 bytes
8 12222 bytes +182 bytes (+1.51%) +163 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 10286 bytes -1754 bytes (-14.57%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 10705 bytes -1335 bytes (-11.09%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11107 bytes -933 bytes (-7.75%)
Brotli (Wikipedia) brotli brotli -q 11 11292 bytes -748 bytes (-6.21%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 11758 bytes -282 bytes (-2.34%)
LZMA2 (Wikipedia) xz xz -9 11980 bytes -60 bytes (-0.50%)
Zstandard (Wikipedia) zstd zstd -19 12024 bytes -16 bytes (-0.13%)

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.